332 



SCIENCE 



[X. S. Vol. XXXII. No. 819 



To the south and west of this area it is 

 dark, from the similar presence of Kim- 

 eridge clay. Yet further west it assumes 

 an intermediate color by having drawn 

 upon the Oxford clay. This boulder clay, 

 whether the chalky or the purple, in which 

 partings of sand sometimes occur, must 

 once have covered, according to Mr. F. W. 

 Harmer, an area about ten thousand square 

 miles in extent. It spreads like a covelet 

 over the pre-glacial irregularities of the 

 surface. It caps the hills, attaining some- 

 times an elevation of fully 500 feet above 

 sea-level;-^ it fills up valleys,-^ sometimes 

 partly, sometimes wholly, the original 

 floors of which occasionally lie more than 

 100 feet below the same level. This 

 boulder clay, often with an underlying 

 sand or gravel, extends to the south as far 

 as the neighborhood of Muswell Hill and 

 Finchley; hence its margin runs westward 

 through Buckinghamshire, and then, bend- 

 ing northwards, passes to the west of 

 Coventrj^ On this side of the Pennine 

 Chain the matrix of the boulder clay is 

 again reddish, being mainly derived from 

 the sands and marls of the Trias ; pieces of 

 chalk and flint are rare (no doubt coming 

 from Antrim), though other rocks are 

 often plentiful enough. Some authorities 



-' Not far from Royston it is found at a height 

 of 525 feet above O.D. See J". W. Harmer, " Pleis- 

 tocene Period in the Eastern Counties," p. 115. 



-At Old North Road Station, on a tributary of 

 the Cam, the boulder clay was pierced to a depth 

 of 180 feet, and at Impington it goes to 60 feet 

 below sea-level. Near Hitchin, a hidden valley, 

 traced for seven or eight miles, was proved to a 

 depth of 68 feet below O.D., and one near Newport 

 in Essex, to 140 feet. Depths were also found of 

 120 feet at West Horseheath in Suffolk, of 120 

 feet on low ground two miles southwest of Sandy 

 in Bedfordshire, of from 100 to 160 feet below the 

 sea at Fossdyke, Long Sutton and Boston, and at 

 Glemsford in the valley of the Stour 477 feet of 

 drift was passed through before reaching the 

 chalk. See F. W. Harmer, Quart. Journ. Geol. 

 Soc, LXIII., 1907, p. 494. 



are of the opinion that the drift in most 

 parts of Lancashire and Cheshire is sep- 

 arable, as on the eastern coasts, into a 

 lower and an upper boulder clay, with in- 

 tervening gravelly sands, but others think 

 that the association of the first and third 

 is lenticular rather than successive. Here 

 also the lower clay can not be traced very 

 far inland, eastward or southward; the 

 others have a wider extension, but they 

 reach a greater elevation above sea-level 

 than on the eastern side of England. The 

 sand is inconstant in thickness, being 

 sometimes hardly represented, sometimes 

 as much as 200 feet. The upper clay runs 

 on its more eastern side up to the chalky 

 boulder clay, and extends on the south at 

 least into Worcestershire. On the western 

 side it merges with the upper member of 

 the drifts radiating from the mountains of 

 North Wales, which often exhibit a similar 

 tripartite division, while (as we learn from 

 the officers of the Geological Survey) 

 boulder clays and gravelly sands, which it 

 must suffice to mention, extend from the 

 highlands of South Wales for a consider- 

 able distance to the southeast and south. 

 Boulder clay has not been recognized in 

 Devon or Cornwall, though occasional er- 

 ratics are found which seem to demand 

 some form of ice-transport. A limited de- 

 posit, however, of that clay, containing 

 boulders now and then over a yard in 

 diameter, occurs near Selsey Bill on the 

 Sussex coast, which most geologists con- 

 sider to have been formed by floating rather 

 than by land ice. 



Marine shells are not very infrequent in 

 the lower clays of East Anglia and York- 

 shire, but are commonly broken. The well- 

 Imown Bridlington Crag is the most con- 

 spicuous instance, but this is explained by 

 many geologists as an erratic — a piece of 

 an ancient North Sea bed caught up and 

 transported, like the other molluscs, by an. 



