362 



SCIENCE 



IN. S. Vol. XXXII. No. 820 



forty or fifty feet above the present Ord- 

 nance datum. But at Manchester it reaches 

 over 200 feet, while near Heywood it is at 

 least 425 feet. In other words, the sands 

 and gravels, presumably (often certainly) 

 mid-glacial, mantle, like the upper boulder 

 clay, over great irregularities of the sur- 

 face, and are sometimes found, as already 

 stated, up to more than 1,200 feet. Either 

 of these deposits may have followed the sea- 

 line upwards or downwards, but that ex- 

 planation would almost compel us to sup- 

 pose that the sand was deposited during 

 the submergence and the upper clay during 

 the emergence; so that, with the former 

 material, the higher in position is the newer 

 in time, and with the latter the reverse. 

 "We must not, however, forget that in the 

 island of Eiigen we find more than one 

 example of a stratified gravelly sand be- 

 tween two beds of boulder clay (containing 

 Scandinavian erratics) which present some 

 resemblance to the boulder clays of eastern 

 England, while certain glacial deposits at 

 Warnemiinde, on the Baltic coast, some- 

 times remind us of the Contorted Drift of 

 Norfolk. 



Towards the close of the glacial epoch, 

 the deposition of the boulder clay ceased''^ 

 and its denudation began. On the low 

 plateaus of the eastern counties it is often 

 succeeded by coarse gravels, largely com- 

 posed of flint, more or less water-worn. 

 These occasionally include small intercala- 

 tions of boulder clay, have evidently been 

 derived from it, and indicate movement by 

 fairly strong currents. Similar gravels 

 are found overlying the boulder clay in 

 other parts of England, sometimes at 

 greater heights above sea-level. Occasion- 

 ally the two are intimately related. For 

 instance, a pit on the broad, almost level, 



" Probably deposits of a distinctly glacial origin 

 (such as those near Hessle in Yorkshire) con- 

 tinued in the northern districts, but on these we 

 need not linger. 



top of the Gogmagog Hills, about 200 feet 

 above sea-level, and four miles south of 

 Cambridge, shows a current-bedded sand 

 and gravel, overlain by a boulder clay, ob- 

 viously rearranged ; while other pits in the 

 immediate neighborhood expose varieties 

 and mixtures of one or the other material. 

 But, as true boulder clay occurs in the 

 valley below, these gravels must have been 

 deposited, and that by rather strong cur- 

 rents, on a hill-top — a thing which seems 

 impossible under anything like the existing 

 conditions; and, even if the lowland were 

 buried beneath ice full 200 feet in thick- 

 ness, which made the hill-top into the bed 

 of a lake, it is difficult to understand how 

 the waters of that could be in rapid motion. 

 Rearranged boulder clays also occur on the 

 slopes of valleys""' which may be explained, 

 with perhaps some of the curious sections 

 near Sudbury, by the slipping of materials 

 from a higher position. But at Old Os- 

 westry gravels with indications of ice ac- 

 tion are found at the foot of the hills 

 almost 700 feet below those of Gloppa. 



Often the plateau gravels are followed 

 at a lower level by terrace gravels,**' which 

 descend towards the existing rivers, and 

 suggest that valleys have been sometimes 

 deepened, sometimes only reexcavated. 

 The latter gravels are obviously deposited 

 by rivers larger and stronger than those 

 which now wind their way seawards, but it 

 is difficult to explain the former gravels by 

 any fluviatile action, whether the water 

 from a melting ice-sheet ran over the land 

 or into a lake, held up by some temporary 

 barrier. But the sorting action of currents 

 in a slowly shallowing sea would be quite 

 competent to account for them, so they 

 afford an indirect support to the hypothesis 



"= For instance, at Stanningfield in the valley of 

 the Lark. 



"' These contain the instruments worked by 

 paleolithic (Aoheulean) man who, in this country 

 at any rate. Is later than the chalky boulder clay. 



