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Chapter I. General account of the district
PROC. GEOL.
Assoc.,
VOL.
XXV.
PLATE
1.
A.-ARTHUR·S SEAT AND SALISBURY CHAIGS FROM
(See page 40.)
[Pho to b,. R . 1.111111. B.-CONTACT OF DOLERITE AND UPPER OLD RED SANDSTONE. SALISBURY CRAIGS. (See page 40.)
To face page
I.
PROCEEDINGS OF THE
GEOLOGISTS'
ASSOCIATION.
GEOLOGY OF THE DISTRICT AROUND EDINBURGH.* CHAPTER I. GENERAL ACCOUNT OF THE DISTRICT. By J. S. FI:.ETT. M.A., D.Sc., LL.D., F.R.S.. F.G.S.
F we take up a small-scale map of the Geology of the Lothians, such as, for example, the one-inch Sheet No. 32 of the Geological Survey of Scotland, we see that the principal structural feature is the presence of a great anticline, along which the older rocks rise to the surface and are flanked on each side by strata of newer age. This anticline is known as the Pentland axis, because the Pentland Hills, which begin about three miles south of the city of Edinburgh, and extend for a great distance to the south-west, are the visible expression of this great subterranean fold. The Pentlands stand out boldly above the low undulating surface of Midlothian, principally because the upheaval has brought up a great series of hard and durable volcanic rocks, which during the long ages through which denudation has been acting on them have acquired prominence through their superior hardness and resistance to erosion. Edinburgh stands on the continuation of this anticline. It passes almost through the Waverley Station, and runs northwards to the shores of the Firth of Forth near Leith. What is probably the same fold appears on the north side of the Firth at Burntisland, in Fife, so that we must regard it as a structure of no small magnitude. The axis of folding, however, is not horizontal, but dips to the north-east. We might compare it, in fact, to the bow of an overturned canoe; or, again, we might express this feature by saying that the fold" noses out." or " pitches" to the north. For that reason the lowest beds exposed on the back of the fold occur in the south-west, while higher and higher beds come in as we travel in a north-east direction. Thus, in the Pentland Hills, about twelve miles from Edinburgh, Wenlock, Ludlow and Downtonian rocks occur in the core of the anticline. These sink beneath the Lower Old Red Sandstone conglomerates, which in turn are wrapped over by the lavas and ash-beds belonging to the same great epoch. These lavas stretch up into Edinburgh, where they are last seen in the Braid Hills and Blackford Hill, on
I
• Issued as a Pamphlet to members taking part in the Excursion to Edinburgh In August, '9'3. PROC, GEOL.
Assoc. VOL. XXV,
PART
I, I914.J
2
j, S. FLETT.
the sou thern borders of the city. O ver them th e Upper Old R ed Sandst on e now sweeps in a great horse-shoe curve, an d th is in turn passes up into th e Lower Carbo niferous, known in Scotland as th e Calciferous Sandstones, on which th e central and northern part s of Edinburgh ar e built. The great Pentl and arc h is by no means simpl e or symmetrical. It is of very old standing, and th ere is evidence that at two differen t periods it -was expose d at th e surface, and great shee ts of gravel were produced by its denudat ion. On e of these is at the base of the Lower Old Red Sandstone, and the other at the base of the Upper Old Re d Sa ndstone, and each of them mark s an important un conformability ; moreover, on its east side the ar ch is broken, and an important fault brings down the higher rocks that form the Mid lothi an Coalfield agains t the vastly old er lavas and ashes of th e Lower Old Red. The effect of this dislocati on is to bring th e Coal Measures, the high est rocks of this district, within two or thr ee miles (measured across th e out crop s) of the Old Red Sandstone. On the western side of the anticline, however, there is no great fault ; the dips are gentle and rolling, and a wide stretch of Lower Carboniferous bed s (Calciferous Sandstones, etc. ) must be crossed before the Coal Measures are reached to the west of Bathgate, sixtee n or eight een miles from Edinburgh. The Midlothian or Pentland ant iclin e, accordin gly, is asymmetrical, being stee p on its easte rn side, while its west side has a gentle dip ; a nd, moreover, it is broken , or perhaps we might say reinforced, by a great fault on its eastern side . Also, as we have said alread y, it " pitches" to the north. On the east sid e of th e Pentlan d fault th e strata lie in a very perfect syncline that forms the Midlothian Coalfield. It contains a nearl y complete success ion of th e Carbo niferous rock s from low down in th e Calciferou s Sandstones to th e top of th e Coal M easur es. Onits west side, where it ab uts againstthePentland arch, the synclin e is ste ep and the beds lie at angles of 45 degrees andover, but soon th ey flatten and after a time rise to the surface and undulat e gently over a wide area in East Lo thian, -till, nea r D unbar, the Old R ed Sandstone again rises to th e surface. On the west of the Pentlands, between Ed inbur gh and Bathgate, the whole of Midlothian and part of East Lothian cons ist of Lowe r Carboniferous rocks which are folded into many little anticlines and synclines. The Carboniferou s Li mestone comes in at Bathgate, where it dips to th e west below th e Lanarkshire Coalfield. The beds that underlie th e limestone s are principally shales and sandstones, with which occur the valuable deposits of oil-shale which have given rise to th e oilshale industry in th e L othi ans. Well-known centres of oil-shale mining ar e Pumpherst on, Broxburn and Dalm eny, an d th e whole of this distri ct is marked by great flat-topp ed mounds of red burnt shale, the refuse of the oil reto rts. The oil-shales lie in litt le basins that are separated by arche s or anticlines, but, on
GENERAL ACCOUNT OF THE DISTRICT.
