Chambly Canal National Historic Site of Canada
The geomorphology of this site was greatly modified by the various
ice ages that affected Canada many thousands of years ago. At that time,
the St. Lawrence Lowlands were covered in a thick layer of ice. As the ice
receded, the sagging of the platform rocks allowed the sea to overrun the
region. During this period, the Atlantic extended an arm as far as the St.
Lawrence Lowlands. Geologists call this the "Champlain Sea Episode".
The maximum depth of this sea was 185 to 215 m, roughly the height of the
peak of Mount Saint-Bruno. We can thus be sure that the whole area was under
water during this period. In the depths of this sea, fine particles of clay
clotted and precipitated, covering the till left by previous ice ages and
the underlying rock.
Generally speaking, the area around the Chambly Canal is covered
with fluvial deposits consisting of clay materials reworked on the marine
clay dating from the post-Champlain period. At that time, isostatic rebound
caused the marine sea to recede, to be replaced by estuarial and fluvial
conditions. These clays are described as reworked because they originated
in the Champlain Sea, were resuspended, then redeposited.
The regions relief is very flat because it originates from a fluvial
valley. The only major relief comes from intrusions of igneous rock forming
the Monteregian Hills. Between the mouth of the Chambly Canal at Saint-Jean-sur-Richelieu
and the Chambly basin, there is only a 25-m drop.