The Labrador Boundary


Privy Council Documents


Volume V
Contents




      Ancient river         channels.

      New channels.

      Extent and         movement of the         ice.

      Probable great         thickness of ice.


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II. GEOLOGY.



No. 1048.

EXTRACTS FROM “ REPORT ON EXPLORATIONS IN THE LABRADOR PENINUSLA.”

BY A. P. LOW, OF THE GEOLOGICAL SURVEY OF CANADA, Vide No. 1047.

FORMATION OF FIORDS.

In his description of the Atlantic coast of the Peninsula (quoted above), Dr. Low states that a fringe of small rocky islands extends almost continuously along the coast, with a breadth of from five to twenty-five miles; that outside the islands, submerged banks extend seaward for an average distance of about fifteen miles, and on them the water is rarely over forty fathoms deep. From this it will be seen, he observes, that the fiords as a rule, have greater depths than the banks outside the island fringe. He then observes as follows :—
“ To account for such an apparent anomaly, it is necessary to consider the formation of both the fiords and banks. The fiords appear to be valleys of denudation of very ancient origin, eroded, at least in part, when the elevation of the peninsula was considerably greater (at least 600 feet) than at present. Their remote antiquity is established by the deposition in their lower levels of undisturbed sandstones of Cambrian age. The banks are likely of comparatively recent formation, and appear to be made from material carried off the higher lands by glaciers and deposited by them as a terminal moraine among and outside the fringe of islands, to be subsequently flattened out by floating ice and currents, thus filling up the deep channels at the mouths of the fiords. ”

ANCIENT RIVER CHANNELS.

The channels of most of the rivers of Labrador are of very ancient origin, apparently dating back to a period before the deposition of the Cambrian

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rocks. These valleys are cut deep into the general level of the plateau, their depth and length apparently depending on the volume of water carried, and thus showing that they have been mainly formed by normal denudation.
The larger rivers flowing southward, have deep valleys cut through the highlands of the coast region, and the streams are often from 500 feet to 1,000 feet below the general level of the surrounding country. The heads of these valleys are from one hundred to three hundred miles from their mouths; and at their 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 extending and deepening the valleys. This erosion is, however, so exceedingly slow, that the change in the heads of the valleys, since glacial times, has been practically nothing, owing no doubt to the hardness and resistance to weathering of the Archæan rocks in which they are cut. The gorge of the Saguenay, With its almost vertical walls rising 1,500 feet above the surface of the water, and its great depth of more than 800 feet in places, is an excellent example of one of these ancient river-valleys. That of the Hamilton River, which is cut back from the head of Hamilton Inlet for nearly three hundred miles, and of which the depth is from 700 feet to 1,200 feet below the general level of the surrounding country, is another fine example of river erosion. The rivers occupying smaller valleys, are all of the same type. The East Main and Rupert rivers, flowing as they do on the gradual slope towards James Bay, where the marine deposits of sand and clay are found inland about one hundred miles, have not the marked valleys found elsewhere, but descend in a number of steps, where they have either cut narrow gorges out of soft Huronian schists, or fall directly over granitic ledges. The ancient valleys of these streams appear to have been filled up during the deposition of these marine beds, and the present river-courses are of post-glacial origin.
Before entering the ancient valleys above described, all the rivers in central Labrador flow almost on the surface of the country, and are broken into chains of lakes often formed by dams of glacial drift, which in other places form low ridges that divide the streams into different channels. These channels wander about on the lower levels of the interior country in a most bewildering manner, and render travel without a guide excessively difficult.


GLACIAL GEOLOGY.

The observations of striæ and other glacial phenomena taken along the different routes followed during these explorations, in conjunction with similar evidence previously obtained on the rivers flowing westward into Hudson Bay, all show that the Labrador Peninsula, with the exception of a narrow strip of highlands along the North Atlantic Coast, was completely covered with ice during a portion at least of the glacial period. The movement of the ice followed the general slope of the country outward in all directions from a central gathering-ground, or nevé, and the thickness of the

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ice was such that in its flow it passed over ridges and valleys unchanged, or with only minor deflections.
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The strong glaciation of the highest hills in the interior, on the edges of the nevé region, the constant directions of the striæ over hill and valley, and the fact that the general slope of the plateau from the interior outwards is very slight and does not exceed two or three feet per mile until within a few miles of the coast, all point to a considerable thickness of ice in the interior such as to cause the strong, radial flow of the ice evidenced by the glaciation of the region.

[1927lab]

 

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