The mineralogy of the glacial deposits of the region between the rivers Neath and Towy, South Wales

433

THE MINERALOGY OF THE GLACIAL DEPOSITS OF THE REGION BETWEEN THE RIVERS NEATH AND TOWY, SOUTH WALES. J.

C. GRIFFITHS, M.Sc., Ph.D., F.G.S.

[Received 1st May, 1939.] [Read 7111 July, 1939.]

I. INTRODUCTION. has long been known that the glacial deposits of South Wales can be subdivided into Older and Newer Drifts indicating a temporal relationship between an earlier and a later period of ice-invasion. Each of these two drifts may in turn be subdivided spatially into.a local (Northern)' drift and a foreign (Western or Irish Sea)" drift. The local drift was deposited by ice-sheets radiating from two independent centres; one, the Central Welsh ice-sheet, had its gathering ground in the region between the Plynlimon and Drygarn mountains and flowed along the Towy Valley; the other, with its ice-shed on the Brecknock Beacons-Carmarthen Fans escarpment, poured southwards over the South Wales Coalfield. These two sheets coalesced in the region between the Loughor and Towy Valleys, and together, at their period of maximum, had their ice-fronts beyond the present coast from Porthcawl to Pendine.

IT

The foreign drift on the other hand was laid down by the Irish Sea Ice, which had its gathering ground in the South-West of Scotland, the Lake District, the North-East of Ireland, and North Wales. This great mer-de-glace filled the Irish Sea Basin to overflowing and, crossing North Pembrokeshire from northwest to south-east [1*, p. 220J, came into contact with the local ice in the region of the Tat Valley around St. Clears. The Irish Sea Ice overcame the local sheets and invaded the coast of south and west Gower and the Bridgend Plain reaching at least as far east as the western suburbs of Cardiff." George [2, p. 208J has traced the limits of these drifts in Gower by the distribution of their pebbly erratics. A local term indicating that its source was to the north. In contrast to the local drift, indicating that its source lay in the west. * For list of References see p- 457. 3 The author has found erratics representative of Irish Sea Ice in the drifts in the Ely Valley and western suburbs of Cardiff. I

2

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434

C. GRIFFITHS,

The deposits laid down by these ice-sheets, the Northern Drift (deposited by the local ice) and the Western Drift (laid down by the Irish Sea Ice) constitute the Older Drift of South Wales. During the second period of ice-invasion the two local centres once more became operative in the region described, but the Irish Sea Ice had its front in North Pembrokeshire [3 and 4]. The deposits of this invasion are termed the Newer Drift; they are nowhere as extensive as the Older Drift in the region now being investigated. An attempt to delineate the limits of these two invasions was made by Charlesworth [4].

II.

DESCRIPTION OF THE MINERALS.

Samples of material (clays, sands and gravels) of from 3-5 pounds were collected and, where possible, the sampling was carried out vertically as well as horizontally. The method of treatment was briefly as follows:The material was boiled and deflocculated with ammonia and then allowed to settle in a beaker [5, pp. 52-53]. The clay grade first obtained was discarded as being too fine for microscopical analysis. The silt grade was screened through a 60 mesh sieve, and the finer material centrifuged in bromoform. The sand grade was screened through sieves of mesh 30, 60, 90 and 120 and the heavy residues obtained by separation in bromoform. The most convenient grain sizes for this investigation were those of the sands which passed the go-mesh sieve and the silts. No acid treatment was found necessary and magnetic separation was avoided because a quantitative estimate by counting the grains was undertaken. The heavy residues were variously coloured red, brown, black, dependent on the relative proportions of the iron ores, hematite, limonite, ilmenite respectively which were nearly always present in varying amounts. The opaque minerals (mainly iron ores) were nearly always the predominant constituents of the heavy residues. The following minerals were identified : CUBIC.

TETRAGONAL.

Fluorite Garnet Magnetite Pyrite

Anatase Rutile Vesuvianite Zircon

HEXAGONAL AND TRIGONAL.

Apatite Calcite Corundum Hematite Ilmenite Pyrrhotite Siderite Tourmaline

ORTHORHOMBIC.

Andalusite Sulphates Brookite Enstatite Hypersthene Staurolite Topaz

~U NER ALOGY

OF GLAC I AL DE P OSIT S, SOU TH WALES.

435

LIGHT Mol" OC LI x rc , Am phib oles Micas Actin olite Mu scovite H ornbl ende B iot ite Trem olite L epidolite B arkev ikite Mon a zite B lue amphiboles P yroxenes Augi te Titaniferous Chl orite a ug ite Diopside Ch lo rito id Epid ot es A egirine-augi te Pistacite Clino zoisi te T hulite T it an it e Zoisit e ? All anite ?

TR IC LIN IC .

Kyanite

A;\lORPHOUS. Le ucoxe ne Li monite Se ricite Sa uss uri te CO;\IPOSITE

I{ E SID U E S .

Quartz Or thocla se P lag iocla se Albi t e Oligo clase Labrador it e Lab ra doritebytownite Micr ocli ne I{O CK -FR AG)!El" TS

Chert Sha le Qua rtzite

FL U ORITE . This is a very rare mi neral oc curring as irregular gra in s with re fract ive index less t h an that of Ca na d a balsam . The cleavag e is not well shown o n a ny of t he grains . GARNET. An u b iqu it ou s m ineral in these glacial deposits, garnet shows a significan t variation fro m pl ace t o p lace (see p . 45 1). The com m onest varie ty is colo u rless a nd occ urs as subang ular t o sub-rounded grains. I ncl usio ns are co m mo n and take the form of opaque bl obs a nd fin e dust , and so me ti m es occur in suc h quantity that the grai ns h a ve a dusky hue . Clear globular in clu sion s are rare, a nd t hei r refringence is somewhat different fr om the su rro un di ng garnet . So me of these inclusion s show a low doub le refracti on u nder crossed-nicols and v ery occasionally rod-like prisms wit h a h igh refractive in dex and d ou ble refraction also occur. They are tentatively referred t o the Zircon group . Etch-marking is common, but the conch oid al fracture is on ly rarely seen . Dodecahedral grains a re very rare. A pink variety occ urs su b or din ate in a mou nt to the colo urless typ e, but m ost of the character istics mention ed above have b een observe d in both forms . On e pink grain show ed a nisot ro pism with low d ou b le refra ction. Grains WIt h rounded margins showing em bayrne nts are of fr equent occu rrence, but the origin of this feature is obscure; it m a y be due to corrosion. Ye llow a n d pinkish-yell ow ga rnets a re very rare . Some of the d eepe r colo u re d grains are p robab ly representative of the spine ls, but no p ositiv e identification has been ob tained. MAGNETITE.-This m ineral occ u rs sparingly in a ll the residues . It is usually smalle r than the ot he r opa q ue m in erals a nd of rounded or subrounded shape . Occa sion al oct ahedra h ave been n oted a n d so me of these . althou gh stron gly m agnetic, show a red h ematitic coat ing. PY RITE. Another opaq ue mineral whi ch has a sporadic occurren ce being co mmon to abund a nt in the drifts d erived from the Low er Palzeozoic rocks . It is irregular in outl in e, b u t occasion ally shows cubic for m a n d so me t imes p yritoh edra . In reflected light the brass- yellow colou r a n d characterist ic striations a re we ll sho w n . Some of the p yritohedra , presumably p yrite or p seudomorphs after p yrite , show a brown -red . colo ur in reflected light . These have a n oran ge-b ro wn streak and d o n ot di ssolve readily in h ydrochlori c acid. They are probably lim onite.

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C. GRIFFITHS,

ANATASE. This mineral has been recorded from the glacial sands of 'West Pembrokeshire [6, p. 243] and a blue variety from similar deposits in Anglesey [7 , p. 758J. In the drifts of the region here described it occurs as yellow and brown broken and worn euhedra showing two characteristic habits. It occurs most commonly as fractured pyramids rarely capped by the basal plane, but nearly always striated parallel to (oo rj-s-the striations being cleavage traces. Quite frequently rectangular basal sections showing the uniaxial negative directions image occur. Crystals of pyramidal anatase growing on yellow rutile are figured (Fig. 39, No. 12). RUTILE. Rutile is an ubiquitous mineral and one of the most abundant non-opaque heavy minerals. Like zircon, it chiefly occurs in the finer grades, being especially plentiful in the fine sands « 120 grade). Its wellrolled character shows that for the most part it is of detrital origin. The two common varieties are golden-yellow and foxy-red, but the colour variations show a complete transition from one to the other and indeed one or two strongly pleochroic grains show brown-red for light vibrating along the length and yellow for light vibrating across the length. The elongation is always positive. In these deposits the yellow rounded form is the most common, the foxy-red nearly always showing some degree of angularity and frequently occurring as fractured prisms with pyramidal terminations. Rarely, a foxy-red grain occurs in the coarse material; grains up to 0.4 mrn. in length have been seen. Twinning is not too common, but geniculate (101), polysynthetic (902) [8, p. 51], and spearhead (301) twins occur in that order of decreasing frequency. The red variety is often found in the magnetic' crops' and is possibly ferriferou , [9, p. 91]' Sagenitic rutile is referred to under muscovite (p. 443). VESUVIANITE. This is a very rare constituent of the drift of some of the localities in West Gower. An almost euhedral basal section is figured (Fig. 39, No. II). It has a yellow-brown colour, high refractive "index and low double refraction, and does not extinguish completely in any position. In convergent light, under crossed-nicols, the slightly excentral uniaxial figure is positive, and, therefore, it is probably the variety Viluite [10, p. 252]. A core of dust inclusions occurs at the centre. The other grains are similar in general character. but show more rounding and the sign could not be determined because of the anomalous behaviour of the directions image and the dispersed isogyres. ZIRCON. One of the commonest non-opaque heavy minerals at all localities is zircon and, like rutile, it shows a definite concentration in the finer grades. The habit varies from rounded, almost spherical to complete euhedra. Inclusions of dust, opaque blobs, negative crystals, and frequent vacuoles are common characteristics of the mineral. Zoning is quite common and basal sections are occasionally seen. The polyvarietal character and ubiquitous distribution of this mineral entirely masks any significance it may possess as an indicator of source. Of the coloured varieties pink and deep purple are most common and attain a large size, up to 0.25 mm. They are nearly always well-worn and often sub-spherical. Yellow and dusky-brown varieties are rarer and occasionally show well-marked zoning. Several of these yellow grains were tested with the eye-piece spectroscope, but no undoubted xenotime was diagnosed. APATITE. This mineral is sporadic in distribution, but occurs throughout the residues. The average grain size is 0.2 mm., but grains up to 0.5 mm, have been seen. Dust and opaque blobs are frequent as inclusions, and are often orientated parallel to the' £ ' axis; other types of inclusion are rare." r Grains similar to figs. r, 3, 5 described by A. W. Groves and A. E. Mourant [11, p. 95] have been seen.