3
the whole, as we travel eastwards from Bathgate to Edinburgh, there is a descending sequence though the same beds are repeated again and again by the folding, particularly in the western part of this district. The Burdiehouse limestone is of fresh-water origin and marks a well-defined horizon near the middle of the oil-shale group. (See Fig. I, p. 5.) Beneath the productive shale measures lies a great thickness of sandstone and shales, and among the former a very pure and durable sandstone was worked for many years at Craigleith quarry on the north-west side of Edinburgh. THE SEQUENCE OF ROCKS IN THE LOTHIANS.
If we turn now to consider the rocks that occur in the Edinburgh district and the order in which they were deposited, we find that the oldest are those that are brought up to the surface in the core of the great Pentland arch. There are shales, grits, and greywackes that belong to the Wenlock and Ludlow divisions of the Upper Silurian and contain their characteristic fossils. They pass upwards into red Downtonian rocks in which are intercalated beds of conglomerate with pebbles of quartzite and of volcanic rocks. The whole assemblage is seen in the North Esk burn on the southern limits of the Edinburgh map. Near the waterfall above Loganlee Reservoir in the Pentlands the grey Ludlow shales and grits are well exposed, and are intensely folded, many .of the beds being in a vertical position. The Lower Old Red Sandstone, which is the next formation in order of deposition, rests unconformably upon these rocks, and it is evident that the Silurian beds had been much folded and had suffered great denudation in the interval between the two series of deposits. This epoch of folding is that which marked the close of Silurian times (the Caledonian folding), and threw the Silurian strata of the Southern Uplands of Scotland into dose-packed isoclinal folds. Evidently the Pentland arch was at this early time a line of pronounced movement, for in other places, notably at Stonehaven in Kincardineshire, and near Lesmahagow in Lanarkshire, there is no break between Silurian and Old Red Sandstone, but the former passes up, perfectly conformably, into the latter. The surface that lies beneath the Lower Old Rea Sandstone of the Pentlands is thus the oldest land surface of which we have evidence in the Lothians. The Lower Old Red Conglomerate is overlain by the great series of lavas and ash-beds that form most of the Pentland chain as seen from Edinburgh. They were, a large part at least, of subaerial origin, and indicate the site of volcanic islands that rose above the surface of the waters of Lake Caledonia. In its geographical boundaries this chain of volcanoes could have borne little resemblance to the Pentland Hills as we see them to-day, for on the east side a great fault has cut through the rocks, and no doubt beneath the Dalkeith Coalfield there are buried masses of lavas and ashes belonging to this group. We may also readily
4
J.
S. FLETT.
believe that they extended much farther to the west, for the principal vents known in the Pentland Hills occur on that side, and it seems as if a considerable part of the lava-flows had emanated from that quarter. The Upper Old Red Sandstone and the Lower Carboniferous probably conceal from our view a large extent of volcanic rocks of the same age and the same kind as form the Pentland Hills at present. There is no true Middle Old Red Sandstone (which corresponds in time to the Middle Devonian) in this part of Scotland, and though great masses of sediment were accumulating in freshwater lakes in Caithness and the Orkneys, there was dry land in the Lothians ; and the volcanoes which we have described above, together with the sandstones and marls that no doubt overlay them, were being rapidly eroded and reduced in height. The strata that rest upon the Pentland volcanics belong to the Upper Old Red Sandstone, and are mostly cream-coloured and brownish sandstone, sometimes pebbly, with bands of clay and here and there thin concretionary limestones, known as " cornstones." These rocks wrap round the Pentland chain, especially on its western side; on the east side they have sunk from view along the great Pentland fault. The south of Edinburgh about Blackford Hill and Morningside districts is built on them, and they are well exposed in the Suburban Railway cutting near Craiglockart Station. The" Cornstone" which occurs near the top of this group has been seen in excavations near the Meadows. Between them and the Pentland lavas there is a strong unconformability, attended by an overlap, but there is no great development of conglomerates at the base of the overlying series. In this unconformability we have the evidence of the second land-surface that was formed in the Edinburgh district, when the much-denuded Pentland ridge was slowly sinking in the waters of the Upper Old Red Lake and was being gradually buried in sheets of sand and mud. The Upper Old Red Sandstone passes up, perfectly gradually and conformably, into the Lower Carboniferous. The red sandstones, cornstones and marls, deposits of fresh water, give place to shales and sandstones that were deposited under estuarine conditions and contain common types of Carboniferous shells, fishes and plants. The line of transition between the two types of strata passes through the south of the city, but is not visible in any good section. It is well known, however, from information obtained through boring operations for supplies of water for the breweries, which depend largely on hard water obtained from the Upper Old Red Sandstone. In the King's Park the junction of the two formations passes through the Hunter's Bog just above the intrusive sheet of Salisbury Craigs. At the base of the Carboniferous lie the "Ballagan Beds" or Cementstones, a group of green, grey and brown clays with seams
GENERAL ACCOUNT OF THE DISTRICT.
