The periglacial rock-stream at Clatford Bottom, Marlborough Downs, Wiltshire

The Periglacial Rock-Stream at Clatford Bottom, Marlborough Downs, Wiltshire by R. J. SMALL, M. J. CLARK and J. LEWIN Received 22 February 1969; taken as read 5 December 1969

CONTENTS 1. 2. 3. 4. 5.

INTRODUCTION THE MORPHOLOGY OF CLATFORD BOTTOM THE SARSENS THE COOMBE DEPOSITS SUMMARY AND CONCLUSIONS ACKNOWLEDGMENTS REFERENCES

page 87

88 90 91 95 97 97

ABSTRACT: This paper examines the morphology and deposits ofClatford Bottom, a valley containing numerous sarsens evidently transported from the surrounding slopes and interfluves by periglacial solifluction. Thirteen pits dug on the slopes and in the valley bottom enabled confirmation of the periglacial hypothesis, and revealed that the maximum movement was from the north-east. Only one major phase of solifluction, tentatively ascribed to the Weichselian, was identified. The asymmetrical nature of Clatford Bottom, and the mechanism involved in its development, are discussed.

1. INTRODUCTION accumulations of sarsens (large blocks of silicified sand and flint conglomerate, usually regarded as of early Tertiary age (White, 1925» in some of the valleys of the chalk: country around Marlborough have long attracted the attention of topographers and naturalists.' An early description was that of W. Cunnington, in the Devizes Gazette for June 1852: 'Few persons who have not seen [the sarsens] can form an adequate idea of the extraordinary scene presented to the spectator who, standing on the brow of one of the hills near Clatford, sees, stretching before him, countless numbers of these gigantic stones, occupying the middle of the valley, and winding like a mighty stream towards the South.' Cunnington wrote further of the 'impression which almost involuntarily forces itself on the mind, that it must be a stream of rocks e'en now flowing onwards'. Professional geomorphologists have, curiously, shown only passing interest in what appear to be excellent examples offossil periglacial rock-streams (Te Punga, 1957).

THE REMARKABLE

1 This paper is taken as a record of a Field Meeting in the area, led by Dr. R. J. Small and Dr. M. J. Clark, on 7 June 1969 (Circular 713).

87

88

R. J. SMALL AND OTHERS

2. THE MORPHOLOGY OF CLATFORD BOTTOM The downland of the Clatford area declines gently southward and south-eastward, in conformity with the dip of the Chalk, toward the Kennet Valley between Fyfield and Marlborough (Ordnance Survey Oneinch sheet 157, Swindon). In the area of study it has been dissected by former streams into two broad-backed interfiuves, Overton Down and Totterdown, which are separated by Clatford Bottom (SU 1569) itself. The latter is a comparatively shallow valley, incised some 30-50 m. below the general level of Overton Down. (a) The Asymmetry of Clatford Bottom One of the most striking features of the upper part of Clatford Bottom is the strong valley asymmetry. The long northern slope, ascending toward Totterdown (Fig. I), is of broadly rectilinear form, and with slope angles generally in the order of only 3-4°. Toward the foot of the slope, where some complication of form occurs, angles slightly in excess of these are found. The shorter southern slope is not only much steeper but is morphologically more complex. Toward the head of Clatford Bottom the slope profile is convex overall, with the lower part showing a pronounced steepening of up to 18°. Further down valley, in the section below the Delling Copse, this basal steepening is accentuated to give angles of up to 29°. The upper part of the slope here remains broadly convex in profile (maximum angles at 5_9°), but at the base of the convexity is sometimes developed a slight concavity which appears to be the remains of a valleyside bench left upstanding by more recent incision of Clatford Bottom. Asymmetrical valleys are known elsewhere in the English chalk country, and are accepted as the product of differential slope weathering and retreat under past periglacial conditions. Ollier & Thomasson (1957) have made a detailed analysis of asymmetrical valleys in the Chilterns, and have demonstrated that slopes facing south-westward and receiving the greater insolation are usually the steeper, whereas the sheltered north-east-facing slopes are the more gentle. From an examination of slope deposits OIlier & Thomasson inferred (i) that the south-west-facing slopes were the more active (in terms of frost weathering and solifluction) and experienced both rapid retreat and steepening of angle, and (ii) that the development of asymmetry accompanied downcutting of the Chiltern valleys. Clearly this hypothesis is not applicable to Clatford Bottom without considerable modification. It is, in fact, possible that the 'reversed' asymmetry of the upper part of Clatford Bottom owes something to structural influence. The valley runs slightly transverse to the dip, so that some uniclinal shifting of the former Clatford stream toward the south might have been fostered. However, there is no doubt that periglacial processes have since greatly accentuated this initial asymmetry. As will be shown, quite thick

t

TO TO TTE ROO W N

,

®..

