4 
MUSEUM BULLETIN NO. 24. 
altitudes of the highest beach, determined at a few localities, 
and the distribution of the marine sediments show that, in general, 
the northern boundary lay along the face of the Laurentian 
Plateau escarpment, which borders the lowlands on the south 
and trends in a nearly east and west direction, roughly parallel 
to the course of the Ottawa river, and at a distance, generally, 
of only a few miles north of the river. Several long, narrow 
embayments, however, extended far up the valleys of the rivers 
which flow from the north and join the Ottawa. The sea 
extended far up the Ottawa valley, possibly as far as the head of 
Lake Timiskaming. Marine fossils are known to occur in the 
clays as far west as Lake Coulonge, about 70 miles west of Ot- 
tawa. Above this point the Ottawa valley is narrow, so that 
during the time of marine submergence the upper part of the 
valley was a long, narrow estuary. 
The southwestern margin of the sea has not been traced, 
but it is known from the altitudes of the raised beaches and from 
the distribution of the marine sediments that it was bounded, 
approximately, by the eastern border of the Pre-Cambrian 
upland area in south-central Ontario. The shore-line extended 
southward from the vicinity of Renfrew to near the eastern end 
of Lake Ontario. At the highest stage of marine submergence, 
the portion of the triangular area between the Ottawa and St. 
Lawrence rivers, lying east of a line drawn from Ottawa to 
Brockville, was entirely submerged except for a few isolated 
hills. It has long been known that the upper as well as the lower 
portion of the St. Lawrence valley was also submerged and that 
the level of the marine waters extended westward into the 
Ontario basin; for, although marine, fossil shells are known to 
occur in the clays only as far westward as Brockville on the 
St. Lawrence, yet, the clays are continuous westward and the 
marine fossils occur at such high altitudes as to make it certain 
that the marine waters entered the Ontario basin. 1 In the upper 
St. Lawrence valley, however, the submergence was not so great 
as in the Ottawa valley; for the Ottawa valley lies at a lower 
level and was depressed to a greater extent, as is shown by the 
higher altitudes of its marine deposits. 
1 Coleman, A. P., "Marine and freshwater beaches of Ontario,” Bull. Geol. Soc. Am., 
vol. 12, 1901. 