5
FEET
gooo
}
Re d Sandst one. Hor izon not proved
E Xira Limest on e
'>"
f '"'"" is: .... } o.l:'l~ ~- ~ "'0
I
"'"
WOOD COAL t"
s:n
S OU T H PAR ROT COAL U
'"
0.'" g,c:
8000
7000
..>
;;::
.. Dalkeith " and Musse lburgh Coalfield
R oslin Sand st one COAL I' Lev enseat" Limestone COALS I I Cal my" or Gair Limestone
o 0. 0
Index " Lim estone
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.. " "
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tr,~
.. Edge Coal s " and Ir onst ones, Loanhead
cr.~
"' 0
:! c: ",(Il
6000
(Il
"Bilston 'Burn JJ Lim est one Vexhim Lim estone and Coal .. N ort h Gr een s " Li mest on e and Coal G ilmerton L imeston e (= Cobbinsh aw)
5000
R AEBURN SHALE MUNGLE S HALE and Mari ne Sh ell Bed T WO· F OO T CO AL
Hou sl on Marls H OUST ON COAL GREY SHA LE
FELLS S HAL E AND LIME STONE
Broxburn Marls HROXB U RN S H AL E C H AMPFLEUR I E SHA L E
4 000
Binn y Sands tone D U N NET SHALE
Barracks Limest on e and Ash Dunnet Sa nds tones CAM PS SHA LE B U RDI EHOU SE LI M E ST ON E
0
;:
u,
'" ;>
e-
too Cl
30 00 PUMPH E RSTON SHAL E S
Marine Shell Bed
..'" 0
c:
o
> o
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:;;
'" °"c: '"
o.~
-" ... '" °" 'e"n ';;;'"" 0
DA L MAHOY SH ALE
2 000
Hailes Sand stone W ardie Sha les
'"
G ran ton Sandstone Abbeyhill Shales 1000
Arthur's Sea t Volcan ic Group
o
"..,''"" " Cem ent stone Gr oup or "Ballagan Beds "
..,
(Il
0
" c: .'" o'"
o
Upp er Old Red Sandstone
0
6
J. S. FLETT.
of impure limestone, but they are not well exposed; upon them rest the volcanic rocks of Arthur's Seat and the Calton Hill, which yield some of the most picturesque scenery in the district. The lavas are principally olivine basalts, dark on fracture but weathering brown, and though they are succeeded by mugearites there is far less variety of petrological types than in the Pentland Hill lavas, which include rhyolites, dacites and many types of andesites and basalts. The Edinburgh volcano was only one of a group, some of which were far larger and more important. Thus at North Berwick and in the Garlton Hills near Haddington the volcanic outbursts began earlier, lasted longer, and were on a much larger scale than at Arthur's Seat. At Burntisland, some time after the volcanoes we have mentioned had become extinct, eruptions began on the site marked by the Binn, the steep hill overlooking the town, and the lavas and ash-beds may be traced in continuous section along the shore from Pettycur to near Kirkcaldy. The great eruptions of the Bathgate Hills belong to the period of the Carboniferous Limestone, because beds of fossil corals occur interstratified with the lava flows. They were at their maximum activity about the time when the Burntisland volcano was waning to its close. The Oil Shale Group succeeds the Arthur's Seat volcanic series, but in the lower part of the strata there is no workable oil-shale. The most important seams lie at a higher level, either below the Burdiehouse Limestone or between that and the base of the Carboniferous Limestone. At Burdiehouse, three miles from Edinburgh, the old quarries so famous for the fossils they have yielded are still being worked. The rock is a fresh-water limestone, and contains no corals or brachiopods. In fact the Lower Carboniferous of Scotland as a whole is a fresh-water or estuarine deposit, and contrasts strongly in this respect with the marine limestones of England. Among the oil-shales there are a few bands containing Orthoceras and goniatites, but these, like the marine bands in the English Coal Measures, indicate only occasional invasions of the district by the sea. The Scottish Carboniferous Limestone is really a series of shales, sandstones and coals, with seven or eight beds of limestone usually only a few feet thick. In the middle of this series there is an important group of coal seams known as the Edge Coals, as they were worked at an early date in the steeply-inclined beds on the west margin of the Midlothian Coal-field. It is by no means uncommon to find a marine limestone resting on a coal or fireclay, a striking example of rapid change in conditions of deposit, as a terrestrial surface covered with dense vegetation was suddenly covered by clear sea-water in which marine animals flourished and left banks of corals and shells. The Carboniferous Limestone Series is well exposed on the shores of the Firth of Forth at Aberlady and at Kinghorn.
GENERAL ACCOUNT OF THE DISTRICT.
7
The Millstone Grit in Scotland is quite thin in comparison with the same set of rocks in England, and is followed by the Coal Measures of which the uppermost part is composed of. red sandstone and has no workable coals. They occur in the Lothian Coal-field, where the Upper Red Beds are also represented, and also on the north of the Firth of Forth at Kirkcaldy. These two coal-fields are probably continuous beneath the sea, and coal seams have been worked for nearly three miles out from the shore on the Fife side, but as yet submarine coal-mining in Midlothian is only in its infancy. In the East of Scotland there is hardly any trace of the richly fossiliferous Secondary deposits that form some of the most interesting and instructive chapters of English geology, and Tertiary strata are altogether wanting. The geological record for these periods is a blank so far as our district is concerned, but it again becomes full of variety when the Glacial period is reached. The physical features of the Lothians owe their present configu ration very largely to the moulding action of the great ice-sheet In pre-glacial times the Forth Valley was already deep and wide; it had much of its present configuration though the land must have stood for a long time at a higher level than now to allow the erosion of the deep channel (almost a canon) spanned by the Forth Bridge. Many of the streams still flow in their old preglacial courses, but here and there, when the ice melted away, great banks of boulder-clay were left in the old valleys, obliterating them altogether for a space. In such a case the rivers found a new way, often across the rock, and in time carved a deep narrow gorge with precipitous cliffs on either hand. At the Dean Bridge in Edinburgh the water of Leith flows through a post-glacial channel that has originated in this way; it bears evidence both to the great transporting power of the ice which cast a dam across the old channel, and to the efficiency of running water in eroding solid barriers of rock. Our Lowland Scotch scenery is of a peculiar type not easily described, but not difficult to recognise after we have once become acquainted with it. The swelling volcanic hills such as the Pentlands and the Ochils, with steep terraced scarps that bear witness to flow after flow of lava and bed upon bed of ashes, pass more or less abruptly into low undulating plains, richly cultivated. On these we often may have long rounded hummocky mounds or "drums" that are scattered not quite irregularly but have a definite parallel orientation, like waves of the ocean after a great storm. The drums, or drumlins are accumulations of boulder-clay, often, however, having cores of rock, and their direction is parallel to that in which the ice-sheet was moving In Midlothian the ice at the time when they were shaped. movement was due east; all the higher hills were buried under the ice-sheet, and on the summits of the Pentlands the striee
8
]. S. FLETT.