' ......... ............

I9 @Y

(9'<.. .

,

,

-'-"-

v IIll ev Po orly d ef i ne d

-~

vll n.y

-( ----4

00

Hang in g vall ey

Oe t ; iu l fan S ite o f S a n ll" q u ad ra t

,//,~ .I ...+

.

7~ 9

' 70 9

Spo t heig ht I FII

-K-

Knick po int

Co nv e.: br e ak of

s lo p.

Con e llV II b re ak

of "op.

..,

..... _.

C o ny e. cha n gll

..Y__ 'k:.

Co n ell VII c he ng. 0 1 s lo p.

®-

S lOp. en glll in

.,

, ,+

r ,

jf) 0-

, ... ...

W IlII - d "flnlld

~

:x... -= I...

of slop.

t!j

~

>-<

o t"'"

de gr . ..

P in dug i n 19 6 " lind 196 5

T. rr ae ll

.••.•.......•••.•..

> o>-< > r~

o

o

F YFIEL D DOWN

~

...,til

I I

~

I

/~

t!j

,

I

lID

I

>

~

>-<

Z

OV ER TON DOWN

ft)

~

>-<

.2

,,

",,//" • 1 o

oI

Fee'

i .M. "t r. . i

\

,

" ...... ......

r..., til

::z:: >-<

~

t!j

1000

~

3 0 00

Fig. 1. The geomorphology of Clatford Bottom

\0

90

R. J. SMALL AND OTHERS

solifluction deposits mask the lower part of the gentle slope, proving that it has been actively modified in a frost climate. Furthermore, the striking recent undercutting of the right-hand valley wall can hardly be attributed to the influence of a dip angle of 3° or less. Some other factor has obviously promoted the lateral migration of the Clatford stream, and this can only have been the large amount of solifluction debris, containing many sarsens, pushing into the valley from the northern side. (b) The Terraces and Knickpoints of Clatford Bottom

Other features of note in Clatford Bottom are the 'terraces' and 'knickpoints' shown in Fig. 1. The valley-side bench on the southern side of the valley has already been mentioned. Shallow pits were dug on this feature, and solid chalk was reached at depths of between 36 and 41 cm., proving that the bench is wholly of erosional origin. On the northern slope of Clatford Bottom three possible terraces were mapped. The lowest and by far the best defined may be traced from near the edge of the Delling Copse south-eastward for some 600 m. The front edge of the terrace, although sloping at only 4-6°, is unmistakable in the field, where its relationship to a break in the valley long-profile 100 m. south-east of the Delling Copse is equally apparent. It is impossible to say whether this terrace is the result of deposition, but there are some pointers to this conclusion. For one thing it is covered by a considerable thickness of coombe deposit (266 em. were recorded in Pit 11). For another, it is noticeable that there are fewer sarsens on the tread of the terrace than on its front riser. This would be explained if the terrace were the remains of a former valley deposit, containing sarsen stones which have been left on the present ground surface as fluvial action has washed away the finer constituents. The two higher terraces are less well-defined,though the lower of the two can be tentatively related to a subdued knickpoint some 200 rn. above the Delling Copse (Fig. I). It is possible that this 'middle' terrace is the northern counterpart of the bench south of Clatford Bottom. 3. THE SARSENS Sarsens are found in large numbers throughout the Clatford area, those remaining on the interfluves being associated with clay-with-flints or Eocene remanie deposits. By far the greatest concentration of stones is found on the lower northern slope and valley floor of Clatford Bottom. A quadrat survey (based on 5 individual quadrats each WOO sq. m. in area) revealed a maximum density of 1 sarsen per 6 sq. m. (B in F ig. l) and a minimum of 1 per 140 sq. m. (E). Intermediate densities were I per 8 sq. m. (A), I per 15 sq. m. (D) and 1 per 55 sq. m. (C). The oversteepened slope to the south of Clatford Bottom is free of sarsens. The 5 quadrats contained 407 stones ; of these 77 per cent had visible major axes within the range

