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). 23
