deposits before reaching the solid strata, yet, perhaps in proximity, the bedded rocks appear near the surface of the country. These depressions are portions of ancient valleys which have been
filled often to depths of five hundred feet, and in some cases probably to a thousand feet. By chains of borings the buried valleys may be traced. Their general course is frequently shown by the surface features; but without the borings their great depth would not be suspected. Thus the Dundas Valley may be taken as an example. It is situated at the head of Lake Ontario and bounded by mountain walls, but is also deeply buried by drift, as shown in Fig. 4. Some of the filled valleys are chiefly occupied with bowlder clay; in other cases with both till and stratified materials; so that their burial is not always alike. Not merely have many of the old valleys been filled with the sweepings of the highlands, but they have been further obscured by the submergence of the district beneath the modern lake waters.
The Course of the Ancient St. Lawrence compared with that of a Modern River.—In Seeking for the explanation of the drowned and buried valleys, discoveries have been made showing that some of them can be connected, and thus is the change in the course of the ancient Laurentian (so named to distinguish the old water way from the modern) River established. The modern St. Lawrence River is characterized by the most remarkable system of lakes in the world. The basins are very deep, and not mere expansions of a river having no noteworthy depth. In the soundings of the lakes and in the buried valleys the connecting links of a great chain of evidence are welded together, showing that the ancient water way did not pursue the present eccentric course, but was an ordinary river valley of large size, yet its course was not everywhere coincident with that of the modern stream.