Since the departure of the ice, or during post glacial time, many of the smaller or more shal low lakes have been filled by sediment, or by deposits of vegetable origin, sometimes forming beds of peat. In the larger lakes deltas and marshy areas have been formed in the same man ner. The streams have resumed their flow along the lowest line of levels that they could find, and in so doing have often cut throng]] the veneer of drift and worn deeply into the underlying rock. This is the origin of most of the gorges of the Northern States. They are young, post-glacial valleys, and the old, buried channels are often to be found not far away. These conditions have not only produced striking scenic resnits,.in the gorges and waterfalls of the Northern States, but have created nearly all the available water power of the same region. Be fore the ice invasion many streams flowed at lower levels, in the deep and mature valleys of the time. But the obstruction of these valleys has compelled the streams to flow at greater altitudes, and the rocky masses and spurs encoun tered in their downcutting have caused a concen tration of descent in rapids or falls, and thus have made the streams a source of power, while at the same time obstructing the otherwise open ways of commerce.
The soils of the country may roughly be di vided into two classes by virtue of their origin. Outside of the glacial region they are known as residual soils, being in the main the product of the local rock disintegrating in place. In this case the more soluble minerals, such as calcium carbonate, have largely been removed. The chief movements of such soils have taken place along the course of rivers and by means of them. The soils of the Lower Mississippi are an illustration of this latter phase, having been gathered from all parts of its basin. The remaining soils are glacial. The ice sheet invaded the residual soils of pre-glacial time, plowed them up. pushed them forward greater or less distances, and thoroughly mingled them with materials often coarse, and mechanically derived from the rocks lying in the track of the glacier. These soils, therefore, par take of the variety of the numerous rock masses from which they have been derived.