MINERALOGY OF GLACIAL DEPOSITS, SOUTH WALES.

437

CARBONATES. Both siderite and calcite have been detected at various localities, but show no significant distribution. The calcite usually occurs as irregular grains and sometimes as perfect rhombs. Occasionally the structures in the calcite suggest that the material is derived from organic sources; this variety has been seen in residues from the drift at Rhossili Bay and Burry Holm. Shells have been recorded from the drift at Rhossili Bay [12, p. 291J, but it would be highly speculative to attempt to relate these 'organic' carbonates to 'shelly drift' without further investigation. HEMATITE. Usually abundant as an opaque mineral, but frequently occurring as translucent flakes of blood-red colour, hematite reaches a maximum frequency in the drifts derived from the Old Red Sandstone. (As inclusions see pp. 441 and 443.) ILMENITE. This opaque mineral nearly always forms the bulk of the heavy residue. When quite fresh the grains are rolled and worn subrectangular prisms and are purple red to dull-black in reflected light, but some sign of decomposition to leucoxene is usually present, when whitish or pinkish patches show up in incident light. On magnetic separation, fresh ilmenite from the heavy residues occurs in the moderately magnetic group, and with increasing decomposition to leucoxene grades into the non-magnetic residues, the bulk occurring in the weakly magnetic group. PYRRHOTITE. The mammillary habit, bronze or orange-yellow colour in reflected light, together with its occurrence in the strongly magnetic group characterise this mineral. Its frequency in the drifts remains consistently low and shows no significant increase indicative of a source nearby." TOURMALINE. Tourmaline is a common constituent of all the deposits examined. Its variety of form and colour is striking and as many of the types have proved significant in suggesting a source for the drifts, a detailed description is appended and the main types are classified ; 1. The commonest variety (Variety I) is the brown or green worn pri~m. The habit varies from angular fractured prisms to worn rounded grams showing little sign of prism edge. The pleochroism is usually marked, the grain often being quite opaque in the dark position. Rarely, these forms show one termination. 2. Variety II is of somewhat similar form, but never so rounded as I. It is pink in colour and the pleochroism is pink-fast to opaque or pink to green. The habit is always prismatic and frequently one termination is developed. 3. A much worn variety rarely showing any sign of euhedrism and having a distinctive foxy-red colour often occurs up to 0.4 mm. in greatest diameter, the largest size exhibited by any tourmaline. The pleochroism is not nearly so intense as in Varieties I and II. It is apparently confined to the Irish Sea Drift. 4- Euhedral prisms capped by pyramids of different crystallographic values at opposite ends of the prism exhibit the trigonal hemihedral symmetry of tourmaline. These crystals are always of small size, usually about .075 mm. in length, but exceptionally reaching .II3 mm. 5. In this class are included certain remaining rare types which usually occur as fractured prisms with or without one termination. They are distinguished by their pleochroism as follows-yellow to colourless, mauve to blue, pink to blue. The blue fluted prisms [14, p. 323J are by no means I An increase in frequency might be expected in the drifts derived from Coal Measure rocks as pyrrhotite is an abundant heavy mineral in Coal Measure sandstones. See [13, p. 13I] and work done by the au thor on Pennant Sandstones not yet published.

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C. GRIFFITHS,

common and one blue-green grain showed striations (or perhaps cleavage) parallel to the length and a basal parting across it. The plcc chroism of all the tourmalines is O>E or light-fast. Inclusions are varied but not typical of anyone group. The most common type consists of fine dust either distributed haphazardly throughout tho crystal or arranged in streams parallel to the vertical axis. Small acicular inclusions of an unknown mineral are arranged parallel to '£ ' in the example figured (Fig. 39, No.6). Inclusions of opaque blobs and rounded crystals are of frequent occurrence. Zonal arrangement of the colours is very rarely seen, but parti-coloured grains are more frequent. The colour combinations in the latter are green and brown, and brown and blue. Certain rounded grains almost non-pleochroic and exhibiting a negative uniaxial interference figure in convergent light are orientated parallel to the basal plane (0001). ANDALUSITE. The occurrence of andalusite in glacial deposits has been described at length [6, p. 243J, but its significance as an indicator of Irish Sea Drift at certain localities in the area at present under discussion, merits some repetition. Colourless rounded prismatic grains, often roughly triangular in habit, lying on the prismatic cleavage (Fig. 39, NO.9) are the most common They rarely show any signs of euhedrism, and the elongation is always negative. The interference figure is usually eccentric. One of the grains figured shows a thin colour band, evidently a fairly typical characteristic of the andalusite in the South Wales (Irish Sea) Drifts [6, p. 243J. The pleochroism, pink to colourless (pink-fast), is not often exhibited. Most of the grains contain inclusions, principally fine dust, or opaque blobs. Occasionally these inclusions are arranged parallel to a crystallographic axis. Globular colourless inclusions are rare and only one example has the inclusions arranged in the semblance of a cross typical of chiastolite (Fig. 39, NO.9). The mean refractive index (Y) of a number of grains both colourless and pink, is 1.64. The grain size of this mineral varies from about 0.3-0.5 mm., but usually ranges between 0.3 and 0.4 mm. SULPHATES. One or two colourless grains possessing evidence of three cleavages and having a low double refraction and biaxial positive interference figures are members of the sulphate family. They have a medium high refractive index and have been referred to barytes, but the occurrence of celestite in the Pennant Sandstones! nearby suggests that perhaps they belong to this species. BROOKITE. This mineral is known to occur in considerable quantity in the Basal Grit of the Millstone Grit of the North Crop of the South Wales Coalfield- and as might be expected it bulks largely in the drifts near that outcrop. The commonest variety is coloured a deep golden yellow, exactly similar to that of the brookite in the Millstone Grit. The usual characters (high refractive index, anomalous double refraction and crossed axial dispersion) are shown by all the examples. ENSTATITE. This is a very rare mineral occurring as non-pleochroic colourless, rectangular prisms with a well-marked cleavage along the length. The elongation, parallel to the cleavage, is slow and the double refraction is low. In convergent light a rather diffuse eccentric isogyre is exhibited. Some pale green grains, otherwise similar in character to the above, may be representative of enstatite or may be monoclinic pyroxenes lying parallel to the (100) face. I

2

Grit.

Author's observations. I am indebted to Mr. F. Pascoe) M.Sc., for the description of the minerals from the Millstone

2

3

5 7

6

10 9

8

12 II

FIG. 39.-HEAVY MINERALS FROM THE GLACIAL DRIFT OF THE REGION BETWEEN THE RIVERS TOWY AND NEATH. PROC. GEOL.

Assoc.,

VOL.

L.,

PART

4.

1939·

29

44°

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C. GRIFFITHS,

HYPERSTHENE. A deep green, very faintly pleochroic rounded prism with traces of a cleavage along the length is probably a member of the hypersthene group. The length is slow and the double refraction moderate, extinction straight, and in convergent light the emergence of a bisectrix is seen, the character of the sign being negative. One or two grains showing pleochroism from pink or green to colourless, pink-fast, and otherwise similar to the above in general character are also representatives of this group. STAUROLITE. Staurolite is a fairly common mineral in the drifts of certain areas (see p. 453). Its characteristic habit is a rounded rectangular prism. The colour varies between deep honey-brown and pale yellow, and the refractive index is always high'. The pleochroism is not usually intense and the pleochroic scheme is always dark-slow. The extinction where determinable is straight, but the interference figure is usually too eccentric to be of diagnostic value. The double refraction is low, but the thickness of the grains gives polarisation colours up to a third order red in some few cases. The grains often exhibit a characteristic hackly fracture. Globular inclusions are of frequent occurrence; they show a refractive index greater than the host mineral in most cases, but one in the example figured (Fig. 39, No. 10) has a refractive index less than that of the host. Other examples contain, in addition, several opaque blobs. These inclusions are of common occurrence in the staurolite of the Irish Sea Drift of South Wales. The cleavage is never well-developed. TOPAZ. This mineral occurs sporadically and is confined to the Irish Sea Drift. It is distinguished only with difficulty from colourless andalusite. The grains are usually rounded and show a platy cleavage. The double refraction is always low and commonly the plates show a good biaxial positive interference figure. The frequent occurrence of opaque inclusions, though never in regular arrangement, adds to the possibility of confusion with andalusite. AMPHIBOLES. In this group are included many varieties; the following types, without being exhaustive, represent the commonly occurring species : 1. A rounded cleavage fragment of a blue-green amphibole is figured (Fig. 39, No.8). It contains a few opaque inclusions. The cleavage is well-developed and the pleochroism is blue-green (slow) to yellow-green. The elongation is positive and the double refraction low. The extinction angle in this example is z,..£ = 7° A similar grain with a brown-green colour has an extinction angle of ZI\£ = 16°. A number of grains, identical in character with these examples, but for slight variations in colour and extinction, have been classed together and are very similar to the variety described as ' actinolite' from the glacial sands of Anglesey [7, p. 758]. The extinction angles on a large number of grains of this type have been measured and the minerals are found to fall into the following groups: i) ZI\£=0-9°. ii) ZI\£=14-16°. iii) Z~£=20o. It is of course obvious that in anyone case the extinction angle measured may not be the maximum for the mineral, but by taking the mean of a number of values the maximum or maxima in the example above, it can be approximately determined and the minerals grouped accordingly. The character of the sign, when determinable, is always negative. A similar type of amphibole is common in the East Anglian Drifts [9, p. 93] and has been recorded from Anglesey and West Wales.s 2. Typical hornblende, as seen in these drifts, has a more intense pleochroism, from deep green (slow) to pale green; it lacks the blue tint. The cleavages are always well-shown and the terminations are sharp and I 2

Mean refractive index determined by immersion methods, Y =1.74. Author's observations.

MINERALOGY OF GLACIAL DEPOSITS, SOUTH WALES.