are to be seen, while boulders of Highland erratics lie on the hill slopes, and are very numerous on the shores of the Firth of Forth where the waves have washed them out of the boulderclay. Very perfect striee may be found on Arthur's Seat just a little below the summit. Above the Lothian plain many lesser eminences rise to heights of a few hundred feet, and are conspicuous objects in the landscape as seen from points of vantage such as the Castle Rock or Arthur's Seat. These are in all cases of volcanic origin, some of them being necks or stumps of old craters such as the two already mentioned, and also North Berwick Law and the Binn of Burntisland; others are great sills of hard intrusive dolerite, among which we may instance Corstorphine Hill, Dalmahoy, Salisbury Craigs, and Mons Hill near Dalmeny. A few are built up principally of lava flows, such as the Calton Hill, Blackford Hill, the Braid Hills and Corston Hill. All these owe their prominence to the resistance they offer to agencies of erosion. They stood much as they do now before the ice invaded the low grounds, and their harder rocks are often closely striated or marked with deep grooves and flutings that show how severe was the pressure of the ice and how stoutly they resisted its attack. Arthur's Seat and Corstorphine Hill present splendid examples of glacial fluting. The phenomena of "crag and tail," so familiar to Scottish geologists, illustrate also the obstruction which these volcanic hills offered to the ice-sheet. The" tail" is a long, gentle slope by which on one side there is a gradual descent to the level of the plain. The "crag" is a steep face which looks in the opposite direction. The" crag" faces the direction trom which the ice-sheet came. In front of it there is bare rock, and in fact there are many cases in which great rock-erosion has taken place in this situation, by the pressure of the ice against the obstruction. No better examples of this need be cited than the horseshoe shaped hollows that lie below the Calton Hill and the Castle Rock and the deep channels which are now occupied by Princes Street Gardens and the Grassmarket of Edinburgh. They are among the most striking lessons of the sculpturing power of a moving ice-sheet. The" tail" on the other hand marks the place where erosion was lessened as the rock was protected by the obstruction in front of it. Sometimes the tail consists largely of boulder-clay like the eastern slope of Arthur's Seat. The Lawnmarket and High Street, which are the "tail" of the Castle Rock, are essentially sandstone and shale. Of course "crag and tail" is especially well developed where there was an old escarpment facing west and a dip slope to the east, as on the Calton Hill. Where the dip slope was to the west the crag is often rudimentary, as at Corstorphine Hill. North Berwick Law and the Castle Rock are volcanic necks where there is nothing in the geological structure likely to give rise to this
GENERAL ACCOUNT OF THE DISTRICT.
9
kind of scenery except under the long-continued movement of a sheet of ice passing to the east. When the ice-sheet melted away the physical configuration of the district around Edinburgh was very different from what it is to-day. Some of the rivers were represented by chains of lakes connected by short stream courses. This was the case, for example, with the water of Leith which flowed through a lake above Colinton and another above Murrayfield. By deepening their channels the intervening streams have drained the lakes and cut the rocky gorges above Slateford and the Dean Bridge. Similarly there was a large lake in the valley of the Esk above Roslin. The irregular surface of the boulder-clay contained also many sheets of water which had no relation to important streams. These have been gradually silted up or filled with peat, but some of them persisted till the end of the eighteenth century. The Meadows on the south side of Edinburgh mark the site of one of these lakes, long known as the Borough Loch. It furnished a supply of water to some of the breweries about IS0 years ago. A shallow lake existed in the King's Park near Holyrood, while another nestled at the foot of the Castle Rock where the railway now passes through Princes Street Gardens. It was known as the Nor Loch, and was drained only when the New Town of Edinburgh was built. A much larger lake covered the flat ground near Corstorphine, through which the railway runs to the Forth Bridge. Near the bottom of the deposits of this lake an arctic plant bed was found, showing the rigorous climatic conditions which prevailed when the water first gathered in this hollow. During the closing stages of the Ice Age this part of Scotland stood at a somewhat lower level than at present, and the old beaches now raised above the tide are exceedingly well preserved in many parts of the Firth of Forth. The highest of these is called the "Hundred-foot Beach," because its elevation is almost exactly 100 ft. at the present time. It forms a broad flat shelf on the north side of Edinburgh, and the steep slope above it, representing the old sea-cliff, is visible in quite a number of places. We find this beach, for example, on the golf course at Duddingston, on the north side of Whinny Hill (Arthur'S Seat), and its deposits almost touch Holyrood Palace. It passes across the east end of the Calton Hill through Gayfield Square and Great King Street out to the west by Craigleith, Below this there are three other raised beaches known as the "7 S-ft., 50-ft., and 2S-ft." beaches. They are best seen about Granton and Trinity, the 2S-ft. beach being very conspicuous in many places on both sides of the Firth of Forth. Associated with these beaches are beds of shelly clay with fossils, which show that the earlier beaches belong to a time when the waters of the North Sea were intensely cold.
