
Parr V. Secr. i. § 1.] PLEISTOCENE. 889 
spreads over the low ground of the glaciated districts and may even 
be traced up the valleys of the smaller groups of hills, whence it was 
not wholly removed by the erosion of the later glaciation. Thus 
it extends all over the low grounds of North Germany, Denmark, 
Holland, Scandinavia, Scotland, and much of England and Ireland, 
resting usually on surfaces of rock that have been worn smooth, dis- 
rupted, or crumpled by ice. It is not spread out, however, as.a uni- 
form sheet, but varies greatly in thickness and in irregularity of 
surface. specially round the mountainous centres of dispersion it 
is apt to occur in long ridges or “drums,” which run inthe general 
direction of the rock-striation, that is, in the path of the ice-move- 
ment. 
| In those areas which served as independent centres of dispersion 
for the ice-sheet, the boulder-clay partakes largely of the local 
character of the rocks of each district where it occurs. Thus in 
Scotland the clay varies in colour and composition as it is traced 
from district to district. Over the Carboniferous rocks it is dark, 
over the Old Red Sandstones it is red, over the Silurian rocks it is 
fawn-coloured. ‘The great majority of the stones, also, are of local 
origin, not always from the immediately adjacent rocks, but from 
points within a distance of afew miles. Evidence of transport can be 
gathered from the stones, for, they are found in almost every case to’ 
include a proportion of fragments which have come from a distance. 
The direction of transport indicated by the percentage of travelled 
stones agrees with the traces of ice-movement as shown by the rock- 
strie. Thus, in the lower part of the valley of the Firth of Forth, while 
most of the fragments are from the surrounding Carboniferous rocks, 
from 5 to 20 per cent. have come eastward from the Old Red Sand- 
stone range of the Ochil Hills—a distance of 25 or 30 miles—while 
2 to 5 per cent. are pieces of the Highland rocks, which must have 
come from high grounds at least 50 miles to the north-west. As each 
main mass of elevated ground seems to have caused the ice to move 
outward from it for a certain distance, until the stream coalesced 
with that descending from some other height, the bottom-moraine or 
boulder-clay, as it was pushed along, would doubtless take up local 
- debris by the way, the detritus of each district becoming more and 
more ground up and mixed, until of the stones from remoter regions 
only a few harder fragments would be left, In cases where no pro- 
minent ridges interrupted the march of the ice-sheet, and where the 
ground was low and covered with soft loose deposits, blocks of hard 
erystalline rocks might continue to be recognizable far from their 

layer of decomposed rock due to prolonged pre-glacial disintegration (p, 338). It is 
difficult to explain by any known glacial operation the accumulation of such deep 
masses of detritus below a sheet of moving land-ice. Another problem is presented by 
the occasional and sometimes extensive preservation of undisturbed loose pre-glacial 
deposits under the till. The way in which the “ Forest-bed ” group has escaped for so 
wide a space under the Cromer cliffs, with their proofs of enormous-ice-movement, is a 
remarkable example. 
