One of the surprising
things of the "Ice Age" is that about 10,000 to 12,000
years ago, a large embayment of the Atlantic Ocean extended
up the St. Lawrence Valley into southern Quebec and eastern
Ontario. The seashore extended across from the Lake Ontario
basin near Kingston northeastward near Carleton Place to the
Ottawa Valley as far as Pembroke. All of Ontario east of that
shoreline, including Ottawa, and much of the St. Lawrence Valley
of Quebec was flooded by sea water to depths as great as 100m
Figure 1 (left): Lake Champlain including areas
where vertebrate remains have been found (white dots).
It has long been debated as to whether the sea extended into
the Lake Ontario basin after glacial Lake Iroquois drained down
to a level far below present Lake Ontario level. The latest
information suggests Ontario basin water got as low as sea level
but the narrow connecting strait at Kingston and runoff from
the land kept the water fresh in the Ontario basin. These conjoined
waters only lasted a few centuries before uplift of the Kingston
sill raised Lake Ontario above sea level.
The great weight of glacial ice that covered Canada pressed
the Earth's crust downward so that part of it was below sea
level. As the ice sheet gradually thinned and melted away, the
St. Lawrence and Ottawa valleys became open to the sea and residual
glacial lakes mixed with sea water and became salty. The slower
process of isostatic rebound as the ice disappeared raised the
area over a 2000-year period and drained off the sea. As a result,
the seashore was slowly moved eastward to where it is today.
At Covey Hill, south of Montreal, these sea beaches are well
displayed down the slope.
We know the waters of the Champlain Sea were marine (salt
water) because the fossil shells in the beaches and deep-water
clays are species found in the ocean today. We know the water
was very cold, as you would expect from the retreating glacier,
because many of the species are found today in the arctic. Analysis
of the porewater in the clay also shows that the water in which
the sediments were deposited was salty; the low permeability
of the clay has preserved some of the salty water in the clay.
2a (left). L. Hiatella arctica, Portneuf.
Que. C. Portlandia arctica, Ste. Genevieve,
Que.* R. Macoma balthica, Ste. Genevieve, Que. *Originally
Leda glacialis, then Yoldia arctica
The kinds of fossils found in the sediments of the Champlain
Sea include molluscs (snails and clams: Fig. 2, most common
and obvious), barnacles, urchins, sponges, foraminifera, ostracodes,
and vertebrates (sorry, no corals-too cold). Many species of
molluscs have been identified from the Champlain Sea, although
only a dozen or so are common. Barnacles are common, fastened
onto rocks, boulders, or molluscs. At some places fossil molluscs
are so abundant hundreds or thousands can be collected in a
2b (right). L. Mya truncata, Ste Genevieve,
Que. R. Neptunea despecta tornata, Ste Genevieve, Que.
The vertebrate fossils of the Champlain Sea include several
species of whales and seals. Their bones are usually found in
gravel pits in beach deposits along the old sea shore, suggesting
they beached themselves as they sometimes do today, or were
carried by waves onto shore after death. Figure 1 shows where
Champlain Sea vertebrate remains have been found. Near Ottawa,
carbonate concretions in the clay contain fossils (fish, molluscs,
even a bird feather!). These have been dated as 10,000 years
old, i.e. from near the end of the Champlain Sea.
Figure 3 (below). Fish in a carbonate concretion
from the Ottawa area.
The clay of the Champlain Sea (referred to by engineers as
Leda clay from a characteristic fossil) is "sensitive"
so that when disturbed by construction, earthquakes, or stream
erosion it liquefies and causes spectacular earthflow landslides.
Areas along the South Nation River in eastern Ontario; and many
others in Quebec, show the ribbed or chaotic landslide topography
that results, particularly evident on air photos, sometimes
with loss of life and often much property damage.
Figure 5 (below). At about 1815 h December
11, 1963, a large landslide took place at Saint-Joachim-de-Tourelle,
Quebec. The earthflow involved sensitive marine clays (otherwise
known as Leda Clay) which flow like a liquid when in the remolded
Marine invasions in other parts of Canada
Other former seas of similar origin have also been named, such
as the Goldthwait Sea east of Quebec City, and the Laflamme
Sea along the Saguenay Valley. The area flooded by the sea inland
from Hudson Bay is called the Tyrrell Sea. Over 100,000 years
ago the Bell Sea occupied the same area at the close of the
second last glaciation - geologists know well that history repeats
itself. Other areas submerged by the sea because of crustal
downwarp from glaciation include southwest British Columbia
(Victoria, Vancouver) and large areas in the arctic islands
of northern Canada.
Figure 4 (below) Earthflow
For further reference: Gadd, N.R. (ed.),
1988. The Late Quaternary development of the Champlain Sea basin.
Geological Association of Canada Special Paper 35, 312p.
Paul F. Karrow PFK16