127 
predominant movement of the ice sheets was in a south- 
west direction is shown by the general trend of striae and 
the frequent occurrence of “‘stossing’’ of glaciated rock 
sutfaces on the north side. Other striae, which are more 
local and generally confined to the valley of the Ottawa 
river, show a movement of the ice in a direction nearly 
at right angles to this. The two sets of striae are rarely 
seen on the same rock surfaces in the vicinity of Ottawa, 
but farther west they are frequently seen crossing each 
other, and the older course is towards the southwest, 
while the more recent courses are towards the southeast 
and appear to have been influenced in their direction by 
the river valleys. 
How many repetitions of glaciation in Pleistocene time 
there were in this area is not known, but the presence of two 
till sheets, separated by stratified sands and gravels shows 
_ that at least two ice invasions occurred, which are presum- 
ably the most recent, and it is possible that there were 
others of which no record remains. 
The boulder clay ascribed to the earlier invasion 
of the ice sheet is the lowest member of the surface deposits. 
The boulders of this deposit, which are usually small,. 
are imbedded in a stiff gritty clay, forming a compact 
resistant material. It occurs in large patches, not in 
a continuous sheet, and is rarely exposed except in excava- 
tions or along river banks. This boulder clay where 
present generally rests on comparatively fresh rock sur- 
faces, often polished, striated and grooved, and no evidence 
of residual clays, soils or gravels of pre-glacial age have 
been found in the district. 
Sections of the drift at Ottawa which include the upper 
boulder clay show it resting on horizontally stratified, 
or crossed bedded, soft, incoherent sand beds. The upper 
boulder clay carries many larger boulders, has a larger 
proportion of rounded stones and contains less clay in 
the matrix than the lower boulder clay. The erosive 
action of the ice sheet during its later advance was feeble, 
for it passed over sand and clay beds with little damage 
to these soft materials. The principal work accomplished 
seems to have been the pushing forward and transporta- 
tion of loose drift matetials derived from the earlier ice 
invasion. 