Assoc.,
VOL.
XXV.
PLATE
1.
A.-ARTHUR·S SEAT AND SALISBURY CHAIGS FROM
(See page 40.)
[Pho to b,. R . 1.111111. B.-CONTACT OF DOLERITE AND UPPER OLD RED SANDSTONE. SALISBURY CRAIGS. (See page 40.)
To face page
I.
PROCEEDINGS OF THE
GEOLOGISTS'
ASSOCIATION.
GEOLOGY OF THE DISTRICT AROUND EDINBURGH.* CHAPTER I. GENERAL ACCOUNT OF THE DISTRICT. By J. S. FI:.ETT. M.A., D.Sc., LL.D., F.R.S.. F.G.S.
F we take up a small-scale map of the Geology of the Lothians, such as, for example, the one-inch Sheet No. 32 of the Geological Survey of Scotland, we see that the principal structural feature is the presence of a great anticline, along which the older rocks rise to the surface and are flanked on each side by strata of newer age. This anticline is known as the Pentland axis, because the Pentland Hills, which begin about three miles south of the city of Edinburgh, and extend for a great distance to the south-west, are the visible expression of this great subterranean fold. The Pentlands stand out boldly above the low undulating surface of Midlothian, principally because the upheaval has brought up a great series of hard and durable volcanic rocks, which during the long ages through which denudation has been acting on them have acquired prominence through their superior hardness and resistance to erosion. Edinburgh stands on the continuation of this anticline. It passes almost through the Waverley Station, and runs northwards to the shores of the Firth of Forth near Leith. What is probably the same fold appears on the north side of the Firth at Burntisland, in Fife, so that we must regard it as a structure of no small magnitude. The axis of folding, however, is not horizontal, but dips to the north-east. We might compare it, in fact, to the bow of an overturned canoe; or, again, we might express this feature by saying that the fold" noses out." or " pitches" to the north. For that reason the lowest beds exposed on the back of the fold occur in the south-west, while higher and higher beds come in as we travel in a north-east direction. Thus, in the Pentland Hills, about twelve miles from Edinburgh, Wenlock, Ludlow and Downtonian rocks occur in the core of the anticline. These sink beneath the Lower Old Red Sandstone conglomerates, which in turn are wrapped over by the lavas and ash-beds belonging to the same great epoch. These lavas stretch up into Edinburgh, where they are last seen in the Braid Hills and Blackford Hill, on
I
• Issued as a Pamphlet to members taking part in the Excursion to Edinburgh In August, '9'3. PROC, GEOL.
Assoc. VOL. XXV,
PART
I, I914.J
2
j, S. FLETT.
the sou thern borders of the city. O ver them th e Upper Old R ed Sandst on e now sweeps in a great horse-shoe curve, an d th is in turn passes up into th e Lower Carbo niferous, known in Scotland as th e Calciferous Sandstones, on which th e central and northern part s of Edinburgh ar e built. The great Pentl and arc h is by no means simpl e or symmetrical. It is of very old standing, and th ere is evidence that at two differen t periods it -was expose d at th e surface, and great shee ts of gravel were produced by its denudat ion. On e of these is at the base of the Lower Old Red Sandstone, and the other at the base of the Upper Old Re d Sa ndstone, and each of them mark s an important un conformability ; moreover, on its east side the ar ch is broken, and an important fault brings down the higher rocks that form the Mid lothi an Coalfield agains t the vastly old er lavas and ashes of th e Lower Old Red. The effect of this dislocati on is to bring th e Coal Measures, the high est rocks of this district, within two or thr ee miles (measured across th e out crop s) of the Old Red Sandstone. On the western side of the anticline, however, there is no great fault ; the dips are gentle and rolling, and a wide stretch of Lower Carboniferous bed s (Calciferous Sandstones, etc. ) must be crossed before the Coal Measures are reached to the west of Bathgate, sixtee n or eight een miles from Edinburgh. The Midlothian or Pentland ant iclin e, accordin gly, is asymmetrical, being stee p on its easte rn side, while its west side has a gentle dip ; a nd, moreover, it is broken , or perhaps we might say reinforced, by a great fault on its eastern side . Also, as we have said alread y, it " pitches" to the north. On the east sid e of th e Pentlan d fault th e strata lie in a very perfect syncline that forms the Midlothian Coalfield. It contains a nearl y complete success ion of th e Carbo niferous rock s from low down in th e Calciferou s Sandstones to th e top of th e Coal M easur es. Onits west side, where it ab uts againstthePentland arch, the synclin e is ste ep and the beds lie at angles of 45 degrees andover, but soon th ey flatten and after a time rise to the surface and undulat e gently over a wide area in East Lo thian, -till, nea r D unbar, the Old R ed Sandstone again rises to th e surface. On the west of the Pentlands, between Ed inbur gh and Bathgate, the whole of Midlothian and part of East Lothian cons ist of Lowe r Carboniferous rocks which are folded into many little anticlines and synclines. The Carboniferou s Li mestone comes in at Bathgate, where it dips to th e west below th e Lanarkshire Coalfield. The beds that underlie th e limestone s are principally shales and sandstones, with which occur the valuable deposits of oil-shale which have given rise to th e oilshale industry in th e L othi ans. Well-known centres of oil-shale mining ar e Pumpherst on, Broxburn and Dalm eny, an d th e whole of this distri ct is marked by great flat-topp ed mounds of red burnt shale, the refuse of the oil reto rts. The oil-shales lie in litt le basins that are separated by arche s or anticlines, but, on
GENERAL ACCOUNT OF THE DISTRICT.