441

saw-edged. The cleavage parallel to "s ' is slow and the maximum extinction angle of this variety is ZI\£=22-24°. The double refraction is considerably higher than that of the first variety, but the other characteristics are essentially similar in both forms. Brown amphiboles of the second variety have been seen. A brown amphibole with ' actinolitic ' habit-that is like variety I above-has an extinction angle of Z,,£ = 16°. 3. Other less well-defined types occur. Certain blue amphiboles may represent the riebeckite-arfvedsonite group, but no undoubted glaucophane has been recorded. This mineral is, however, present in the drifts of Pembrokeshire [6, p. 243, and 10, PI. XI, Fig. 47, No. 6J and the Bridgend Plain." 4. A colourless variety showing good cleavage, medium to low double refraction and similar in other characters to hornblende, has been referred to tremolite. It has also been recorded from the glacial sands of Haverfordwest [10, PI. XXII, Fig. 72, Nos. 2 and 3]. 5. One or two brown grains with intense pleochroism, extinction of 12-14° and moderate double refraction have been recorded as barkevikite [14, p. 324]. The commonest form is the first variety, followed by hornblende. The other types are relatively rare. The average length of variety I is about .15 mm., but the hornblende frequently reaches 0.2 mm. CHLORITE.-Chlorite is a common, sometimes abundant, species of ubiquitous occurrence. Several different varieties are present, but the commonest type is a rounded, detrital, pale green cleavage plate. It is usually non-pleochroic and isotropic. Many grains give interference figures in convergent light, both uniaxial and biaxial, positive and negative varieties being observed. The uniaxial types are probably related to the Penninites and are considerably rarer than the biaxial ones. Very frequently, however, no directions image can be obtained. Another variety of deeper green colour exhibits aggregate polarisation under crossed-nicols and is usually not so well-worn. It is probably a secondary decomposition product of more recent origin than the detrital type. Various brown and yellow chlorites, the colour in some cases due to iron-staining, are common in the drifts derived from the Lower Palzeozoic rocks. Some grains of chloritic appearance, possessing faintly pleochroic haloes and a pseudo-uniaxial negative interference figure, represent altered biotites, while others are pseudomorphous after amphiboles, but these types are of rare occurrence. The inclusions in the chlorites may be classified into six divisions :_2 I. Opaque inclusions of carbonaceous matter or iron-ore, with or without fine dust. 2. Inclusions of zircon, apatite and rarely tourmaline. 3. Inclusions of rutile, as rods, twins, or minute hair-like needles (Fig. 39, NO·3)· 4. Inclusions of hematite, either opaque or translucent, often in hexagonal plates. 3 5. Inclusions of limonite. 6. Inclusions absent. Author's observations. During a recent investigation by the author, the chlorites in the Pennant Sandstones were classified in a similar manner. An attempt has also been made to classify the chlorites in the drift and the Pennant Sandstones on the basis of optical characters and inclusions, bu t has met with no success. Remarks upon the derivation of some of the chlorites of the drifts from the Pennant Sandstones are included in the Table II, p. 450. 3 These inclusions are identical with those described under muscovite p. 4+3. I

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C. GRIFFITHS,

It is highly probable that group 5 has been derived from group 4 by decomposition of the hematite to limonite, the limonite of some chlorites showing a rounded hexagonal outline. CHLORITOID. This is a very rare constituent of the drift of certain localities in Gower (Irish Sea Drift). A deep green non-pleochroic platy crystal, showing two cleavages and lying on a (third) cleavage face is figured (Fig. 39, NO.4). It has a low double refraction and a biaxial positive interference figure. The extinction direction is parallel to a cleavage direction and the optic plane is perpendicular to the same cleavage. The angle between the two cleavages is approximately 70°. One or two grains with a faint pleochroism, blue-green (slow\, to grassgreen, and ill-defined directions images, but possessing a faint crosscleavage similar to that described above, may also be members of this group. EPIDOTE. The epidote family is represented in the glacial deposits by a number of varieties as follows : 1. Pistacite [8, p. 314]. A deep yellow-green (pistachio-green) intensely pleochroic variety with a rounded irregular form. The pleochroism is usually dark-fast and the grains are frequently elongated along (b), Y. The interference figure is commonly of the' compass' type, but is more often eccentric, always showing the emergence of an optic axis. The optic angle (2E) is very high and the isogyre straight; the occasional determination of the character of the sign proved it to be negative. The double refraction is high and the optic plane perpendicular to the length. Inclusions of opaque 'blobs' are of frequent occurrence. Boswell [9, p. 93J has described a similar epidote from the drifts of East Anglia, where there also occurs another variety of paler colour and more angular form iloc, cit.). These two varieties have been recorded in the glacial sands of Anglesey [7, p. 758J, but in the similar deposits from West Wales only the deeper coloured variety is present [6, p. 243J. 2. A colourless variety is subordinate in amount to the green (pistacite) type described above. It usually possesses some indication of prismatic form and frequently shows a cleavage parallel to the length of the crystal. The double refraction is always low and the extinction straight or nearly so. In some cases, however, extinction angles up to 30", have been observed. In covergent light the figure is biaxial, usual!y somewhat eccentric, and the character of the sign is positive. Optic axis figures with straight isogyres and occasionally figures of negative sign have been seen. The optic plane in nearly all cases is perpendicular to the elongation [10, p. 259J1. These examples are considered to belong to the rather illdefined clinozoisite family [8, p. 312J. Certain pink varieties otherwise similar to the above are probably thulites [14, p. 325J. 3. Certain brown and green varieties with well-marked cleavage or cleavages, and possessing a taint pleochroism, are probably representative of other epidote species, but have not been classified as yet. 4. Distinct from all these a pale yellow variety usually irregular in habit, non-pleochroic, and showing aggregate polarisation or ultra-blues and no sign of an interference figure, occurs sporadically throughout the drifts. Its cloudy appearance and ill-defined optical properties strongly suggest derivation by decomposition of other ferro-magnesian minerals. Very rarely this variety is associated with muscovite and is then probably a saussuritic product [15, p. 84J. MICAS. Muscovite, biotite, leached biotite and lepidolite have been identified. The commonest type is a colourless cleavage plate lying on the perfect cleavage (001). The double refraction is low and the extinction t It is probable that there is more than one variety ofclinozoisite present in these drifts aud this would offer an explanation for the wide range of optical properties observed in this species.

MINERALOGY OF GLACIAL DEPOSITS, SOUTH WALES.

443

is frequently undulose. The flakes give a symmetrical biaxial negative interference figure of low to moderate optic angle (2E). The inclusions are characteristic and similar to those of the chlorite group (p. 441) : 1. Opaque inclusions as rounded blobs, minute hair-like needles, or dust. The opaque blobs may be arranged in streams. 2. Inclusions of zircon, apatite and other rod-like crystals. 3. Rutile inclusions as rods, twins, or minute hair-like needles. In one example the rutile inclusions show a tendency to assume a sagenitic habit. 4. Hematite inclusions, either as hexagonal crystals or irregular flakes. 5. Epidote inclusions suggesting saussuritic decomposition products. 6. Inclusions absent. One or two brown cleavage flakes showing a pseudo-uniaxial negative interference figure are probably biotite. When pseudo-uniaxial negative but lacking colour, they are considered to be leached biotites. Some of these contain pleochroic haloes. Chloritised varieties have been referred to on p. 441. One grain, with pleochroism from pale blue to pink and having a symmetrical biaxial negative interference figure and otherwise resembling the micas in its cleavage and refractive index, is referred to the variety lepidolite. MONAZITE. This is an exceedingly rare mineral found only in the , Western Drifts' of the region described. A typical grain of somewhat rounded badly worn habit, possessing a high refractive index and a low (anomalous, non-extinguishing) double refraction has a slow length (?). The interference figure is too dispersed to be of diagnostic value in this example, but another grain of similar character shows a biaxial positive figure of low optic angle (2E). PYROXENES. This group is well represented in the glacial deposits of Gower and has been recorded in similar deposits farther west [6, p. 243], and in Anglesey [7, p. 758]. In the glacial deposits of Gower and the Burry Estuary they are subordinate in amount to the amphiboles, but they apparently bulk more largely in the glacial deposits of Anglesey

[7, loco cit.]. 1. The commonest variety is the brown or green stumpy prism. The cleavage is not often exhibited and the grains are always much worn.' This variety is rarely pleochroic, but always possesses a high double refraction and a high extinction angle (Z~£=45° is the maximum observed). In convergent light a titled optic axis figure of high optic angle presents little opportunity for determination of sign. A complete transition in colour from green to brown is displayed in this group and the crystals always possess a smoky or dusky hue. A purple variety has been referred to titaniferous augite. 2. Another well-defined group has diopsidic affinities. The grains of this group are usually colourless or pale green and have a high double refraction. The extinction angle varies Z~£ = 29-380. The interference figure is of similar type to the augite described above. An example of this type is figured (Fig. 39, No.2). 3. Certain green pyroxenes with intense pleochroism and low extinction angles may be representative of the zegirine-augite species. TITANITE. A few denuded diamond-shaped crystals, colourless or dusky brown, with high refractive index and double refraction show a biaxial positive interference figure of low optic angle. In one example the dispersion, p > v, was sufficiently strong to be ascertained by Wratten filters. I The augite in the drifts of Pembrokeshire has a much more angular habit, indicating a fresh source of supply near at hand.

J.