PERIGLACIAL ROCK-STREAM IN WILTSHIRE

91

3D-150 em., and 8 individual sarsens had major axes in excess of 210 em. From the quadrat counts it can be estimated that there are still approximately 8000-10,000 sarsens lying in a strip some 60 m. wide and extending 750 m. south-eastwards from the Delling Copse. It was hoped initially to make an orientation study of the sarsens in Clatford Bottom, on the grounds that persistent alignment of stones would support the hypothesis of transportation by solifluction. However, the attempt proved abortive for three reasons: (i) most of the sarsens are partly buried, so that their true major axes cannot be determined, (ii) the stones are in any case often very irregular in shape, and (iii) many sarsens have been shattered by stonemasons. It was also thought that a detailed analysis of the attitude of individual sarsens might indicate whether they have been affected by frost heaving, but again no meaningful measurements could be made. Rather the impression was gained from general observation that many sarsens, and in particular the larger slab-like stones, lay flat on or near the surface of the slope or valIey floor. It seems, in fact, that if the sarsens were moved under periglacial conditions, some form of rafting, with the stones being borne along at or near the top of the soliflucting layer, operated. 4. THE COOMBE DEPOSITS In order to examine the deposits on which the sarsens lie or in which they are embedded, thirteen pits were dug in Clatford Bottom (Fig. 1).Those in the valIey bottom and on the lower part of the gentle slope revealed up to 3.5 m. of coombe deposit. Some variation from pit to pit was found, but the general sequence (Fig. 2) was as follows: (i) A shalIow surface loam (rarely more than 20 em. in thickness). This is often diversified by angular fragments of sarsen taken to be industrial debris deriving from the past working of sarsens. (ii) A brown flinty loam (generalIy 60-120 em. in thickness). The matrix of this deposit is dark or strong brown (7.5YR 4{4) in colour, though at times it becomes yellowish red (5YR 5{6). There is a high content of large angular flints, small flint chips, sarsen fragments, and some quite large rounded sarsens. (iii) Coombe rock (up to 200 em. or more in thickness). This comprises a white (I0YR 8{1) or light grey (lOYR 7{2) solifluction rubble, with a few scattered flints and smalI sarsens set in a pasty or powdery chalk matrix, grading downward into weathered chalk in situ. In its upper part the coombe rock is stained to give a very pale brown (I0YR 8{3) or pale brown (I0YR 6{3) horizon, in which flints are sometimes common. The junction between the coombe rock and brown flinty loam is complicated by the presence of deep and well-defined pipes of circular or elliptical cross-section (Fig. 2). These are up to 40 em. wide at the top, but

PIT 7

PIT6

PIT9

PIT8

s

::~:~i~:~:~~~~~~~~~::

2

2

j::!!IIIIII;II:;

2

...... FLINTy······

3

3

' i!::;;j~;i: 1r!

3

4

4

5

5

6

6

6

7

7

4

5

COOM BE ROCK

??f

ROCK

:::::::::

:j!i!?

7

:;: 8

8

8

CHALK ? COOM BE ROCK

9

9

HARD NODULAR CHALK 10

10

SARSEN 11

Fig. 2. Summ ary diagram of selected pit sections. All section s are generalised, especially in horizontal plane. Minor local variations (e.g. of flint concent rations) are not shown

PIT 10

PIT 12

PIT 11

DEPTH Fe et

m81ijm~mi§.J"

C ms

SURFACE

.:. :<:

20

' U g h l coloured

40

2

2

3

3

60 80

100 4

4

5

5

120 140

160 6

COO MBE ROCK

7

7

6

, COOMBE" .:.;. ROC K ",,.' :;: "