3
the whole, as we travel eastwards from Bathgate to Edinburgh, there is a descending sequence though the same beds are repeated again and again by the folding, particularly in the western part of this district. The Burdiehouse limestone is of fresh-water origin and marks a well-defined horizon near the middle of the oil-shale group. (See Fig. I, p. 5.) Beneath the productive shale measures lies a great thickness of sandstone and shales, and among the former a very pure and durable sandstone was worked for many years at Craigleith quarry on the north-west side of Edinburgh. THE SEQUENCE OF ROCKS IN THE LOTHIANS.
If we turn now to consider the rocks that occur in the Edinburgh district and the order in which they were deposited, we find that the oldest are those that are brought up to the surface in the core of the great Pentland arch. There are shales, grits, and greywackes that belong to the Wenlock and Ludlow divisions of the Upper Silurian and contain their characteristic fossils. They pass upwards into red Downtonian rocks in which are intercalated beds of conglomerate with pebbles of quartzite and of volcanic rocks. The whole assemblage is seen in the North Esk burn on the southern limits of the Edinburgh map. Near the waterfall above Loganlee Reservoir in the Pentlands the grey Ludlow shales and grits are well exposed, and are intensely folded, many .of the beds being in a vertical position. The Lower Old Red Sandstone, which is the next formation in order of deposition, rests unconformably upon these rocks, and it is evident that the Silurian beds had been much folded and had suffered great denudation in the interval between the two series of deposits. This epoch of folding is that which marked the close of Silurian times (the Caledonian folding), and threw the Silurian strata of the Southern Uplands of Scotland into dose-packed isoclinal folds. Evidently the Pentland arch was at this early time a line of pronounced movement, for in other places, notably at Stonehaven in Kincardineshire, and near Lesmahagow in Lanarkshire, there is no break between Silurian and Old Red Sandstone, but the former passes up, perfectly conformably, into the latter. The surface that lies beneath the Lower Old Rea Sandstone of the Pentlands is thus the oldest land surface of which we have evidence in the Lothians. The Lower Old Red Conglomerate is overlain by the great series of lavas and ash-beds that form most of the Pentland chain as seen from Edinburgh. They were, a large part at least, of subaerial origin, and indicate the site of volcanic islands that rose above the surface of the waters of Lake Caledonia. In its geographical boundaries this chain of volcanoes could have borne little resemblance to the Pentland Hills as we see them to-day, for on the east side a great fault has cut through the rocks, and no doubt beneath the Dalkeith Coalfield there are buried masses of lavas and ashes belonging to this group. We may also readily
4
J.
S. FLETT.
believe that they extended much farther to the west, for the principal vents known in the Pentland Hills occur on that side, and it seems as if a considerable part of the lava-flows had emanated from that quarter. The Upper Old Red Sandstone and the Lower Carboniferous probably conceal from our view a large extent of volcanic rocks of the same age and the same kind as form the Pentland Hills at present. There is no true Middle Old Red Sandstone (which corresponds in time to the Middle Devonian) in this part of Scotland, and though great masses of sediment were accumulating in freshwater lakes in Caithness and the Orkneys, there was dry land in the Lothians ; and the volcanoes which we have described above, together with the sandstones and marls that no doubt overlay them, were being rapidly eroded and reduced in height. The strata that rest upon the Pentland volcanics belong to the Upper Old Red Sandstone, and are mostly cream-coloured and brownish sandstone, sometimes pebbly, with bands of clay and here and there thin concretionary limestones, known as " cornstones." These rocks wrap round the Pentland chain, especially on its western side; on the east side they have sunk from view along the great Pentland fault. The south of Edinburgh about Blackford Hill and Morningside districts is built on them, and they are well exposed in the Suburban Railway cutting near Craiglockart Station. The" Cornstone" which occurs near the top of this group has been seen in excavations near the Meadows. Between them and the Pentland lavas there is a strong unconformability, attended by an overlap, but there is no great development of conglomerates at the base of the overlying series. In this unconformability we have the evidence of the second land-surface that was formed in the Edinburgh district, when the much-denuded Pentland ridge was slowly sinking in the waters of the Upper Old Red Lake and was being gradually buried in sheets of sand and mud. The Upper Old Red Sandstone passes up, perfectly gradually and conformably, into the Lower Carboniferous. The red sandstones, cornstones and marls, deposits of fresh water, give place to shales and sandstones that were deposited under estuarine conditions and contain common types of Carboniferous shells, fishes and plants. The line of transition between the two types of strata passes through the south of the city, but is not visible in any good section. It is well known, however, from information obtained through boring operations for supplies of water for the breweries, which depend largely on hard water obtained from the Upper Old Red Sandstone. In the King's Park the junction of the two formations passes through the Hunter's Bog just above the intrusive sheet of Salisbury Craigs. At the base of the Carboniferous lie the "Ballagan Beds" or Cementstones, a group of green, grey and brown clays with seams
GENERAL ACCOUNT OF THE DISTRICT.
