328 



SCIENCE- GOSSIP. 



fell on the frozen soil, and thus led to extensive 

 sheets of gravel. 



Let us put Prestwich, Lyell, and Clement Eeid 

 into the melting-pot and adopt something from 

 each. As one who has been familiar with the dry 

 Sussex coombes for many years, I may perhaps 

 express an opinion as to their mode of excavation 

 and the formation of rubble di'ift. First of all, 

 the dry coombes must have been caused by water 

 action. Everybody is so far agreed. It is also 

 allowed that the water must have been in motion. 

 The coombes emerge towards the sea with but few 

 exceptions ; therefore the moving water must have 

 excavated and moved southwards. The Weald 

 may have been covered with its beds continuously 

 from north to south. If so, the coombes which 

 then existed had not been denuded sufficiently 

 deeply to cut down below the level of the highest 

 part of the existing downs, except perhaps where 

 now are the gaps at Lewes, Devil's Dyke, Saddles- 

 comb, and the southerly flowing rivers now exist- 

 ing. Surely, however, the carving of these valleys 

 would commence long before the chalk finally 

 protruded its head out of the sea at the close of its 

 last period of submergence. I think with each 

 change of level, as the bed of the sea approached 

 the surface, there would be a perfect tumult of 

 currents, and the more rapid the rise the more 

 complicated and unsettled would be the courses 

 taken by such currents. As the land emerged, 

 which we all admit it did finally, however 

 we may disagree on other points, the marine 

 currents would subside, but would in some cases 

 be replaced by fresh-water streams, or even torrents, 

 owing to the saturation of the chalk beneath. I 

 cannot account for the absence of marine shells in 

 the valleys, but they may have all been decomposed 

 during the "pluvial" period, a process which, 

 indeed, goes on over the Downs in the case of dead 

 land shells even now. The marine-shell question 

 need not trouble us. They are not found here on 

 the Do\^-ns, yet no one ventures to say that the 

 chalk was not, all of it, under the sea at some time 

 or other, in Quaternary times. Up some of the 

 valleys formed under the sea,-the tides would still 

 act and erode. Nothing is more porous than a 

 beach of rounded flints. Yet the tides breaking on 

 the Brighton beach now form miniature hollow 

 coombes with rounded heights between, like, 

 indeed, to downs in miniature. There the sea does 

 the work, although in an instant it has sunk 

 through the porous beach. Where is the necessity 

 to argue for a frozen soil? Eepeated tides will 

 often flow through just the same ' miniature 

 " coombes " over and over again, and these will 

 not necessarily be parallel to one another. Another 

 point, too : the coombes which they make are gene- 

 rally clear of detrital matter, which in their case is 

 carried away to form a "rubble drift." perhaps 

 beyond low tide, with mammalian remains, occa- 

 sionally in the shape of a dead dog or cat. 



As the tides finally retired and the "pluvial"^ 

 period ceased most of the coombes would be run 

 dry, the rainfall being not greater than could be 

 assimilated by the porous chalk. That marine 

 shells are absent is true. If not explicable, as 

 suggested above, it cannot be denied that they 

 may yet be foimd. 



A j)oint in favour of this theory is the fact that 

 in some of the chalk rubbles there are found land 

 shells, for in such cases we should have the waves 

 of retiring tides drawing denizens of the land inta 

 the sea and actually depositing rubble strata con- 

 taining land shells. Where " head " has been 

 formed to any large extent there must have been, 

 after the retiring sea had done its work, a com- 

 paratively large river in existence. In the case of 

 the Brighton " head " there were two rivers which 

 joined before the cliffs were reached. If, as 

 geologists believe, the Weald was denuded during 

 the course of an upheaval or upheavals over the 

 central ridge, surely it is not assuming too much 

 to believe that, after another submergence and 

 another upheaval, that part of the Downs that 

 remained and was nearest the Weald continued to 

 uprise over and above the degxee to which the 

 existing coast was raised. In that case the angle 

 of the fall of torrential waters would possibly be 

 sufficient, with the occasional aid of ice-floes, to 

 deposit the head containing flints, angular because 

 of the shortness of their journeys, together with Ter- 

 tiary sandstones, with their rough corners rounded. 

 It may be that some gravel has been formed in 

 the way brought forward by Mr. Clement Reid. 

 The great fault, however, with modern gravel 

 geologists is to " overburden the boat," to ride 

 their own particular theories to the death by 

 endeavouring to cause them to explain too much, 

 or, as in the case of Prestwich, by classing too 

 many different deposits together under one name. 



The most sensible view of rubble dejjosits seems 

 to be that of Mr. J. Allen Brown, who claimed that 

 these had been formed at all times since the last 

 period of emergence. He also, very reasonably as 

 I think, attributed the formation of much of the 

 angular gravel to the removing action of subter- 

 ranean waters upon the- (?halk, the latter being 

 removed both in solution and mechanically. See 

 also Whitaker's " Guide to the Geology of London," 

 ed. 1901, p. 72, concerning the formation of " clay 

 with flints." 



Given a pluvial period, a period of floods, there 

 seems no reason why Lyell should not, after all, 

 IDrove to be correct in recognising, in some of the 

 rubble drifts of Sussex at least, the deltaic work 

 of rivers piercing the Downs, in this way doing 

 away with the necessity of imagining a sub- 

 mergence of the land here to at least 1,000 feet 

 (Prestwich), the lieight of the highest occurrence 

 of such drift elsewhere. 



2?> Campbell lloacl, Crojidon, 

 March 1902. 