180

200 7

220 8

HARD tlUBBlE- - -

8

COOMB E ROCK

9

240 260

9

280

; ~ ~~: : :;: : :: COOMBE ROCK

300

320 340

360

94

R. J. SMALL AND OTHERS

narrow downward. Most of the pipes stop within the coombe rock, but some penetrate to the solid chalk at a depth of 3.5 m. Closely spaced pipes coalesce at the top, being separated by narrow aretes often capped by sarsen fragments which appear to act as watersheds for percolating acidulated rainwater. Around the walls of the pipes chalk pellets from the coombe rock interdigitate with small masses of brown clay loam. The general form of the pipes and the nature of their walls is clear evidence that their formation is due to solution of the coombe rock. This solution is evidently active today, and it may be surmised that selective solution did not begin until the main episode of valley infilling had been completed. The precise relationship between the brown flinty loam and the coombe rock is somewhat problematical. The superposition of the loam suggests at first sight that its deposition postdates that of the coombe rock, though the depositionary surface cannot have been the present piped junction. An hypothesis based on such reasoning would presuppose a major phase of transport and deposition of coombe rock with a few small sarsens embedded in it. There was then a sharp break in deposition, before the largescale influx of non-calcareous flinty loam (presumably derived from the capping of clay-with-flints on distant Totterdown) heavily laden with sarsens. Later the top of the coombe rock would have been modified by decalcification, and the upper part of the flinty loam removed by fluvial action. Such an interpretation is certainly possible, but there remain some important problems. It is not easy to envisage how two such different deposits could be derived from a single source area (the slopes and interf1uve north of Clatford Bottom). One might argue that the lower coombe rock resulted from soliflual modification of the lower bare chalk slopes at an early stage of periglaciation, and that the flinty loam, traversing a much greater distance from the clay-capped interfluve, inevitably arrived at the valley bottom at a comparatively late stage. However, the far-travelled clay-with-flints would still hardly have formed a separate and totally distinct deposit at the valley floor, for in being transported over the long and gentle slope north of Clatford Bottom it would surely have become incorporated in an amorphous chalky sludge. Again, if the two deposits are indeed of different origin, one might expect a rather greater contrast in composition between them. Detailed mechanical analyses (Clark, Lewin & Small, 1967) have shown that there is a continual textural transition from pure coombe rock, through stained coombe rock, to brown flinty loam, and that all the constituents of the flinty loam may be found in the coombe rock, albeit in different proportions. For these reasons we are inclined to view the flinty loam and coombe rock as being originally one and the same deposit. This material was laid down as coombe rock in a major solifluction episode, with a concentration

PERIGLACIAL ROCK-STREAM IN WILTSHIRE

95

of sarsens toward the surface of the deposit achieved by some form of rafting. Rejuvenation followed this main phase of deposition, and outwashing of the fines produced the very strong concentration of sarsens seen today on the lower terrace face and valley floor. A period of decalcification has subsequently converted the upper part of the remaining coombe rock into a flinty loam of smaller volume , thereby explaining the abundance of angular flints and prov iding a further reason for the vast number of sarsens lying today at or near the surface of the remaining deposit.

5. SUMMARY AND CONCLUSIONS It is suggested, on the basis of available morphological evidence, that Clatford Bottom has experienced the following history. (a) At some time after the breaching and fragmentation of a sarsen layer (resting at approximately 700-900 ft. a.D. (215-275 m.)) there developed a shallow and slightly asymmetrical valley, the slopes of which would have been occupied by some sarsens let down vertically from the former hill-top surface (Fig. 3, A). (b) An episode of rejuvenation, prompted by climatic, base-level or lithological factors, seems to have occurred subsequently. Evidence of this is afforded by the bench on the southern side of Clatford Bottom and possibly by the middle terrace on the northern side. Valley asymmetry may have been exaggerated at this stage, to judge by the angles of up to 18° occurring in C1atford Bottom above the Delling Copse knickpoint (Fig. 3, B). This could hardly be explained in terms of mild structural influences, but may have resulted from the first significant arrival from the north of soliflucting chalk debris containing a few sarsens in an 'early' periglacial phase. (c) At a later date, the effects ofthis rejuvenation were partially offset by an important aggradational episode, perhaps the result of intensification to 'full ' periglacial conditions, and marking the arrival of much larger quantities of solifluction detritus containing numerous sarsens. This solifluction layer travelled as a broad sheet over the gentle northern slope of C1atford Bottom, with some tendency toward concentration along the lines of minor tributary valleys (Fig. 1). The valley-floor infill may eventually have reached a thickness of some 10 m. (Fig. 3, C). (d) A second rejuvenation, indicated by the knickpoint below the Delling Copse and the lowermost terrace of Clatford Bottom, has taken place quite recently. Possibly this rejuvenation was allied to a decline in the amount of chalky debris and sarsens moving into the valley during a 'late' periglacial phase. The previously overwhelmed Clatford stream seems to have reasserted itself and further undercutting of the southern slope occurred, to give angles of up to 29°. With the ultimate cessation of

I.D 0\

s.w.