5
FEET
gooo
}
Re d Sandst one. Hor izon not proved
E Xira Limest on e
'>"
f '"'"" is: .... } o.l:'l~ ~- ~ "'0
I
"'"
WOOD COAL t"
s:n
S OU T H PAR ROT COAL U
'"
0.'" g,c:
8000
7000
..>
;;::
.. Dalkeith " and Musse lburgh Coalfield
R oslin Sand st one COAL I' Lev enseat" Limestone COALS I I Cal my" or Gair Limestone
o 0. 0
Index " Lim estone
",;> ", ,, ... ",
.. " "
0. 0 0
tr,~
.. Edge Coal s " and Ir onst ones, Loanhead
cr.~
"' 0
:! c: ",(Il
6000
(Il
"Bilston 'Burn JJ Lim est one Vexhim Lim estone and Coal .. N ort h Gr een s " Li mest on e and Coal G ilmerton L imeston e (= Cobbinsh aw)
5000
R AEBURN SHALE MUNGLE S HALE and Mari ne Sh ell Bed T WO· F OO T CO AL
Hou sl on Marls H OUST ON COAL GREY SHA LE
FELLS S HAL E AND LIME STONE
Broxburn Marls HROXB U RN S H AL E C H AMPFLEUR I E SHA L E
4 000
Binn y Sands tone D U N NET SHALE
Barracks Limest on e and Ash Dunnet Sa nds tones CAM PS SHA LE B U RDI EHOU SE LI M E ST ON E
0
;:
u,
'" ;>
e-
too Cl
30 00 PUMPH E RSTON SHAL E S
Marine Shell Bed
..'" 0
c:
o
> o
e-
:;;
'" °"c: '"
o.~
-" ... '" °" 'e"n ';;;'"" 0
DA L MAHOY SH ALE
2 000
Hailes Sand stone W ardie Sha les
'"
G ran ton Sandstone Abbeyhill Shales 1000
Arthur's Sea t Volcan ic Group
o
"..,''"" " Cem ent stone Gr oup or "Ballagan Beds "
..,
(Il
0
" c: .'" o'"
o
Upp er Old Red Sandstone
0
6
J. S. FLETT.
of impure limestone, but they are not well exposed; upon them rest the volcanic rocks of Arthur's Seat and the Calton Hill, which yield some of the most picturesque scenery in the district. The lavas are principally olivine basalts, dark on fracture but weathering brown, and though they are succeeded by mugearites there is far less variety of petrological types than in the Pentland Hill lavas, which include rhyolites, dacites and many types of andesites and basalts. The Edinburgh volcano was only one of a group, some of which were far larger and more important. Thus at North Berwick and in the Garlton Hills near Haddington the volcanic outbursts began earlier, lasted longer, and were on a much larger scale than at Arthur's Seat. At Burntisland, some time after the volcanoes we have mentioned had become extinct, eruptions began on the site marked by the Binn, the steep hill overlooking the town, and the lavas and ash-beds may be traced in continuous section along the shore from Pettycur to near Kirkcaldy. The great eruptions of the Bathgate Hills belong to the period of the Carboniferous Limestone, because beds of fossil corals occur interstratified with the lava flows. They were at their maximum activity about the time when the Burntisland volcano was waning to its close. The Oil Shale Group succeeds the Arthur's Seat volcanic series, but in the lower part of the strata there is no workable oil-shale. The most important seams lie at a higher level, either below the Burdiehouse Limestone or between that and the base of the Carboniferous Limestone. At Burdiehouse, three miles from Edinburgh, the old quarries so famous for the fossils they have yielded are still being worked. The rock is a fresh-water limestone, and contains no corals or brachiopods. In fact the Lower Carboniferous of Scotland as a whole is a fresh-water or estuarine deposit, and contrasts strongly in this respect with the marine limestones of England. Among the oil-shales there are a few bands containing Orthoceras and goniatites, but these, like the marine bands in the English Coal Measures, indicate only occasional invasions of the district by the sea. The Scottish Carboniferous Limestone is really a series of shales, sandstones and coals, with seven or eight beds of limestone usually only a few feet thick. In the middle of this series there is an important group of coal seams known as the Edge Coals, as they were worked at an early date in the steeply-inclined beds on the west margin of the Midlothian Coal-field. It is by no means uncommon to find a marine limestone resting on a coal or fireclay, a striking example of rapid change in conditions of deposit, as a terrestrial surface covered with dense vegetation was suddenly covered by clear sea-water in which marine animals flourished and left banks of corals and shells. The Carboniferous Limestone Series is well exposed on the shores of the Firth of Forth at Aberlady and at Kinghorn.
GENERAL ACCOUNT OF THE DISTRICT.
7
The Millstone Grit in Scotland is quite thin in comparison with the same set of rocks in England, and is followed by the Coal Measures of which the uppermost part is composed of. red sandstone and has no workable coals. They occur in the Lothian Coal-field, where the Upper Red Beds are also represented, and also on the north of the Firth of Forth at Kirkcaldy. These two coal-fields are probably continuous beneath the sea, and coal seams have been worked for nearly three miles out from the shore on the Fife side, but as yet submarine coal-mining in Midlothian is only in its infancy. In the East of Scotland there is hardly any trace of the richly fossiliferous Secondary deposits that form some of the most interesting and instructive chapters of English geology, and Tertiary strata are altogether wanting. The geological record for these periods is a blank so far as our district is concerned, but it again becomes full of variety when the Glacial period is reached. The physical features of the Lothians owe their present configu ration very largely to the moulding action of the great ice-sheet In pre-glacial times the Forth Valley was already deep and wide; it had much of its present configuration though the land must have stood for a long time at a higher level than now to allow the erosion of the deep channel (almost a canon) spanned by the Forth Bridge. Many of the streams still flow in their old preglacial courses, but here and there, when the ice melted away, great banks of boulder-clay were left in the old valleys, obliterating them altogether for a space. In such a case the rivers found a new way, often across the rock, and in time carved a deep narrow gorge with precipitous cliffs on either hand. At the Dean Bridge in Edinburgh the water of Leith flows through a post-glacial channel that has originated in this way; it bears evidence both to the great transporting power of the ice which cast a dam across the old channel, and to the efficiency of running water in eroding solid barriers of rock. Our Lowland Scotch scenery is of a peculiar type not easily described, but not difficult to recognise after we have once become acquainted with it. The swelling volcanic hills such as the Pentlands and the Ochils, with steep terraced scarps that bear witness to flow after flow of lava and bed upon bed of ashes, pass more or less abruptly into low undulating plains, richly cultivated. On these we often may have long rounded hummocky mounds or "drums" that are scattered not quite irregularly but have a definite parallel orientation, like waves of the ocean after a great storm. The drums, or drumlins are accumulations of boulder-clay, often, however, having cores of rock, and their direction is parallel to that in which the ice-sheet was moving In Midlothian the ice at the time when they were shaped. movement was due east; all the higher hills were buried under the ice-sheet, and on the summits of the Pentlands the striee
8
]. S. FLETT.
