Ancient Glacial Lake Aggasiz
 

(Courtesy of)
Manitoba History, Number 19, Spring 1990

    In 1823 W. H. Keating, the geologist with the first scientific expedition to the Red River Valley wrote: “In some places pebbles were as abundant as if we had been travelling upon the bed of some former river or lake; the mind endeavours in vain to establish limits to the vast expanse of water which certainly at some former day overflowed the whole of that country.” Later explorers and geologists such as David Owen (1848) confirmed Keating’s view: “Pembina Mountain is, in fact, no mountain at all, nor yet a hill. It is a terrace of table-land—the ancient shore of a great body of water, that once filled the whole of the Red River Valley.” So striking were the remnants of this lake that in later years they drew comment from such noted explorers as John Palliser and Henry Hind.  Striking too, was the energy displayed by those who mapped the area. Canada’s first direct involvement in this endeavour came in 1888 when J. B. Tyrrel of the Geological Survey of Canada, though suffering from typhoid fever, was dispatched to chart the waterways of Manitoba. His field notes for that expedition are sprinkled with such comments as “had a touch of fever this morning so only went 20 miles.”

    By the close of the nineteenth century it was generally accepted in scientific circles that the lake had formerly existed. The most common explanation for its origin was that it had formed from the meltwaters of a great glacier which had once overridden much of northern Canada. Although he had never visited the area and had not been directly involved in the research, the lake was named in honour of naturalist Louis Agassiz—the most vocal advocate of glacial theory.

    At the height of the last Ice Age some 20,000 years ago, all of what is now Manitoba lay beneath a sheet of ice which in places was as much as four kilometers thick. Calculations indicate that it covered over 13,000,000 square kilometers and was composed of 25,000,000 cubic kilometers of ice. The massive weight of this ice sheet was sufficient to compress the earth’s crust by as much as one hundred meters. So much of the earth’s water was “locked-up” in the continental glaciers that ocean levels dropped dramatically, exposing coastal shelves which had previously been submerged. The shallow sea floor between Alaska and Siberia was also exposed, thus forming a 1500-kilometer-wide land bridge between the Old and New Worlds. It is believed that it was by this route that people first entered the Americas.

    More is known about the last and most recent glacial advance than earlier ones because each partially “erased” the evidence of its predecessors. From its centre in Hudson Bay, the ice sheet advanced southwards, carrying soil and stones within it and planing the landscape ahead of it. This debris was deposited in long ridges called “terminal moraines” which mark the point of maximum advance. Unlike the glacial advance, the “retreat” was not characterized by a physical movement of the ice. Rather, with an improvement in climate, its southern edge began melting at a rate faster than “new” ice was being forced towards it. Nor was deglaciation a continuous event. Manitoba and the neighbouring provinces and states emerged from beneath the ice sheet in a series of advances and retreats.