On the preglacial hillsides the humus layer must have been comparatively thin, while the broad glacial plains have gathered deep black soils, rich in carbon and nitrogen taken from the atmosphere. To these soils and subsoils a large part of the wealth and prosperity of the glaciated regions of our country must be attributed. The ice invasions have also added very largely to the water power of the country. The rivers which in preglacial times were flowing over graded courses for the most part, were pushed from their old valleys and set to flow on higher levels, where they have developed waterfalls and rapids. This power will probably be fully utilized long before the coal beds of the country are exhausted, and will become one of the chief sources of the national wealth. =The Recent epoch.= The deposits laid since glacial times graduate into those now forming along the ocean shores, on lake beds, and in river valleys. Slow and comparatively slight changes, such as the warpings of the region of the Great Lakes, have brought about the geographical conditions of the present. The physical history of the Recent epoch needs here no special mention. The Life of the Quaternary During the entire Quaternary, invertebrates and plants suffered little change in species,--so slowly are these ancient and comparatively simple organisms modified. The Mammalia, on the other hand, have changed much since the beginning of Quaternary time: the various species of the present have been evolved, and some lines have become extinct. These highly organized vertebrates are evidently less stable than are lower types of animals, and respond more rapidly to changes in the environment. =Pleistocene mammals.= In the Pleistocene the Mammalia reached their culmination both in size and in variety of forms, and were superior in both these respects to the mammals of to-day. In Pleistocene times in North America there were several species of bison,--one whose widespreading horns were ten feet from tip to tip,--a gigantic moose elk, a giant rodent (Castoroides) five feet long, several species of musk oxen, several species of horses,--more akin, however, to zebras than to the modern horse,--a huge lion, several saber-tooth tigers, immense edentates of several genera, and largest of all the mastodon and mammoth. [Illustration: Fig. 369. Megatherium] [Illustration: Fig. 370. Glyptodon] The largest of the edentates was the Megatherium, a. clumsy ground sloth bigger than a rhinoceros. The bones of the Megatherium are extraordinarily massive,--the thigh bone being thrice as thick as that of an elephant,--and the animal seems to have been well able to get its living by overthrowing trees and stripping off their leaves. The Glyptodon was a mailed edentate, eight feet long, resembling the little armadillo. These edentates survived from Tertiary times, and in the warmer stages of the Pleistocene ranged north as far as Ohio and Oregon.