are to be seen, while boulders of Highland erratics lie on the hill slopes, and are very numerous on the shores of the Firth of Forth where the waves have washed them out of the boulderclay. Very perfect striee may be found on Arthur's Seat just a little below the summit. Above the Lothian plain many lesser eminences rise to heights of a few hundred feet, and are conspicuous objects in the landscape as seen from points of vantage such as the Castle Rock or Arthur's Seat. These are in all cases of volcanic origin, some of them being necks or stumps of old craters such as the two already mentioned, and also North Berwick Law and the Binn of Burntisland; others are great sills of hard intrusive dolerite, among which we may instance Corstorphine Hill, Dalmahoy, Salisbury Craigs, and Mons Hill near Dalmeny. A few are built up principally of lava flows, such as the Calton Hill, Blackford Hill, the Braid Hills and Corston Hill. All these owe their prominence to the resistance they offer to agencies of erosion. They stood much as they do now before the ice invaded the low grounds, and their harder rocks are often closely striated or marked with deep grooves and flutings that show how severe was the pressure of the ice and how stoutly they resisted its attack. Arthur's Seat and Corstorphine Hill present splendid examples of glacial fluting. The phenomena of "crag and tail," so familiar to Scottish geologists, illustrate also the obstruction which these volcanic hills offered to the ice-sheet. The" tail" is a long, gentle slope by which on one side there is a gradual descent to the level of the plain. The "crag" is a steep face which looks in the opposite direction. The" crag" faces the direction trom which the ice-sheet came. In front of it there is bare rock, and in fact there are many cases in which great rock-erosion has taken place in this situation, by the pressure of the ice against the obstruction. No better examples of this need be cited than the horseshoe shaped hollows that lie below the Calton Hill and the Castle Rock and the deep channels which are now occupied by Princes Street Gardens and the Grassmarket of Edinburgh. They are among the most striking lessons of the sculpturing power of a moving ice-sheet. The" tail" on the other hand marks the place where erosion was lessened as the rock was protected by the obstruction in front of it. Sometimes the tail consists largely of boulder-clay like the eastern slope of Arthur's Seat. The Lawnmarket and High Street, which are the "tail" of the Castle Rock, are essentially sandstone and shale. Of course "crag and tail" is especially well developed where there was an old escarpment facing west and a dip slope to the east, as on the Calton Hill. Where the dip slope was to the west the crag is often rudimentary, as at Corstorphine Hill. North Berwick Law and the Castle Rock are volcanic necks where there is nothing in the geological structure likely to give rise to this
GENERAL ACCOUNT OF THE DISTRICT.
9
kind of scenery except under the long-continued movement of a sheet of ice passing to the east. When the ice-sheet melted away the physical configuration of the district around Edinburgh was very different from what it is to-day. Some of the rivers were represented by chains of lakes connected by short stream courses. This was the case, for example, with the water of Leith which flowed through a lake above Colinton and another above Murrayfield. By deepening their channels the intervening streams have drained the lakes and cut the rocky gorges above Slateford and the Dean Bridge. Similarly there was a large lake in the valley of the Esk above Roslin. The irregular surface of the boulder-clay contained also many sheets of water which had no relation to important streams. These have been gradually silted up or filled with peat, but some of them persisted till the end of the eighteenth century. The Meadows on the south side of Edinburgh mark the site of one of these lakes, long known as the Borough Loch. It furnished a supply of water to some of the breweries about IS0 years ago. A shallow lake existed in the King's Park near Holyrood, while another nestled at the foot of the Castle Rock where the railway now passes through Princes Street Gardens. It was known as the Nor Loch, and was drained only when the New Town of Edinburgh was built. A much larger lake covered the flat ground near Corstorphine, through which the railway runs to the Forth Bridge. Near the bottom of the deposits of this lake an arctic plant bed was found, showing the rigorous climatic conditions which prevailed when the water first gathered in this hollow. During the closing stages of the Ice Age this part of Scotland stood at a somewhat lower level than at present, and the old beaches now raised above the tide are exceedingly well preserved in many parts of the Firth of Forth. The highest of these is called the "Hundred-foot Beach," because its elevation is almost exactly 100 ft. at the present time. It forms a broad flat shelf on the north side of Edinburgh, and the steep slope above it, representing the old sea-cliff, is visible in quite a number of places. We find this beach, for example, on the golf course at Duddingston, on the north side of Whinny Hill (Arthur'S Seat), and its deposits almost touch Holyrood Palace. It passes across the east end of the Calton Hill through Gayfield Square and Great King Street out to the west by Craigleith, Below this there are three other raised beaches known as the "7 S-ft., 50-ft., and 2S-ft." beaches. They are best seen about Granton and Trinity, the 2S-ft. beach being very conspicuous in many places on both sides of the Firth of Forth. Associated with these beaches are beds of shelly clay with fossils, which show that the earlier beaches belong to a time when the waters of the North Sea were intensely cold.