EXTRACTS FROM “ PHYSIOGRAPHY OF THE ARCHEAN AREAS OF CANADA.”
BY PROF. A. W. G. WILSON, MCGILL UNIVERSITY, MONTREAL, CANADA.
REPORT OF THE 8TH INTERNATIONAL GEOGRAPHIC CONGRESS, U.S., 1904. Pages 116-135.
The present paper offers a brief general description of the more salient physiographic features of the Archean areas of Canada. The paper is based upon the reports of the many explorers who have traversed this area in its different parts. The writer's own field work in several parts of the area has served as a basis of interpretation where his method of description departs from that of the authorities from whose work this summary was prepared.
No attempt is made to discuss the imperfectly known geology of the very large area here under consideration. The purpose of the paper is rather to draw attention to the remarkable unity of the physiographic features of the region in its whole extent and to present a brief description of their most marked characteristics.
AREA AND EXTENT.
The Archean areas of Canada, the Canadian Shield of Suess, extend around Hudson Bay in a U form, reaching from Hudson Strait on the north-east, southward through Labrador, Quebec, northern Ontario, and then north-westward through the district of Keewatin and part of the district of Mackenzie to Coronation Gulf, in the extreme north-west.
2. Depressions containing sedimentary deposits.—The occurrence of valleys partly occupied by sediments which have been called Cambrian has been reported by Low in Labrador and by Tyrrell in northern Keewatin and eastern Mackenzie. Outlying areas of Paleozoic sediments are found in the basins occupied in part by Lakes Nipissing, Temiscaming, St. John, and Mistassinni.
Low, in describing certain of these valleys in Labrador, mentions that the streams are often from 500 to 1,000 feet below the level of the plateau. The heads of the valleys are from 100 to 300 miles from their mouths, and at the upper ends the rivers descend from the level of the interior in a succession
of heavy falls through narrow gorges, where processes of erosion are at present slowly extending and deepening the valleys. The valley of the Hamilton River lie describes as from 700 to 1,200 feet below the level of the plateau. Some of these ancient valleys have been more or less filled with glacial débris, and the modern streams for part or all of their lengths have taken new courses.
Some of the depressions in which these sediments lie seem to be ancient broadly open valleys and are considered by those who have had the opportunity of studying them in the field as of pre-Cambrian age.
In the Labrador region the margins of the depressions in which the sediments occur are well defined. This is strikingly true of the lower and partly submerged valley of the Hamilton River.
3. Gorge and canyon valleys.—Narrow, steep-sided valleys and gorges, some many miles in length, some extending only for a short distance, are of frequent occurrence in various parts of the peneplain. Many of them are found in the Labrador area, through which the drainage of the interior upland passes down to Hudson Bay, to the St. Lawrence River, or to the Atlantic Ocean. One of the most interesting of these is the gorge of the Hamilton River, described by Low as occurring above the more open, partly submerged valley of the second type here described, in which certain sediments lie and which forms the lower part of the valley of the same river.
Origin of the basins, valleys, and gorges.
Given time enough there can be no question that the normal processes of river erosion could produce these deep canyons or steep-sided valleys. So far as we know at present, this seems to have been the process by which most of the deep gorges and canyons cut below the level of the Labrador peneplain were excavated. Mr. Low notes, with respect to the canyon of the Hamilton just below the Grand Falls, that the river in its erosion of this gorge has been guided by two series of joint fractures, so that the canyon has a somewhat zigzag course. Mr. Low has also drawn the writer's attention to the fact that there are several instances where an old valley has been blocked by glacial débris, and the streams flowing in the upper portion of the valley are turned aside and have already cut well-defined canyons, in some cases of considerable length, in the crystalline rocks. It is to be noted that the canyon of the Hamilton River enters a larger, broader valley, to which reference has already been made, from the north side. The old valley continues inland for a considerable distance beyond the junction of the present Hamilton River via the canyon, with the lower part of the stream in the older valley. Mr. Low regards this canyon as of post-Glacial origin and as due to the erosion by the large stream which now rushes through it.