VALLEYS AND OP COOMBE ROCE:. 



365 



This deposit seldom extends far up the valleys, but it can be traced 

 as much as eight miles south of the old cliff, over a surface of Chalk, 

 Eocene, and marine Pleistocene beds. 



The Coombe Rock, though of singularly uniform character 

 considering its coarseness, changes as the distance from the Downs 

 increases. In the Coombes, and for three or four miles south of the 

 Downs, it consists of a mass of unstratified, or obscurely stratified, 

 flints, battered, but not rolled, and imbedded in a matrix of chalky 

 paste and pieces of chalk. Close to the old cliff, as at Brighton, 

 large masses of chalk are found in it, and also, locally, numerous 

 greywethers. Pebbles only occur in places where they might be 

 obtained from older beds immediately to the north. 



As the distance from the old cliff increases, the Coombe Rock 

 changes laterally into a deposit known locally as " shrave." This 

 consists of angular flints in a loamy matrix, the proportions being 

 such that the mixture is worthless either for gravel or brick-earth. 

 Still further from the Chalk, as at Selsey, the shrave changes into 

 almost clean brick-earth, though it still contains scattered augular 

 flints. Thin brick-earth also overlies a considerable portion of the 

 Coombe Rock around Brighton and Worthing. 



The fossils of the Coombe Rock consist almost entirely of teeth of 

 Horse and Elephant, broken, and apparently also decayed, before 

 they were imbedded. A few Palaeolithic implements have also 

 occurred. Though careful search was made I could not find a 

 single mollusk, nor any plant-remains, except two or three pieces of 

 decayed wood. 



It seems evident, from the peculiar character of the Coombe Rock, 

 that it was not formed by agents now at work in the district. It 

 is not of glacial origin, for none of the stones are striated, and the 

 few from distant sources are such as we know occur in the under- 

 lying marine Pleistocene deposit. It is not marine, for it is almost 

 unstratified, the stones are not rolled, and the marly matrix contains 

 no shells, though full of undissolved chalk. It cannot be a gravel 

 formed by ordinary fluviatile action, for there are no valleys to 

 contain the streams, and no river transports soft chalk-detritus more 

 than a short distance, or lays down sheets of gravel of this singularly 

 unworn and unstratified character. 



The enormous sheet of Coombe Rock just described has evidently 

 been derived from the Downs, and I believe that a study of the 

 coutours of the Downs 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 a drop of 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. ISTo 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 



Q. J. G. S. No. 171. 2 c 



