1877*] The Great Ice Age and Origin of the “ Till .” 225 
glacier would slide without either grinding it lower by erosion 
or raising it higher by deposition. 
But what must be the nature of this deposit ? It is evi- 
dent that it cannot be a mere moraine consisting only of the 
larger fragments of rock such as are now deposited at the 
foot of glaciers that die out before reaching the sea. Neither 
can it correspond to the glacial silt which is washed away 
and separated from these larger fragments by glacial streams, 
and deposited at the outspreadings of glacier torrents and 
rivers. It will correspond to neither the assorted gravel, 
sand, or mud of these alluvial deposits, but must be an 
agglomeration of all the infusible solid matter the glacier is 
capable of carrying. 
It must contain, in heterogeneous admixture, the great 
boulders, the lesser rock fragments, the gravel chips, the 
sand, and the slimy mud ; and these settling down quietly 
in the cold, gloomy waters, overshadowed by the great ice- 
sheet, must form just such an agglomeration as we find in 
the boulder clay and tills ; and lie just in those places where 
these deposits abound, provided the relative level of land 
and sea during the glacial epoch were suitable. 
I should make one additional remark relative to the 
composition of this deposit, viz., that under the conditions 
supposed, the original material detached from the rocks 
around the upper portions of the glaciers would suffer a far 
greater degree of attrition at the glacier bottom than it ob- 
tains in modern Alpine glaciers, inasmuch as in these it is 
removed by the glacier torrent when it has attained a certain 
degree of fineness, while in the greater glaciers of the 
glacial epoch it would be carried much further in association 
with the solid ice, and be subjected to more grinding and re- 
grinding against the bottom. Hence a larger proportion of 
slimy mud would be formed, capable of finally indurating 
into stiff clay such as forms the matrix of the till and 
boulder clay. 
The long journey of the bottom debris stratum of the 
glacier, and its final deposition when in a state of neutral 
equilibrium between its own tendency to repose and the 
forward thrust of the glacier, would obviously tend to 
arrange the larger fragments of rock in the manner in which 
they are found imbedded in the till, i.e., the oblong frag- 
ments lying with their longer axes and their best marked 
striae in the direction of the motion of the glacier. The 
“ striated pavements ” of the till are thus easily explained, 
as the surface upon which the ice advanced when its depo- 
vol. vii. (n.s.) q 