N .E. OLD TOTTER DOWN (HIG H SARSEN DENSITY )

OV ERTON DOWN (MEDIUM SARSEN DEN SITY)

® ~-~

®

~~~

_________ ____ J.!~EJ. _~~~~~S!~ _~O..!l~!_!IQ~

~~-.~~~. J.!t.~j

T~~~~~ ;f_ · - . ,J1 ~ '

...,.,:uw .. ~ BENCH

.-

~

•

~--

.. -

~ •

••

-

en

~.Q -~~

~ ;I> t"" t"" ;I>

_

Z t:l

..

©

~ :-<

.Q

0 ...

®

o...,

:r:tTl

;;0

en

NOT

to

SCA Lf

Fig. 3. Hypothetical development of Clatford Bottom. Th e mar ginal parts of Section A, and to some exten t of B and C, illustrate claywith-flints incorporatin g Eocene rernanie and sarsens

PERIGLACIAL ROCK-STREAM IN WILTSHIRE

97

solifluction, stream rejuvenation continued for a while, leading to further removal of the chalky infill and leaving an increasing number of sarsens lying at or near the surface of the valley floor and on the front riser of the low terrace (Fig. 3, D). The evidence of the pit sections in general supports this interpretation. The bulk of the coombe rock and the brown flinty loam appear on the basis of mechanical analysis to represent one deposit, the upper part of which has been partially removed by fluvial action and further reduced in volume by decalcification. Unfortunately no evidence has been discovered to support an absolute chronology of the development ofClatford Bottom, but it is to be presumed that the major episode of solifluction and sarsen transport occupied the Weichselian main glacial period. Obviously a major problem is posed by the apparent absence of earlier Quaternary deposits in Clatford Bottom. The complete removal of an older infill of a chalky nature could perhaps be accounted for, but large sarsens transported to the valley floor during earlier periglacial phases should have remained, to be buried by the younger infill. Whether such stones could have been heaved to the surface of the younger deposit, perhaps as solifluction along the valley took place, is impossible to tell. Another possibility is that shortly after the onset of the final solifluction episode the sarsens already resting on the valley floor were themselves removed by down valley solifluction flows, prior to the arrival of 'new' coombe rock and sarsens. The last alternative is to assume that the sarsens were transported at a very slow rate down the gentle slope of Clatford Bottom during several phases of solifluction, arriving at the valley floor only during the final cold period. However, this thesis of late arrival seems to rely rather too much on coincidence, and the implied derivation of sarsens only from distant sources (Williams, 1968)seems to us unjustified.

ACKNOWLEDGMENTS

The authors wish to express their gratitude to the Nature Conservancy for giving them permission for pits to be dug at Clatford Bottom and for providing financial and other assistance, and to the many students from the Department of Geography, University of Southampton, who did the digging. REFERENCES CLARK, M. J., J. LEWIN & R. J. SMALL. 1967. The Sarsen Stones of the Marlborough Downs and their Geomorphological Implications. Southampton Research Series in Geography, 4, 3-40. OLUER, C. D. & A. J. THOMASSON. 1957. Asymmetrical Valleys in the Chiltem Hills. Geogr.J., 123,71-80. PROC. GEOL. ASS., YOLo 81, PART 1,1970

7

98

R. J. SMALL AND OTHERS

TE PUNGA, M. T. 1957. Periglaciation in Southern England. Tijdschr. K. ned. aardrijksk, Genoot., 74, 400-12. WHITE, H. J. O. 1925. The Geology of the Country around Marlborough. Mem, geo!. Surv. U.K., 74-8. WILLIAMS, R. B. G. 1968. Some Estimates of Periglacial Erosion in Southern and Eastern England. Bluletyn peryglacjalny; 17,311-35. Department of Geography The University Southampton