444

C. GRIFFITHS,

KYANITE. Kyanite has been rec orded from the m ore westerly d rifts in Pembrok eshire [6 . p . 243] a nd near P endine [16 . p . 147J. In the drifts of West Gower and around the Burry Estuary it occurs as st u m p y grains, which sh ow traces of the two cleavages-or of one cleavage and the b as al p artin g . The doubl e refraction is always low. On e ex ample has straight extin ct ion and its interference fig ure shows the emer gence of an optic axis. The optic plane is slow. A number of grains h ave extinction angles o f 30° from the vertical cleavage a nd possess symmetrical biaxial negative int er ference figures. In the drifts t o the west of the Towy, elongated cleavage prism s with opt ica l properties lik e those d escribed above are m ore typical. LEUCOXENE. This miner al has b een referred t o under ilmenite. p . 437. LIMONITE. Us ua lly occu rri n g in the n on-magnetic p ortion of the heavy-residue. limonite . in v arying qu antity , has be en re cognised in all the drifts exam in ed . Its ea rthy-b ro w n or ye llow co lour in refle cted light a n d its so lubi lity in dilute hydrochl ori c aci d have be en used as criteria for re cognition . As a coati ng on other gr a ins lim onite is di stributed throughout the m agneti c grad es. SERICITlC and SUASSURITlC products h ave also b een see n . QUARTZ . T he most ab u n da nt mineral in the deposits is u su ally qu artz (see , h owever. p. 446) ; it occurs in a multitude of for m s cont ain in g a great variety of inclus ions. of which, d ust p arti cles. opaque b lobs. tourmaline rod s. a n d sm all zirc ons a re the m ost co m m on . The only significa nce which can be attached t o this mineral is d ependent on the variou s forms of staining. Clear grains when in bulk usu all y occ ur a t no g rea t distance from the Millstone Grit out cro p s. Red staini n g is oft e n directly related to the proximity of an ou tcrop of Old R ed Sandston e. but this is by no means invariable. The m aj ority of quartz grains show a brown limonitic staining of n o apparent significance . In the drifts deriv ed from the Lower Palaiozoic rocks the com parative scarcity of free quartz gr ains is of particular sign ificanc e (see p . 446) ; their pl a ce is taken b y co m posite rockfr agments. usually shale. F ELSPAR. This group is fairly well re p res ented in the ligh t residues. Representatives of the albite , albite-oligocl a se. labradorit e a n d labradoriteb yt ownit e groups have been observed. On e or two grains of microcline , showing ' cross-hatched' twin ning also occu r . Certain decomposed aggrega t es showing n o sig n of twinn in g a nd possessing a refractive index less than that of Canad a b alsam are the on ly re mainin g ev iden ces of the presence of orthoclase .t In a dd it ion to the m ineral s d escr ib ed above. chert a nd co m p osit e rockfragments of sh a le and q uart zite have b een seen .

III.

RELATIVE ABUNDANCE , DISTRIBUTION AND ORIGIN OF THE MINERALS. It should be emphasized at the outset t hat the sedimentary petrology of the different members of the Upper P alzeozoic rocks . the Old Red Sandstone, Millstone Grit, and Coal Measure sandstones and shales have all been investigat ed by A. Stuart, and by various students under his guidance at Swansea Univer sity College, and based on t his work it is com paratively simple to assign a source to the glacial deposits derived from these format ions . In the case of the Lower Palreozoic rocks the mineralogy is not nearly so well known , but can be computed from published work [17] and from information I

317fi] .

Th e determi na tion oft he Ielspars is based on the groups as given by N. H . WincheIl [8. pp .

445

MI:-iERALOGY OF GLACIAL DEPOSITS, SOUTH WALES.

obtained during the investigation of the sources of the .Upper Palseozoic rocks at Swansea University College. My thanks are therefore due to Mr. Stuart, Mr. F. Pascoe and others for permission to make use of this unpublished information (see also [13J). In estimating the percentages of the heavy minerals the method advocated by the Dutch School [15, p. I7 J was adopted, and in expressing the results the graphical method evolved by the Burma Oil Company was followed [18 and 19, pp. 476-7J. Table I represents a generalisation based on the quantitative estimation of the minerals in the drifts derived from the local Palseozoic formations and deals essentially with the commoner heavy minerals. The figures for the' Lower Palseozoic' drifts are taken from localities in Cardiganshire because the deposits are there derived from Lower Palaeozoic rocks exclusively. These might be considered as the type-associations 115, pp. 10-13] or petrological provinces of the glacial deposits of the region described. TABLE 1. Drifts derived from MINERALS.

OLD RED LOWER PALJEOZOICS. SANDSTONE.

MILLSTONE GRIT.

COAL MEASURES.

-'

o·

o'

80

80

80

9°

10

5

Zircon

8

7°

7°

Rutile

<2

10

10-15

Tourmaline

<2

<10

<10

Garnet

-

0- 15

-

Apatite and Muscovite

-

0-7

?

Anatase

-

-

I

X

Brookite

-

-

x

x

%

0/

Opaques

35

Chlorite

/0

/0

,'0

10-l.5

0-I 10

7- 8

1

f

o-{

Each of these drifts reflects the mineralogy of the parent rock, but as one proceeds up the sequence from Lower Palreozoic to Coal Measures it will be seen that two factors tend to mask the direct origin of the drifts. The first is that the ice passed over the rocks in an ascending stratigraphical sequence and hence the drift derived from the Coal Measure rocks always contains a variable proportion of the Lower Palreozoic to Millstone Grit

J.

C. GRIFFITHS,

material west of the Loughor, and Old Red Sandstone to Millstone Grit material east of the Loughor ; secondly, the Coal Measure sandstones are derived from the reworking of the earlier sediments in great part, so that the original constitution is not very different from the final product. The Lower Palreozoic drift deposits are characterised by a low percentage of opaque minerals, of which pyrite is usually most abundant, a very high percentage of chlorite and very little other material. The' light' residues of these drifts are also characteristic. They contain little detrital quartz, and a high proportion of rock-fragments, chiefly shale with some quartzite. The opaque minerals are predominant III the heavy residues of the drift derived from the Upper Palseozoic rocks and nearly always contain a very high proportion of ilmenite. The heavy residues derived from the Old Red Sandstone usually contain considerable quantities of hematite, while the limonite content gradually increases and displaces both hematite and ilmenite away from that outcrop; the residues change in colour from red through black to brown as the predominant iron-ore changes from hematite to limonite. Leucoxene increases in proportion along with the limonite. The chlorite in the drifts derived from the Old Red Sandstone is very variable in amount as will be seen from the graph (Pi. 27), and probably depends on the variable area of outcrop of the green bands in the different divisions of the Old Red Sandstone. There is generally a decrease in the quantity of chlorite in the drifts derived from the Millstone Grit, though this is often masked by the presence of material from the Old Red Sandstone already present in the drifts. The percentage of chlorite remains more or less constant in the drifts on the Coal Measure outcrops, except for accidental variations mentioned below (p. 447). Zircon is quite constant in proportion throughout the drifts derived from the Upper Palseozoic rocks, and the slight decrease in proportion in the Coal Measure drifts is correlated with the slight increase in percentage of the chlorite. This may be taken to indicate that the Lower Palzeozoic rocks were being tapped during Coal Measure times. Rutile remains remarkably constant in amount throughout, indicating its stability, both chemically and mechanically, as a detrital mineral. Tourmaline on the other hand is more variable and this may be due to its tendency to fracture along planes of weakness parallel to the basal plane and its larger grain size initially, tending to confine it to the coarser grades. These remarks are amply borne out by the graphs. Some additional information, however, may be obtained by comparing the curves for the different minerals. It will be readily seen

MDIERALOGY OF GLACIAL DEPOSITS, SOUTH WALES.

447

that the opaque minerals, zircon, rutile and tourmaline have sympathetic curves, while that for chlorite is antipathetic. This emphasises the contrast between the drifts derived from the Lower Paleeozoic and Upper Palseozoic rocks. Again, it will be noted that the curves for rutile and tourmaline are very similar, although the more variable proportions of tourmaline discussed above tend to mask the similarity. The close parallel between these two curves clearly indicates the sedimentary origin of the two minerals, as it is extremely unlikely that they would occur in similar proportions in igneous or metamorphic rocks. Certain variations in the proportions of the minerals shown by the graphs are classed as accidental variations and are discussed below:Chlorite occurs in unusually low proportions at certain localities, particularly when the deposits are sands and gravels, that is, have been water-borne. Localities lI8, 173, 21 and 23 are examples. It will be noted, therefore, that fluvio-glacial material is unusually rich in the opaque, zircon, rutile and tourmaline suite of minerals. This is not, however, an infallible criterion and needs confirmation from lithological evidence. It has been found that the proportions of tourmaline, garnet and apatite increase in the coarser grades at the expense of zircon and rutile, probably resulting from the initial size difference -tourmaline, garnet and apatite being usually larger in the source rocks than the zircon and rutile-and from the fact that the zircon and rutile have passed through more cycles of sedimentation than the other minerals. That the second reason is operative in the case of the glacial deposits of South Wales is indicated by the comparatively high proportions of rounded and worn zircons and rutiles to fresh euhedra and fractured prisms. The light residues of the drift deposits derived from the Upper Palaeozoic rocks consist of quartz with a little felspar and some rare rock-fragments. The only observed variation which proved of any significance was the nature of the iron-staining on the quartz grains; the quartz from the Old Red Sandstone is stained a bright red, from the Coal Measures, yellow-brown, and from the Millstone Grit being water-clear, that is free from iron-staining. This is by no means without exception, however, and it was found that investigation of the light material was somewhat unremunerative when compared with the results obtained from the heavy residues. The variation in the percentages of the minerals derived from the Irish Sea Drift were found to be somewhat erratic, and the writer feels that, until a sufficiently large area has been dealt with. any conclusions other than those based on qualitative evidence are open to extreme doubt. It will be readily understood that any marked increase in the proportions of these 'foreign' minerals results in a corresponding decrease

J.

C. GRIFFITHS,

in the proportions of the commoner minerals and so tends to mask the origin of the drift which, in the region described, is almost always predominantly local. In discussing the distribution of the minerals it will be seen that the 'Lower Palreozoic ' drift overlaps the Old Red Sandstone solid outcrop and the 'Old Red Sandstone' drift overlaps the Millstone Grit solid outcrop and even reaches the Coal Measures in places; this clearly indicates that the ice which transported the material had a southerly component, a fact abundantly confirmed by the evidence of strise and erratic pebbles. Ice-movement through gaps is also clearly indicated by a more marked overlap southwards, as at Llanarthney, Llandebie, and along the Towy Valley below Carrnarthen. These are the most obvious generalisations obtained from mapping the distribution of the mineral assemblages, but it will be seen that an exception occurs in the north-east corner of the region where the' Old Red Sandstone' drift overlaps the Lower Palseozoic outcrop, indicating that the ice from the north-facing scarp of the Old Red Sandstone in this region poured northwards into the Towy Valley between Llangadock and Llandilo. The resulting contamination of Lower Palaeozoic material by the Old Red Sandstone mineral assemblage is brought out in the graphs by the contrast between the curves for the Cardiganshire drifts-exclusively Lower Palreozoic material-and those for the Towy and Tat Valley. It is shown by the increase in opaque minerals, zircon, rutile and tourmaline, and the decrease in chlorite. Superposed on this difference it will be seen that the curves for zircon, rutile and tourmaline show a minimum between Abergwili and Carmarthen and rise once more along the depression between Carmarthen and S1. Clears to a maximum in the Tat Valley. In other words the contamination by Old Red Sandstone material decreases from Llandilo westwards along the Towy Valley as would be expected, and a fresh source for zircon, rutile and tourmaline supplied the contamination in the Tat Valley. Quite clearly this source must lie to the west and the material was supplied by the Irish Sea Ice which contacted the Central Welsh Ice (Towy Glacier) in the Taf-St. Clears depression. Following the graphs from west to east the influx of Old Red Sandstone material along the Llandebie gap is well seen (253, I27)1 and these deposits form a transition between the drifts derived from the Lower Palseozoic rocks and the almost pure Old Red Sandstone drifts. The curves for the Old Red Sandstone material have been discussed above. It will be noted that the drifts derived from the Carboniferous Limestone and Millstone Grit are very similar to those from the Old Red Sandstone and indeed most of the differences are masked by the large quantity I

The numbers in brackets here refer to the exposures on the graph.

