Page:Popular Science Monthly Volume 49.djvu/173

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159
HOW THE GREAT LAKES WERE BUILT.

the table lands to a depth, of five hundred feet below sea level, as is shown in the Ontario basin at this day, and the upper lakes to nearly as great a depth (Lake Erie alone being shallow, but with deep buried channels running through it), but high enough to allow for the necessary slope down the St. Lawrence Valley, not merely to the present gulf, but to the edge of the continent, some eight hundred miles from the present outlet of Lake Ontario. In short, the lake region was elevated more than twelve hundred feet higher than now, which amount itself is indicated by the soundings of the Gulf of St. Lawrence.

While the lake district was thus moderately elevated for long ages, there was an extraordinary altitude of the continent lasting for a comparatively short time, as is seen in the drowned valleys near the coastal margin of the continent; but this elevation did not last long enough for the great canons to be cut back to Lake Ontario.

The lake basins are simply fragments of the old valleys of the St. Lawrence River and its tributaries. These normal but ancient depressions have since been obstructed so as not to allow a free drainage and are thus turned into lake basins with the district further depressed, partially below sea level. The manner in which these things were accomplished is now our theme.

Drowned and Buried Valleys. — Fragments of the ancient valleys which existed in the lake region are discovered by the

PSM V49 D173 Section of lake ontario showing a submerged valley.jpg

Fig. 3. — Section across Lake Ontario from Point Peter to Pultneyville, showing the submerged valley with the bounding escarpment.

soundings in the lakes. Throughout or across some of them great, broad channels, resembling old land valleys, such as are seen in every country, extend, and are bounded on one side or another by the steep slopes of some drowned mountain or escarpment, three hundred or four hundred feet high. Such a valley occurs in the Ontario basin, of which Fig. 3 is a cross-section. An equally good example may be seen in Lake Huron and other lakes. But at the surface these drowned valleys do not appear connected. What do they mean? We shall see.

In the lake district wells have been sunk for considerable depths for water, oil, and gas. On the now level plains the borings have often penetrated great depths of loose rock and dirt