Page:Popular Science Monthly Volume 9.djvu/569

Rh It may be urged that these beds of clay, with their striated stones and huge bowlders, are found over a large section of our country, and are not confined to the region of the lakes. This is very true; and from it we conclude that where now so many happy homes are scattered, from Maine to the far West, the snows and frozen mists of a great winter once accumulated to many thousand feet in thickness, and formed a great glacier, like that which covers the interior of Greenland at the present day, which flowed southward, grinding down the country and acting as a ploughshare to prepare the land for a new harvest. Gradually this great winter began to pass away, and the spring-time in which we now live, to draw near. As the great glacier retreated northward, it left the country covered with beds of bowlder-clay and strewed with huge erratics from northern regions, which together with other débris form the surface material of all our northern country, where it has not since been swept away or covered by other and more recent deposits. It is often well exposed along our lines of railroads, and may be known at a glance by the great number of worn and rounded stones of all sizes that are scattered promiscuously through it. These evidences of glacial action are found as far southward as Cincinnati and the central portion of New Jersey, showing that here was the border of the icy mantle that was spread over all the northern regions. After this great continental glacier passed away, or had retreated far northward, smaller and detached streams of ice still flowed southward to complete the task of moulding the valleys and lake-basins. It is to these smaller glaciers that we attribute the formation of the multitude of lakes filling rock-basins that are scattered through the northern part of the United States and over the whole of the British possessions, many of which have been hollowed out in nearly horizontal beds of rock in the same manner as lake-basins are now forming under existing glaciers. Nor are the lakes which fill glacier-worn rock-basins confined to our own continent, but they form the most common and grandest lakes of temperate latitudes, which might be called the lake latitudes, so completely are the lakes of the world confined to these regions.

The theory of the glacial origin of certain lakes was first proposed by the distinguished English geologist, Prof. Ramsay, and, after being tested in nearly every glaciated region in the world, is now held, by those best qualified to understand it, as the simple and true history of the formation of many of our lakes.

2. The lakes of our second class, those which are confined by banks of gravel, bowlders, etc., owe their origin, like the ones we have been considering, to the action of ice. Lakes of this class are most commonly found in the deep Alpine valleys of mountainous regions, where the material which accumulated on the surface of the glaciers that once flowed through them, in the form of lateral and medial moraines, was carried down and deposited at the extremity of the