PRO C. GEOL. ASSOC.. VOL. L. (1939). WEST.

I C

LOwER

Card19on~h l reo 3 84

0p A

~

E 5

'1 I.

~ [ zI

R

c

0 N

3~{..

eo

M

... Old Red 5Qndsl'one

£~

TaF Valle~

l1and·

.szs "'" s ' 'L? ~ ~"O N roc

II

6 ~

...

To wy Valley

tltie.

OLD RED

(o:rmt\ythen eQ~ w.~ r

EClst

6 ~~ ~ ~ ;~ ~~"t::; !') ~ ~ S ~ ~ ~ N ...eN ...... N..-f-.l..T~ r-, ", ''3 'T ........ N T

~

Ca.tb. LmJt., o.~d G: MObt' . rit

~AN o·noNE .

-

TilE FAf'l5

.........

-.0 .... "

MEASUR es

pnoom,nQnt , with vllrYlnq

amount"~

of- Old ff~d Sandst'one, Carbonlfel"ou, L l me,tOlle 4nd

Mil15t ollt Gne: .

EI\ST.

Rnna.n t' Sdst. Gwnd~th·F(l.wr.

VQr~ous.

.... -.0 ..,

COAl

N°'-<-O _

..... t"l-o ..-.c 0 ..... 0 ~'O"ClO~~~ ... ~O\O\ N.:::_

.-- P"o4 _

P,nnant~

.... .t of Lou"or.

Lougnor HOlt!.

V1lI1e9'

south West

LhIJ-L1Cln

GOLiJe,...

(ast

Sa.sLn

To.we

Vo.l1e~

Neath

T;;
00" ..... 0-..

VQlIe~.

"'vtn'O\C") <"o(

::1 ~

~ ;::;~O'\

<\. 2. 0

C

3 ~3

~'l:'M'·c

"c>

PAlAEOZOIC

Pu re.

loco

PLATE 27.

e 6 4

-

-

-

-

<,

~

2.

8

6 4 10

,r--

~ ~

~ - -

v------

~

<,

~-,,-

-

-

-

B R u

6

..E

4-

Y I

'T 0

u R

M

..

10

~~~

~

~

e {,

A

4

I H

1..

(

~

A

-

r: ~

0

r-:.:

~

1/ ~ ~ ~ ~ .-J

~

<, ~

V ARIATIONS I N THE FRE QUE NCIES OF THE COMMONER MINERALS FROM THE GLACIAL D EPOSITS OF THE RE GION BETWEEN THE RIVERS TOWY AND N EAT H. (Base d on counts of the

< 120

grade only).

[T o face p. 4+ 8

MINERALOGY OF GLACIAL DEPOSITS, SOUTH WALES.

449

of Old Red Sandstone debris already in the drift. The decrease in chlorite, typical of the Millstone Grit, is seen at exposures 162,' 101, and 106. The ice travelled southwards obliquely across the northeast-south-west valley of the Gwendraeth-fawr : thus little variation is seen in the mineralogy of the drifts along that valley. Between the Gwendraeth-fawr and Loughor Valleys, however, the Pennant Sandstone forms a small plateau at a height of 900 feet. As the curves are followed onto this upland (exposures 249 and 95T) they show an increase in zircon and a decrease in rutile and tourmaline, that is, an almost exact reflection of the differences in mineralogy between the Old Red Sandstone and the Pennant Sandstone. The Survey Officers [20, pp. 190, 207, 210J, describing the drift on this plateau, state that it consists mainly of large boulders of Old Red Sandstone and Millstone Grit. Hence, the Brecknock Ice [see 21, pp. 120-121J while carrying large boulders of Old Red Sandstone and Millstone Grit material onto this upland was entirely free from the finer material which is entirely local in character. The deposits in the Loughor, Tawe and Neath Valleys and the Lliw-Llan Basin show no significant variation and the mineral assemblage clearly indicates a mixed origin from the Old Red Sandstone, Millstone Grit and Coal Measure sediments. In Gower, on the other hand, the deposits show two significant variations. The first is the high proportion of chlorite in the drift of Broughton Bay (207) on the north-west coast. It is clear that the ice carrying Lower Palzeozoic material reached this locality; and it is suggested that when the Irish Sea Ice met the Towy Glacier in the Tat Estuary region, the latter was forced round from a westerly course to a south-easterly one and impinged on the north-west coast of Gower in the region of Broughton Bay. Confirmation of this suggestion has been obtained from the distribution of erratic pebbles in this region. The second variation in the graphs occurs at exposure 135 (Langland Bay) East Gower. There a succession of deposits occurred, and it has been found that the basal deposit (B)which also contains Irish Sea Drift material-consists chiefly of Millstone Grit debris, derived from the outcrop to the. immediate north-west, and that the topmost bed (A) was deposited by the Tawe Glacier and contains Old Red Sandstone, Millstone Grit and Coal Measure material derived from the north-east. The intermediate beds show the gradual transition caused by partial incorporation of the basal layers by successive ice advances. The increase in chlorite from less than 1% (nearly pure Millstone Grit) to over 10% is clearly shown in the graph. The distribution of the Irish Sea Drift material at this exposure is discussed later (p. 456).

J.

45 0

C. GRIFFITHS,

TABLE II Drifts derived from

.

MEASURES. I S~~~S~~·I GRIT. ---------------:-----QUARTZ R A A I A -------------I--R-e-d·--I-----I----MINERALS.

LOWER PALlEOZOIC.

MILLSTONE

COAL

Yellow to brown

STAINED QUARTZ

Roc K

FRAGMENTS!

A

shale

R quartzite

I R quartzite

R various

chert

- - - - - -- - - - - eHL ORITE

Wit h inclusions of he matite limonite ru tile rods, etc. op aque hairs ZIRC ON

Purp le

A

C

R

e

? VR ? VR

F

R

-

R? R R -

A

A

C

C

A

F -

I

F

VR

ZIRCON

RUT ILE,

I

yellow red

R ?

-

" -----

e e

e

F

F

F

R VR

I

I

F

e

R?

C F F

I

e e R e

-

I

R

R

I

I APATITE I -----------

e e e

-

I

Tau RMALINE brown " green " pink " blue

-

GAR NET

-

R F

R

"

M USCOVITE with inclusions of hematite limonite rutile rods, etc. opaque hairs

VR

I

I

-

I

-

I

-

BROOKITE

I

-

R blue

ANATASE

I

-

eALCITE

-

I

VR

very rare.

R

=

I

I

-

e

I

VR yellow

?

I

I

I

-

? F

R R

Variable

SULPHATES

e

-

F

I

I

R F

e e

F

-

Variable

--

I

I

F F

SIDERITE

e

I

C F

R F

R R F

R yellow

yellow ?

F

?

F

-

rare. F = frequent. A = abundant.

C

I

R

=

common.

1\1I ~ ERALO GY

OF G LA CI AL DEPOSITS, SO UTH W ALES.

451

The varietal features of the gra ins confirm the conclusions based on the quantitative estim at ions dis cussed above. The essent ial differences between the drifts derived from the Old Red Sand st on e, Millstone Grit and Coal Measur e sed iments are clearly emphasised, and an attempt has been made to express them in Table II. The frequen cies expressed by letter are purely qu alitative. The distribution of these mineral assemblages is shown on the map PI. 28 , and it will be readily seen that the local drift exte nded at one time over the whole of the present land-area and probably beyond. The lin e (_ . . - ) On the map represen ts the southern limit of the mineral assemblage cha ra ct eristic of the drift derived from the Low er P alseozoic ro cks, while the line (- . - ) represents the southern limit of the drift ch aract erised by the Old Red Sandstone mineral assemblage. To the south and east of that line the drifts, though chiefly derived from the Coal Measure sediments, contain a mixed assemblage. The line (- . . . -) divides the local drift into an eastern half in which no Low er Palseozoic material is pr esent from the western half, whi ch shows a fairly strong admixt ure of

it. In T abl e III the mineralogical const it ut ion of the drifts in Pembrokeshire, West Carmart henshire , and certain localiti es in the region between the T owy and Nea t h Valleys a re compa red . The a ssemblages in Pembrokeshire and West Carma rt henshire represent undoubted Irish Sea Drift , and by comparison the depo sit s at Rhossili Ba y (34), Machy nys Bank (18) and Lan glan d Bay base (135) are also representative of Irish Sea Drift. The rem aining localities represen t local drift , an d the a bsence or presen ce of the' foreign ' or Irish Sea Drift minerals is sufficiently obvious to enable these t wo drifts to be easil y di fferentiated. In the region between the Ri vers Towy an d Neat h t he , for eign' mineral s n ever exceed 20 % of the non-opaqu e h eavy re sidue. Therefore, by far the great er proportion of the m ateri al in the Irish Sea Drift of this region is der ived from eit he r immediately local sources or from similar sources lyin g to the west . For exa m ple, pink t ourmaline, garne t and apatite gra dually decrease in gra in size and frequency sout h and east of the line (-. -) on the map, reaching a minimum in the L oughor Valley around P ontardulais and graduall y in creasing aga in to a maximum, sout h and west of the Wll line . T his p eculiar distribution indicates that these minerals had at least two sources, the one the local Old Red Sandstone and t h e ot her the Irish Sea Ice whi ch very probably derived the bulk of this m aterial from the Old Red' Sandstone of Pembrokeshire. In seeking a source for the minerals of the Irish Sea Drift it is first essent ial t o compute the cons tan t ass em blage t ypical

TABLE III.

To

SHOW DIFFERENCE IN MINERALOGICAL CONSTITUTION BETWEEN NORTHERN AND WESTERN DRIFTS.

Derivation

IRISH SEA

(' WESTERN ') DRIFT.

-'

Type of Drift. -,

Localities.

Mixture

I

I

Mixture

Castle Toch Pendine

Var.locs. in Pemb,

I

Mixture 26 Rhossili Bay Gower

I

Mixture

I Mixture

II

LOCAL (' NORTHERN ') DRIFT. Millstone Grit

"

Central Welsh

I

18 Machynys L1anelly

135 Langland Bay Base ' Top

---

MINERALS. Zircon Rutile Chlorite Muscovite Anatase Brookite Apatite Tourmaline Garnet Andalusite Staurolite Kyanite Topaz Amphibole Pvroxene Epidote, green colourless

Lower Palaeozoic

I Old

Red Sandstone 142

144 Duffrvn Hafod Towy Valley

Sugar Loaf

x

x

x

x

x

x

x

x

X X

X X

X X

X X

X X

X X

X X X X

X X

x X X

x x

x X

X

x

x X X

I

X

X X X

x x

X

X

x X

x x x

x x x

X

X X

X X

x

X X

X

x

x -

x x x X

X

x

x x x

X X

x

x x

-

-

X

X X X

X X

-

x

-

X X

x X

x x x

I

I

-

-

x -

-

X X X

-

-

-

-

-

-

Measure

I

Measure

Glais Tawe Valley

is I[Aberdula Neath Valley

------------

X

X X X

I

Measure

'-100 -1-~;--1--92~

Pont AntwnlN. of Hendy GwendGwili raethlach Valley

x x

I-----;,~-i~-,- Co~~

I

-

-

X

-

-

-

-

-

x x X X

>< x

-

-

-

X

>< X

-

-

-

-

-

x I i

-

-

-

-

-

I

I

x

,

X

;<

x

X

-

X X

X X X

I

X X

x -

x

X

X -

I

I I

-

-

-

MINERALOGY OF GLACIAL DEPOSITS, SOCTH WALES.

453

of that deposit, and Table IV is an attempt to do this, and it appears that the Irish Sea Drift usually consists of green epidote garnet, blue-green amphibole, tourmaline, staurolite, kyanite, zircon, rutile, apatite and chlorite. Garnet, tourmaline, zircon, rutile, apatite and chlorite are also present in the local drifts of the region between the rivers Towy and Neath; hence, the critical assemblage consists of green epidote, blue-green amphibole, staurolite, and kyanite, to which may be added andalusite'. This mineral group indicates fairly obviously an origin in a metamorphic rock series, but may be derived at second-hand from some outcrop of Triassic sediments [22, pp. 291 ft.]. A more detailed discussion of the origin of the mineral assemblage of the Irish Sea Drift will be left for another occasion, as the writer is still investigating the glacial deposits of regions in South Wales where the Irish Sea Drift is more typically displayed". This group of minerals has a restricted distribution in the drifts of the region between the rivers Towy and Neath. They do not occur north and east of the unbroken line on the map. Their frequency and grain size show a gradual increase to the south and west of this line and reach a maximum near the 11W line. The latter is also the northern and eastern limit of the 'foreign' erratic pebbles. Between the dotted and UllL lines these minerals are found in the sand and silt grades, but north and east of the dotted line they are only found in the silt grade. .They not only show a decrease in frequency but also a well-marked gradation between the two un broken lines. Reconsidering the origin of these minerals, it is obvious that in whatever deposit they are now found they could only have been introduced into the area during the invasion by Irish Sea Ice, that is, in Older Drift times. The actual ice-front of the Irish Sea Ice during the period of maximum ice-invasion may be taken to coincide with the lULl line on the map, that is, along the limit of the pebbly erratics typical of Irish Sea Ice. It is obvious that whatever region this glacier invaded it would carry both minerals and pebbly erratics, hence the illli line may be taken as the absolute limit of invasion by Irish Sea Ice, as far as the region between the rivers Towy and Neath is concerned. Manifestly no gradation similar to that shown by the minerals could be brought about by ice alone, and therefore it is suggested that this phenomenon is best explained by the existence of a lake in the region to the immediate northI H. H. Thomas, describing the minerals in the drifts of Anglesey, states that I I in all such sands examined from Bagilltin Flintshire to Pembrokeshire, augite, green hornblende, pink garnet. and, above all, andalusite are characteristic constituents. See E. Greenly [7, p. 758]. 2 Recently the author has had the privilege of examining a number of slides of Irish Sea Drift from different localities in Professor Boswell's collection at the Imperial College of Science. and it has become abundantly clear that a discussion of the origin of the Irish Sea Drift is a subject in itself.

TABLE I V. To

S HOW MI NER ALOGI CAL ASSE MBLAGE S OF THE IRI SH S EA DRIFT.

Regions beyond t he Tow y a nd Nea t h . ,. . . .,

Minerals

\ ~~

i"

a

>, &: ~

~ >-l t:~

0 .;

~.~

:;0 I'l

Iron Ores

