~A' 
| 
238 B. N. A. BOUNDARY COMMISSION. 

K 
one at which the causes producing the drift dey osit acted for a very long 
period, or whether its accumulation marks a stage of depression at which 
the Laurentian axis, and its flanking Silurian rocks were most favorably 
situated for degradation, are general questions best discussed in the sequel. 
The Coteau belt is not exactly a shore deposit, and its position during the 
period of greatest subsidence must have been deeply submerged, though 
this may have been for a comparatively short time. 
548. Here on the foot of the Tertiary plateau, from the North Sas- 
katchewan to the Line, heavy ice from the north and east must have 
grounded, bearing material from the Laurentian hills, a distance of at least 
four hundred miles; while at the same time from the north-west or, 
perhaps, even from the south-west, came lighter ice bearing debris from 
the glaciers and coast line of the Rocky Mountains, and moving probably 
under the influence of superficial currents, or prevailing winds. The 
material from the latter source, must either have been originally shore 
gravel, or have lain for a long time subject to the action of the waves 
on the shallow surface of the plateau, and been there impressed with the 
character of shingle—a character not assumed by the debris of the 
Coteau. It is worthy of remark in this connection that while fragments 
of siliceous rocks from the Rocky Mountains, which are mingled with the 
Laurentian matter of the lower levels, frequently retain marks of severe 
glaciation, those of the summit of the plateau and higher levels, rarely 
show traces of it. South-westward of the Boundary-line, the Tertiary 
plateau ‘being in most places absent, and the general surface of the 
country lower, it would seem that the Coteau region is broader and 
more scattered, and that much of the northern ice may have passed 
over it southward, There is nothing to show that the siliceous drift is 
in any sense confined to the higher levels. It appears most abundantly 
there, and decreases eastward, but in many western localities where the 
northern drift preponderates, as great a quantity of the western material 
may exist asin the thin covering of the plateau ; but it loses its pro- 
minence, from its insignificance relatively to the north-eastern material. 
The total quantity of drift, area for area, on the third as compared with 
the second steppe, is probably not over one twentieth. It would 
appear from their mingling, that the two deposits if not exactly 
contemporaneous, are very nearly so. 
