GEOLOGY 



more loamy and less chalky as we leave the rising Downs and cross the 

 coastal plain. It is particularly well seen in the cHff at Black Rock, east 

 of Brighton, where it overlies the raised beach, in the Portslade gravel 

 pits, and in the enormous ballast pit by the side of the Brighton railway 

 near Chichester. Selsey and Bognor cliffs show the loamy modification 

 of this singular deposit, locally called ' shrave ' ; while when traced into 

 the river gorges it tends to pass into a more stratified and cleaner river 

 gravel, forming a terrace well above the present river level. The horse 

 and the mammoth are everywhere the most common fossils in it ; but 

 the teeth are always much battered and decayed, as though they had 

 lain on the surface for some time before they reached their present resting- 

 place. Implements used by man occur in it ; but these also may be 

 of older date, for they are not nearly so plentiful as in the Bournemouth 

 or Southampton gravels, which belong to a somewhat earlier period. 

 Nothing like the Coombe Rock is now being formed in Britain, and we 

 must go to regions having a more rigorous climate to find anything 

 closely analogous. It is not however directly of glacial origin, for none 

 of the stones are striated, and the few from distant sources are such as 

 we know occur in the underlying marine Pleistocene deposit. The 

 enormous sheet of Coombe Rock has evidently been derived from the 

 Downs, and a study of the contours of the Downs (see orographic map) 

 gives us the key to its mode of formation.^ 



The peculiar rolling outline of our Chalk Downs, the steep-sided 

 valleys winding for miles among the hills, yet never, even in the wettest 

 season, containing running water, are familiar types of English scenery. 

 But, perhaps because so familiar, it does not at first strike one that these 

 outlines point to conditions which have now entirely passed away. No 

 streams now fill these upland valleys, and where streams do occupy the 

 bottoms of coombes, their beds fall very gently, so that they do not 

 assume the character of mountain torrents, as any stream in the steeper 

 coombes must necessarily do. It is impossible, under present conditions, 

 for any stream to exist in these dry valleys ; for the Chalk is so porous 

 that the heaviest rain sinks in directly, and the most continued rainfall 

 merely causes new springs to burst out at some point rather higher up 

 the valley than usual. The upper and steeper portion of the valley still 

 remains perfectly dry, and no running water can be found where the 

 incline of the bottom of the valley exceeds the slope of the plane of 

 saturation in the Chalk. The characteristic contour of these valleys is 

 well shown in the Downs near Brighton (see fig. 3). 



Though in Sussex the contemporaneous fossils of the Coombe Rock 

 are insufficient to indicate the climatic conditions that held while it was 

 being deposited and while the coombes were being excavated, yet in 

 other districts this evidence can be obtained. At Fisherton near Salis- 

 bury corresponding beds yield many species of high northern mammals, 

 such as the reindeer, musk ox and lemming, while at Bovey Tracey in 



1 Reid, ' On the Origin of Dry Chalk Valleys and of Coombe Rock,' Quart. Joum. Geol. Soc. 

 xliii. 364 (1887). 



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