Page 52 - Annuaire Statistique Québec - 1918

GEOLOGY

is broken by severa] hills of igneous rocks, which rise abruptly from the
level plain, and constitute stl'iking features of the landscape.

Glacial period and.Modern deposils.
At the close of the Devonian period, practically the whole of the
Province of Quebec had emerged above the level of the ocean, and
remained thus uplifted and uncovered by water during the succeeding
geological ages. Therefore J not only di.d no sedimentation take place
over the al'ea which now con;-;titutes our Province, but the whole surface,
on the contrary, was subjected to continuous erosion and denudation,
both atmospheric and fiu viatile, from the Devonian to the close of the
Glacial period. Hence the absence of the Carbo~iferous and Permian,
as weIl as of aIl Mesozoic and Tertiary rocks.

During the glacial period, the whole of the Province of Quebec was
buried under 3. sheet of ice, whieh extended to the south into the United-
States. This immense glacier, 0,1' ice-sheet, which had a continuous,
slow, fiowing motion, seems to have had its gathering ground, or centre
of radiation, in the central part of the Labrador Peninsula, and for this
reason is usually caUed the Labradorean Ice-Sheet.

The striae, groove:,; and ice-transported material indicate that the
ice moved out\vard in aIl directions from thi;;; centre, carrying off the soil,
the "\veathered and decomposed rock matcriaJ, and ,vearing down,
scouring and poJishing the exposed surfaces of the rocks. It is to the
action of this ice-sheet J that our Laurentian HiUs, as weil as most
elevations in the southern part of the Province, owe their rounded
outlines, and their uniformity of Jevel on the sky-line.
At the close of the glacial epoch, there was a subsidence which
l' sulted in an invasion of the sea along the valleys of the St. Lawrence and
the Ottawa. Moreover, the retreating ice-front of the glacier impounded
or dammed the waters, in an unbroken 11ne to the north, and this gave
rise to the formation of large sheets of water, until the ice-sheet had
sufficiently retreated to allo",: drainage by the way of Hudson Bay.
During this period of submergence, heavy mantles of marine clays and
sands '\'ere deposited J which, in the Province of Quebec, attain their
greatest development in the Lowlands region, south-east of the St.
Lawrence River. The clay, which is heavy and usually blue in colour,
is caUed the Leda clay, from a very prevalent marine shell found in it.
Large deposits are noticed along the shores of the St. Lawrence River,
as well as in Montreal city itself. The sand, large banks of which are
seen along the north shore of the River, is the Saxicava sand. This
period, which immediately followed the Glacial epoch, is usually
referred tO in Eastern Canada, as the Champlain period, and the sea
J
which submerged the southern part of the Province, as the Champlain
Sea.