264 



KNOWLEDGE. 



[November 1, 1897. 



hood of Weymouth, especially near Sandsfoot Caatle. Both 

 these clay plains are exceedingly like the (fault plain, 

 except that the number of oaks is usually less. It is the 

 plain of the Oxford Clay that forms the greater part of the 

 Cambridgeshire fens ; the reason why this area has become 

 fen being that the rivers have cut down to within a very 

 slight elevation of the sea-level — or, as it is called, to their 

 base of erosion — and the natural drainage of the country is 

 consequently very imperfect. In such tracts water stag- 

 nates, peat is formed, and the soil is unsuitable for the 

 growth of any trees but willows. 



Crossing the Cotswolds, similar escarpments of massive 

 limestones on to Hat plains of clay are again met with in 

 rocks of still older geological age. lu the neighbourhood 

 of Bath the chief escarpment is formed by tlie massive 

 limestone of the Great Oolite on to the clay of the under- 

 lying Fuller's Earth. But in the neighbourhood of 

 Cheltenham it is the limestone of the Inferior Oolite, 

 situated beneath the FuUer's Earth, which makes the great 

 escarpment on to the wide, open plain of the Lower Lias 

 clay of the Vale of Gloucester. This Lias plain is one of 

 the largest of all such tracts, and supports in many places 

 a thick growth of oaks. Viewed from the heights of 

 Leckhampton Hill, or Cleave Hill, in the neighbourhood 

 of Cheltenham, it is diificult to realize that we are not 

 looking down upon the plain of either the Gault or the 

 Weald clay of the South-East of England, although we are 

 really on a geological horizon many thousands of feet 

 below that of either of the two latter. 



This Lias plain of the Vale of Gloucester is extended to 

 the foot of the Malvern Hills by the red and green marls 

 and sandstones of the Trias series, which, although really 

 situated beneath the Lias, have such a gentle inclination 

 as apparently to form but a single plain with the latter. 

 At Malvern the Trias plain is brought to an abrupt end 

 by the great fault which brings up the igneous and meta- 

 morphic rocks of the Malvern Hills. And, looking down 

 from the summit of this range towards Gloucester, the 

 great plain below is almost exactly similar to that of the 

 Lias, with the exception that it carries fewer oaks. But 

 even this last by no means exhausts the list of the 

 great clay formations which constitute plains in different 

 parts of the country. If we proceed westwards from 

 Gloucester, and cross the Severn in the direction of the 

 Forest of Dean, we first traverse one or more high ridges, 

 composed of the red sandstones and clays of the Devonian 

 system, followed by the Carboniferous Limestone and the 

 Milestone Grit, and then suddenly find ourselves on the 

 open plain of the Forest of Dean, formed by the clays and 

 shales of the Coal Measures, interstratified among which 

 are the seams of workable coal. The forest is a level 

 tract of somewhat swampy ground thickly carpeted with a 

 coat of mossy turf, and roofed above with a dense growth 

 of oaks, to the exclusion of almost every other kind of 

 tree. And so like are the two areas that the non- geological 

 observer might well believe himself to be in the uncultivated 

 portions of the Weald of Kent, the soil of which Sedgwick 

 years ago named the " oak-tree clay." But, whereas the 

 clay of the one period was laid down far back in the 

 Palaeozoic epoch, when the forests consisted mainly of 

 gigantic club-mosses, huge mare's-tail-like plants, tree- 

 ferns, and araucarias, that of the latter was deposited late 

 in the Secondary epoch, at a time when the vegetation 

 approached that of the tropical zone of to-day. Struc- 

 turally the clay plain of the Forest of Dean differs from 

 all those mentioned above in that the high ground forming 

 its boundary is not an escarpment, the Coal Measures 

 being newer than the Devonian rocks forming the high 

 ground ; whereas, in the case of an escarpment, the clay 



plain at its foot is the older of the two. Nevertheless, 

 the general effect is very much the same in the two 

 cases. 



We thus find that, exclusive of the Tertiary formations, 

 no less than seven great clay plains occur on different 

 geological horizons in various parts of England. These, 

 in descending order, include the plain of the Gault, of the 

 Weald Clay, of the Kimeridge Clay, of the Oxford Clay, 

 of the Fuller's Earth, of the Lias and Trias, and of the 

 Coal Measures. And so like are these to one another, 

 that even the experienced geologist may frequently at 

 first sight mistake the one for the other, if the form 

 of the escarpment of harder rocks be concealed from 

 view. 



That such plains should be very similar in physical 

 appearance and in the vegetation they support is only 

 natural, seeing that clay of one geological epoch is very 

 like that of another. The curious fact of the matter ia 

 how these plains came to be exposed on so many different 

 geological horizons, seeing that in all cases they were 

 originally covered by the harder limestone rocks which 

 now form their bordering escarpments. Of course the 

 laying bare of the clays is the work of denudation, and 

 that chiefly effected by subaerial agencies — that is, by 

 rivers, rain, wind, " weathering," etc. But looking down 

 from the heights of the Cotswolds on to the Vale of 

 Gloucester, or from the North or South Djwns on to the 

 Weald of Kent, it seems difficult to realize that the hard 

 Umestone of the former range, or the solid chalk of the 

 latter, is capable of being worn away faster than the soft 

 clay at its base. The important factor in the matter is, 

 however, undoubtedly the circumstance that while lime- 

 stone, or sandstone (which constitutes some escarpments), 

 is permeable by water, clay is not. Consequently the 

 limestones (or sandstones) are worn away with comparative 

 rapidity, while the clays beneath remain to form the plain 

 at the base. And as Chalk is softer and more easily worn 

 away than the hard Oolitic limestone of the Cotswolds, it 

 is easy to see why the Chalk escarpments of the North and 

 South Downs are now far removed from the centre of the 

 Weald, over which the Chalk once extended, while the 

 escarpment of the Oolites at Cheltenham is less distant 

 from the westerly limits of the underlying Lias. 



Still this does not explain the origin of escarpments as 

 such ; and I confess that I have hitherto seen no explan- 

 ation of the origin of these features which is altogether 

 satisfactory, neither have I been able to evolve one for 

 myself. That escarpments as they now exist were not 

 formed by the sea is evident ; although marine action 

 doubtless aided in planing down the land as it was being 

 first raised above the ocean level. By some means or 

 other the lines of escarpment were, however, definitely 

 marked out after the upheaval of the land ; and when 

 once defined, it is perfectly easy to understand how they 

 are maintained. 



The number of escarpments bordered at the foot by a 

 plain of clay is a peculiarity of the geology of England 

 (and to a certain extent of the adjacent parts of the Con- 

 tinent as well), due in the first place to alternate risings 

 and sinkings in the old ocean floor, and in the second 

 place to the consequent alternation of impervious beds of 

 shallow water clays with layers of limestones deposited in 

 deeper water. Limestones of different ages nearly always 

 differ from one another in hardness or compactness, and 

 consequently weather into escarpments distinguished by 

 some peculiarity of contour ; but clays present no such 

 distinctive differences, with the result that the various 

 plains formed by those of widely different ages are almost 

 indistinguishable in physical features from one another. 