~~~~ 0

........

~

Cl)

J

I

';:

Region bet ween the T owy a nd Xe ath.

''' '= 1~ .C!-~"215 1 co

~ .=: ~ 11

~

·... "O u·:::: ~ [; .... ..c: C .!I:llll

:a

~

~~.£ _ ~

==

>.

~8d.lCl)

.0

s: :5

0

~ S"C ~

Q)

lV ~

J~:; S

~I'l

~

:g UI

.!9 C1:) ~.-i

bLl"'O

_ 0

~

.r: .!l

a:::

"t o ~ i5

~

C -.

;z:Jl

~",... 1'0

=-

;;::-

-----'-: ~I'-~-,--X-.--~-~:-: - - :-

B rown Augite Othe r P yroxen es Green Epid ot e Ga rne t Bluegreen a mp hibo le Ot her a mp h iboles Hypersthene T ourmaline Andalusit e Staurolite Kyani te Zircon Rut ile Anatase Gla uco pha ne Mo nazite Apati te Chlorit e

X X X

X X X X

X

X X

-

x

-

X X X X X

X X X X X X X X X

X

X

X

X

X

X

X

X

X

X

X

X X X X

X X

_

X X X

I

-

x X -

-

x x -

X !

-

X

-

X X

blue -

I -

X

X X X X X

-

x x ?

-

X

I

X

-

x

brown X X X X

-

X

x x

X

X

- -

'"

t::

~-

~

~

- 1- - -- 1-

---

X X X X X

X X X X X X X

X X X X X X X

x

X

X

x

X X X

X X X

X

x

X X

X X

X

X

X

X

~-

~-

,- -

X X X X X

X X

brown X X

X

br own X X

X

brown ?

"'" ;~ ~~~

':""' 00 1:01'": 1

-,...

c

~

~

...l

- - - - - -,- - -X X

X X

x x

x

x

X X

X X

X X X X X

~

x

x

x

X

i !

.§'

o

I'l

>.

~ '"t:: ~ O r:-1

>,-

X

X

-

I-

X X X X X X

~~

2 £" ,...00-

",""

en

~

sn

x

x x

.,

X X

X

X

X X X X X

X X

I

X

I

brow n lx bro wn

x

X

X

X

I

X

I

x

br own X

x

MINERALOGY OF GLACIAL DEPOSITS, SOUTH WALES.

455

east of the ice-front. The distribution of the detritus would then definitely show the described gradation away from the I JILl line (the Irish Sea Ice-front) towards the north-east. The impounding of the land drainage by Irish Sea Icc has been described along the whole of its eastern ice-front, in the Lake District [23], the Midlands [24J, Anglesey [7, op. cit.], Cardiganshire and North Pembrokeshire [4, op. cit.]. In the last-mentioned publication Charlesworth described Lake Teifi as being 560 feet deep during one stage of the retreat of the Irish Sea Ice. I It is not surprising, therefore, to find that lakes existed in the South Wales Valleys at least as far east as the Neath during the retreat of the Irish Sea Ice. From the evidence of striae and the distribution of pebbly erratics it may be safely assumed that the ice, whether local or , foreign,' was over I,SOO feet thick in this region and, at the period of maximum, had its surface-level nearer 2,000 feet above O.D! Considering, therefore, the surface-level of these lakes (at least 880 feet above O.D.) it will be seen that the greater part of the region at present being described would be inundated, but as there is no evidence in support of such a wholesale flooding, it is believed that the limits of the lakes were marked by the position of the local ice-fronts during this stage of the retreat. With further retreat of the ice-fronts the lakes had lower surface levels, and an excellent series of overflow gaps indicates the successive stages in the lowering of the levels of these lakes. During the second invasion of the region between the Towy and Neath Valleys, only the local ice-sheets were operative, and along the valleys of the Neath and Tawe it is evident that the ice from the Black Mountains and Carmarthen Fans was present in great force, and the combined ice-front of these glaciers lay beyond the present coast in the region of Swansea Bay. Similarly, there is evidence indicating that the Towy Glacier was well developed and reached the neighbourhood of the Tat Estuary around St. Clears. Between the Towy and Tawe Valleys, on the other hand, the glaciation was not nearly so severe, and it seems that the ice which invaded this region during the period of the Newer Drift left little in the nature of deposits, but was mainly concerned in incorporating material already laid down (chiefly the sands and gravels deposited by the retreating Older Drift ice), and reshaping their topography. Hence many of the mounds (drumlinoid, morainic, etc.) arc clearly topographically young, whereas the mineralogical evidence dates them as Older Drift. I Charlesworth considered that tbis was part of the Newer Drift retreat phenomena! but a preliminary examination of the evidence in Cardiganshire strongly suggests to the author that this lake, at least, is of the age of the Older Drift. 2 It is the author's intention to publish the evidence from pebbly erratics, striae and topography in a later paper.

PROC. GEOL. Assoc., VOL. L.,

PART

4, 1939.

J.

