LUBBOCK ON THE ANTIQriTT OP MAX. 265 



turned it every spring into a, roaring torrent. These floods were pro- 

 bably more destructive to animals even tlian man himself ; while, 

 however rude they may have been, our predecessors can hardly be 

 supposed to have been incapable of foreseeing and consequently es- 

 caping the danger. While the water, at an elevation of 150 feet 

 above its present level, as for instance at Liercoiu't, had sufficient 

 force to deposit coarse gravel ; at a still higher level it would part 

 with finer particles, and would thus form the loess which, at the same 

 time, would here and there receive angular flints a,nd shells brought 

 down from the hills in a more or less transverse direction by the 

 rivulets after heavy rains. 



As the valley became deeper and deeper the gravel would be 

 deposited at lower and lower levels, the loess always following it ;* 

 thus we must not consider the loess as a distinct bed, but as one 

 which was being formed during the same time, though never at the 

 same place as the beds of gravel. Fig. 3, I have given an imagi- 

 nary diagram, the better to illustrate my meaning ; the loess is 

 indicated by letters with a dash and is dotted, while the gravels 

 are represented as rudely stratified. In this case I suppose the 

 river to have run originally on the level («), and to have deposited 

 the gravel (a) and the loess (a) ; after a certain amount of erosion 

 which would reduce the level to (b), the gravel woidd be spread 

 out at b, and loess at (b'). Similarly the loess (c') would be contem- 

 poraneous with the gravel (c). 



Thus while in each section the lower beds would of course be the 

 oldest, still the upper-level gravels as a Avhole would be the most 

 ancient, and the beds lying on the lower parts of the valley the most 

 modern. 



For convenience I have represented the sides of the valley as 

 forming a series of terraces ; and though this is not actually the case, 

 there are several places in which such terraces do occur.f 



It is, however, well known that rivers continually tend to shift 

 their courses ; nor is the Somme any exception to the rule ; the valley 

 itself indeed may be comparatively straight, but within it the river 

 winds considerably, and when in one of its curves, the current crosses 

 " its general line of descent, it eats out a curve in the opposite bank, 

 " or in the side of the hills bounding the valley, from which curve it 

 " is turned back again at an equal angle, so that it reci-osses the line 

 " of descent, and gradually hollows out another curve lower down in 

 " the opposite bank, till the whole sides of the valley, or river-bed, 



* See Mr. Prestwich's paper read before the Royal Society, June 19th, 1862. 



■f While attributing the excavation of these valleys to the action of the existing 

 rivers, Mr, Prcstwich doubts wliether they could have produced such an eftect 

 without an elevation of the land. INIarine shells occur at Abbeville about 25 feet 

 above the sea-level ; this bed Mr. Prestwich coiTclates with some of the raised 

 beaches round our coasts, and with the lower level valley gravels. The higher 

 level valley gravels correspond in his opinion witli tlic raised beaches which occur 

 at a higher level. 



