HOW THE GREAT LAKES WERE BUILT. 159 



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 sound- 

 ings 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 



Fig. 3. — Section across Lake Ontario feom 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 bor- 

 ings have often penetrated great depths of loose rock and dirt 