C. GRIFFITHS,

Just north of Pontardulais in the Loughor valley an inlier of drift containing 'foreign' minerals occurs, protected from removal during the later glaciation by its position in the bend of the valley at this point. Elsewhere, in the Tawe Valley, another inlier occurs. but hue the' foreign' minerals are found in the sand grade. The position of this deposit near an overflow gap which was used as the lake outlet during the Older Drift period gives it an obvious significance. In the Morlais Valley, just north of Bynea, in the Gwendraeth-fawr Valley at Pont-yberem and again in the TaJ Estuary near Laugharne, outliers of local drift lie on and are surrounded by deposits containing Irish Sea Drift material. The topographic form of these mounds suggests that in the case of the Morlais and Gwendraeth-fawr Valleys they represent halt-stages in the retreat of the local Newer Drift glaciers; while in the case of the TaJ Estuary the deposits represent the dissected remains of the outwash fan of the Tow)' Glacier during its maximum advance in the Newer Drift period. Finally, at Langland Bay (exposure 135) in East Gower there is the succession of deposits, already referred to on p. 449, in which the basal boulder clay represents the ground moraine of the Older Drift period laid down by the combined local and Irish Sea Ice. This deposit is rich in ' foreign' minerals in all its grades, and pebbly erratics indicative of Irish Sea Ice also occur. Immediately above it a boulder clay containing a few' foreign' minerals shows that some of the underlying material has been incorporated by the successive advance of the Tawe Glacier. Above this boulder clay occurs a coarse gravel entirely free from 'foreign' minerals representing the outwash fan of the retreating Newer Drift Tawe Glacier. Hence, at this exposure a complete sequence of deposits representative of both glaciations can be traced in the correct chronological and spatial order.

IV. SUMMARY AND CONCLUSIONS. The mineralogy of the glacial deposits between the rivers Towy and Neath has been examined in detail, both qualitatively and quantitatively. A varied suite of over 55 mineral species have been found and their distribution recorded. The origin of the glacial deposits can be accurately traced from their included minerals as follows : Mineralogical differences enable the local and Irish Sea Drifts to be accurately differentiated. The former may be subdivided into (i) drifts derived from the Central Welsh Icesheet (Towy Glacier) and (ii) drifts derived from the South Welsh Ice-sheet (from the Brecknock Beacons and Carmarthen Fans). based on their differing mineralogical assemblages. The

:\UNERALOGY O F GLACI AL DE POSITS , SO UTH W ALE S .

457

quantitative evidence of the assemblages indicat es that the region between the Tow)' and Loughor Valleys was a region of mixed drifts during Older Drift times. Similar evid ence shows tha t in the region of the Tat E stuary during this period the Irish Sea Ice contacted the Central Welsh Ic e, both being deflected in a south-easterly direction and impinging on the north-west coast of Gower. The grading of the ' foreign' (Irish Sea Drift) miner als over the Gwendraeth-fawr-Loughor Valley region and in the mouths of the Neath and Tawe Valleys indicates that during the recession of the Older Drift Ice the land drainage was impounded in lakes in the lower reaches of the se valleys, the grea test depth of the lak e in the Burry Estuary region being approximately 880 feet. Outliers of drift containing only local minerals ena ble the limits of the Newer Drift to be delineated. Mineralogical evidence, together with that of the topographic form of the deposits, strongly suggests that the Newer Drift Ice incorporated material of Older Drift age. This incorpor ation, taking place over the region between the Towy and Lough or Valleys, clearly indicates that t he glaciation of this region during the period of the Newer Drift was not nearl y so severe as in Older Drift times. Th e mineralogy of an almost complete sequ ence of depo sits a t Langland Ba y in E ast Gower exhibits the correct spat ial and temporal sequence of eve nts. V. ACKNOWLEDGMENTS. Mention ha s already been made of the author's ind ebtedn ess to Mr. A. Stuart , of Swansea University College, under whose tuition mu ch of this work was carried out. The author 's thanks ar e also du e to Professor T. N. George, whose stimulating discussions first awakened his broader interest in the subject of glaciology. During the lat er stages of the work Professor Boswell , Professor Read and Dr. Brammall have examined th e evidence, and the aut hor is ind ebted to them for much useful inform ation and various refinements in th e methods of inv estigation which have in turn led to greater accuracy in the re sults. REFERENCES. 1. STRAHAN, A., and Other s. 1914 . The Country around Haverfordwest. Mem. Geol. Suru., p . 216. ~ GEORGE,T. N. 1933 . T he Gl acial Deposits of Gower. P roc. Geol. A ssoc., lxx, p . 208. 3. J EHU, T. J. 1904. The Gla cial D ep osits of North P em brok eshire. T rans. R oy . S oc., E d in burgh, x li, p . 53. 4. CHAR LESWORTH, J . K . 1929 . T he Sout h Wal es E nd- Mora ine. Quart. J ourn . Geol, S oc., lx x x v , p . 335.

458 5· 6. 7. 8. <). 10. II.

12. 13. 14. 15. 16. 17.

18. 19. 20. 2I. 22. 23. 24. 25. 26. 27. 28. 29.

J. c.

GRIFFITHS,

HATCH, F. H., H. H. RASTALL and M. BLACK. 1<)38. The Petrology of the Sedimentary Rocks, London, 3rd Edn. THOMAS, H. H. 1909. On Detrital Andalusite in Tertiary and Post-Tertiary Sands. Mineralog. Mag., xv, p. 241. - - - - I n E. GREENLY. 191<). The Geology of Anglesey. Meni, Ceol. Surv., ii, p. 758. WINCHELL, A. N. and N. H. \VINCHELL. 1933. Elements of Optical Mineralogy, II, 3rd Edn. BOSWELL, P. G. H. 1916. The Petrology of the North Sea Drift and Upper Glacial Brickearths in East Anglia. Proc, Geol, Assoc., xxvii, p. 79. MILNER, H. B. 1929. An Introduction to Sedimentary Petrography znd Edn., London. GROVES, A. W. and A. E. MOURANT. 1929. Inclusions in the Apatites of some Igneous Rocks. Mineralog. Mag., xxii, p. 92. PRESTWICH, J. 18<)2. On the Raised Beaches and "Head" or Rubble Drift of the South of England, etc. Quart. Journ. Geol. Soc., xlviii, p. 263. STUART, A. : In D. F. DAVIES, E. DIX and A. E. TIWEMAN. 1<)28. Boreholes in Cwmgorse Valley, Appendix; The Petrology of the Sandstones. Proc. South Wales Inst. Eng., p. 131. 1924. The Petrology of the Dune Sands of South Wales. Proc. Geol, A SSOC., XXXV, p. 316. BAAK, J. A. 1936. Regional Petrology of the Southern North Sea, Wageningen. STRAHAN, A. and Others. 1<)09. The Country around Carmathen. Mem. Ceol. Suru., p. 143. DAVIES, K. A. and J. 1. PLATT. 1<)33. The Conglomerates and Grits of the Bala and Valentian Rocks of the District between Rhyader (Radnorshire) and Llansawel (Carmarthenshire). Quart. Journ. Ceol. Soc., lxxxix, p. 202. EVANS, P. and Others. 1933. Graphical Representation of Heavy Mineral Analyses, World Petroleum Congress, 1., p. 251. KRUMBEIN, W. C. and F. J. PETTIJOHN. 1<)38. Manual of Sedimentary Petrography, London. STI~AHAN, A. and Others. 1<)07. The Country around Amrnanford. Mel'l1. Ceol. Surv., p. 188, et seq. - - - - and others. (1917. The Country around Pontypridd and Maesteg. Mem. Ceol. Suru., 2nd Edn., p. 120, et seq. DOUBLE,!. S. : In G. SLATER. 1930-31. The Structure of the Bride Moraine, Isle of Man. Proc. Liverpool Ceol. Soc., XV, p. 2<)1, et seq. SMITH, I3. 1932. The Glacier Lakes of Eskdale, Miterdale and Wasdale. Cumberland, and the retreat of the lee during the Main Glaciation. Quart. Journ. Geol, Soc., lxxxviii, p. 5. \VILLS, L. J. 1924. The Development of the Severn Valley in the neighbourhood of Ironbridge and Bridgnorth. Quart. Journ. Geot, Soc., lxxx, p. 274. JEHU, T. J. 1909. The Glacial Deposits of Western Cardiganshire. Trans. Roy. Soc., Edinburgh, xlvii, p. 17· SMITHSON, F. 1<)28. Geological Studies in the Dublin District : I, The Heavy Minerals of the Granites and the Contiguous Rocks in the Ballycorus District. Geol Mag., lxv, p. 12. BILLlNGHURST, 5.1\. 1<)2'.). Mineral Analyses of some Ordovician Ror ks from Carnarvonshire. Gcol: Mag., lxvi, p. 2. COPE, T. H. 1902. Note on the Titaniferous Iron Sand of Porth Dinlleyn. Proc. Liv. Ceol. Soc., ix, pp. 208-219. FLETT, W. R. 1925-26. Petrography of the Hungryside Sands. Trans. Ceol. Soc., Glasgow, xvii, pp. 398-40I.

MINERALOGY OF GLACIAL DEPOSITS, SOUTH WALES.

459

LIST OF LOCALITIES INCLUDED IN THE GRAPH PI. 27 (See also map, PI. 28). GWENDRAETH-FAWR.

CARDIGANSHIRE.

384. 383. 382. 370. 362.

Monks Cave, Aberystwyth. Llanrhystyd. Aber-arth. New Quay. Lampeter.

TAF VALLEY.

267. 201. 20 3.

120. 255.

Towv 144.

254. 149.

Morfa Bach. south of St. Clears. Mylet Farm. St. Clears. Banc-y-felin. Pond-ddu, It miles west of Coomb. Wernddu, 2 miles southwest of Carmarthen.

252. 9(J.

97· 98. 250. 112. 130.

PENNANT UPLAND, 'NEST OF THE LOUGHOR RIVER.

250. 249. 95T. 100.

VALLEY.

Dyffryn-hafod Farm, 1 mile south of Llanllwch. Carmarthen. Abergwili, near Carmarthen. Llanegwad, Nantgaredig. Golden Grove, Llandilo. Maerdv Farm, 2 miles south -of Llandilo.

OLD I{ED SANDSTONE.

2 I.

157. I 14. 121.

105. 18. 23. 31. 9. 24. GOWER. 207.

54· 208. 67· 29·

CARBONIFEROUS LIMESTONE MILLSTONE GRIT. 112.

130. 162. 106. 101.

AND

Cwm Cerig Farm, Cross Hands. Maes-y-coed, I mile south of Porth-y-rhyd. Cwm Llynfell, Afon Twrch. Graig-Fawr, It miles northeast of Pontardulais. Kidwelly.

i-mile east of Pontyberern. Llannon. Mynydd Sylen. Gwili Valley, I mile north of Porrtardulais. Hendy, Pontardulais.

LOUGHOR RIVER.

2 I.

(a) Coast, Carmarthen Bay. 73. North of Pendine. 258. Black Scar, near Laugharne. Llanstephan. Coast south of Llansaint. Pont Antwn, Llandefeilog. Glyn Abbey, 2 miles northeast of Kidwelly. 26. Rhossili Bay, Gower. (b) Cannarthen Fans. 167. Pont Clydach. 176. Glan Toddeb, I mile northwest of Pont Clydach. 173. Trapp, Afon Cennen.

Glyn Abbey, 2 miles northeast of Kidwelly. Trimsaran. Pontyates. Pontyberem. i mile east of Pontyberem. Cwm Cerig Farm, Cross Hands Maes-y-cocd, I mile south of Porth-y-rhyd.

135.

Ammanford. Pantyffinon, t-mile south of Ammanford. Plas-rnawr, 2 miles south of Ammanford. Glyn-hir, I mile north of Pontardulais, Hendy, Pontardulais. Machynys, Llanelly. Salthouse Point. Peinbrey. Achddu, Pembrey. Landimore, It miles west of Llanrhidian. Gower Broughton Bay, I} miles east of Burry Holm. Burry Holm. Llangennith, I mile southeast of Burry Holm. Ilston, 3 miles north-east of Oxwich Bay. Mayals, 3 miles south-west of Swansea. Langland Bay, 2 miles west of Mumbles.

Luw-LLAN BASIN.

150. 151.

209. 154.

Pont-lliw. Penllegaer. Railway cutting near Penllegaer. Pant-lasau, Rhyd-y-pandy.

J.

C. GRIFFITHS,

TAWE VALLEY.

164. 153. 187. 227.

Godr'ergraig, 3 miles northeast of Pontardawe. Glais Moraine. Llansamlet, t mile northeast of Landore. Swansea Docks.

239·

244· 243·

NEATH VALLEY.

238.

Drummau Ho, 2 miles north of Crymlyn Bog.

92.

Trallwn, 1 mile west of Crymlyn Bog. Reservoir, Crymlyn. Shell Offices, Briton Ferry. Jersey Marine, Briton Ferry. Rhyddings. Bryncoch, t mile north of Rhyddings. Aberdulais.

DISCUSSION. Dr. I{. O. JONES congratulated the Author upon the completion of an excellent piece of work. He asked whether the Author had any information as to the eastern limit of the Western Drift. It appeared to the speaker that the gap between the Taft and the Rhymney at Nantgarw, and possibly the gap between the Rhymney and the Ebbw at Machen at one time carried overflows from lakes ice-dammed on the south. He also inquired whether the Author could fix the southern extension of the Newer Drift in the neighbourhood of Swansea, as it seemed to him that the dams at Glais and Aberdulais were not End Moraines, but were formed during halt stages in the recession of the Newer Drift. THE AUTHOR, in reply, stated that the gaps mentioned by Dr. R. O. Jones were beyond the region with which he was familiar and he did not, therefore, feel qualified to make any statement with regard to that question. The southern limits of the Newer Drift at Swansea probably lay well beyond the present coast and the contemporaneous dams at Glais and Aberdulais represent stadial halts in the retreat of the Newer Drift Tawe and Neath Glaciers respectively, and were younger than the dam at Landore which in turn was younger than the actual" end-moraine."

EXPLANATION 2

3 4 5 6 7 S 9 IO

II 12

OF

FIGURE

39.

Garnet, rounded grain, showing embayments, X 100. Diopside, colourless prismatic grain. ZIIr;=2g0, X 200. Chlorite, green rounded cleavage flake, with inclusions of rutile hairs, X 75. Chloritoid, cleavage plate showing traces of two cleavages, X 200. Clinozoisite, colourless cleavage prism, optic plane slow, perpendicular to length. Extinction low, X 250. Tourmaline, pink rounded prism, with rod-like inclusions elongated parallel to ' r;,' X 200. Andalusite. (IIO) cleavage flake, containing in addition to a thin colour band, a number of opaque inclusions, X 75. Amphibole, blue-green cleavage fragment, extinction, ZIIr; = 7,0 X 200. Chiastolite, colourless triangular cleavage flake, with inclusions arranged in the semblance of a cross, X 75. Staurolite, honey-brown almost euhedral prism, with rounded globular inclusions, X 100. Vesuvianite, yellow brown basal section (Viluite), with central core of dust inclusions X 200. Anatase, pyramidal crystals growing on yellow rutile, X 200.

i\UNERALOGY OF GLACIAL DEPOSITS, SOUTH WALES.

46r

EXPLANATION OF GRAPH, PLATE 27. (i.) CONSTRUCTION AND USE. Percentages of the minerals-opaques, chlorite, zircon, rutile and tourmaline-arc expressed according to the scale used by the Burma Oil Company as follows (18, p. 255, and 19, p. 473) : Frequency. Percentage. Frequency. Percentage. 8+ 90-100% 614-17% 8 75-89% 5 7-13% 860-74% 4 4-6% 2-3% 7+ 45-59% 3 7 35-44% 2 1-2% 728-34(>~) I -!-I~'~ 6+ 23-27% 1* one grain only 6 18-22% 0 0% The localities are numbered and arranged horizontally in groups from west (Cardiganshire) to east (Neath Valley). Each group represents as near as possible a geographical unit-usually a valley. The terms Lower Pakeozoic, Old Red Sandstone, etc., in bold print represent the source of origin of the drift and below in smaller print the contamination, if any, is indicated. For example, localities 144 to 253 occur in the Towy Valley and the drift at each of these is derived from the Lower Pala-ozoic rockseries with some contamination by Old Red Sandstone material. (ii.) DESCRIPTION. The curves represent the variations in percentages of the minerals cited above and indicate (a) the source of origin of the drifts and (b) direction of ice-movement. (a) Origin of the Drifts. The curves for the opaque minerals, zircon, rutile and tourmaline are sympathetic, that for chlorite antipathetic; hence the chlorite is derived from a different source (Lower Pala-ozoic) from the opaques, zircon, rutile and tourmaline (Upper Palrcozoic). In the graphs this is emphasised by the marked change in slope of the curves between localities 148 and 73, the drift at localities west (left) of this break being derived from Lower Palreozoic rocks (containing abundant chlorite, little opaque material, and rare zircon, rutile and tourmaline). while that to the east (right) is derived from Upper Pala-ozoic rocks (containing some chlorite, abundant opaque material and zircon, and much rutile and tourmaline). The marked similarity both in actual proportion and in the nature of the variations of the curves for rutile and tourmaline enphasise that their source lies in pre-existing sediments. Minor differences in the proportions of zircon, rutile and tourmaline can be used to differentiate drifts derived from the different members of the Upper Palaiozoic rock series (Old Red Sandstone, Millstone Grit and Coal Measures.). (b) Ice-movement. The mapping of the distribution of these mineral assemblages (Plate 28) shows that the' Lower Pala-ozoic ' drift overlaps southwards on to the Old Red Sandstone solid outcrop, and, similarly, the , Old Red Sandstone' drift overlaps southwards on to the Coal Measure outcrops in places. From this it may be deduced that there was a strong southward component in the Towy Ice west of the Loughor Valley. East of that valley in the north-east corner of the map the 'Old Red Sandstone' drift overlaps northward on to the Lower Palaeozoic solid outcrop, the contamination of a pure chlorite drift by the zircon, rutile, tourmaline suite being shown at localities 253 and 127. The contamination can be traced along the Towy Valley (localities 148, 149, 254, 144), gradually dying away westwards as shown by the descent of the zircon, rutile and tourmaline curves to a minimum near Carmarthen (254)' From there westwards these curves rise once more towards Banc-y-felin (203), indicating a new source for zircon, rutile and tourmaline; this source, lying to the west, is the Irish Sea Ice which contacted the Towy Glacier in

462

MINERALOGY OF GLACIAL DEPOSITS, SOUTH WALES.

the neighbourhood of St. Clears. These two glaciers were mutually deflected, the Towy Glacier from a westerly course to a south-easterly one, impinging on the north-west coast of Gower at Broughton Bay (207). The marked peak in the chlorite curve at this locality clearly shows a Lower Palreozoic source for the material and amply confirms the direction of ice-movement in this region. At Langland Bay (135) in East Gower a succession of deposits is exposed, a coarse gravel (A) lying on a brown boulder clay (2 and 3), which in turn rests on a yellow boulder clay (B). The basal deposit contains mainly Millstone Grit debris and the low percentage of chlorite mineralogically confirms this origin. The gradual rise in frequency of the chlorite frem the base (B) to the top (A) shows the incoming of material from another source, and it is found that the brown boulder clay (2 and 3) and gravel (A) contains a mixed assemblage from Old Red Sandstone, Millstone Grit and Coal Measure rocks. The basal layers, consisting almost exclusively of Millstone Grit material, were derived during the period of the Older Drift from the immediate north-west, the overlying boulder clay is the ground moraine of the Newer Drift Tawe Glacier which had its source a considerable distance away to the north-east and the gravel represents part of the outwash fan of this glacier during an early retreat stage. Hence here is a complete sequence arranged in correct chronological and spatial order. It will therefore be seen that quantitative mineralogical evidence enables not only a correct source to be assigned to the drift deposits, but also indicates the nature and direction of ice-movement. In addition, having computed a graph of representative deposits, any new locality may be compared with this, and its source obtained by reading off its position on the prepared graph.

PLATE 28.

PROC. GEOL. ASSOC., VOL. L. (I939).

..".

......

,

'.'

.:

e

. ... ,.. ....

............... :

-;'

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MINERAL PROVINCES IN THE GLACIAL DEPOSITS BETWEEN THE RIVERS TOWY AND NEATH. LOCAL

IRISH SEA DRIFT.

DRIFT.

Eastern limit of Lower Palseozoic minerals. Southern limit of Lower PalreozoicassembJage. " "Old Red Sandstone assemblage.

I II I I

Eastern limit of ' foreign' erratic pebbles and ice-front of Irish Sea ice. Eastern limit of 'foreign' minerals in Sand grade. ,t "}~,, " " Silt ,t Numbers, e.g. 252, refer to Localities.

CONTOURS. 200 feet. 600 I,OOO [To face p, 462.