The slope of the land is here toward the north， and as the ice retreated it formed a barrier to the drainage and dammed back a great sheet of water in front of it. When the ice melted， the lake was drained， leaving the flat fertile plain through which the Red River now flows. The ancient glacial lake has received the name of Lake Agassiz in honor of the great scientist who did so much toward the explanation of glacial phenomena. Glacial lake plains of this kind are found not infrequently. They now form fertile areas of great agricultural value.
172.Waterfalls Due to Glaciation. -As the ice spread over thecountry it filled the river valleys in many places with débris. When the ice melted away， some rivers could no longer find their old courses and were forced to seek new ones. It frequently happened that in deepen-ing these new channels the r i v e r c a m e u p o n b u r i e d ledges， and in wearing these down， rapids and falls were developed. In this way many of the water powers of New England and the northernNIAGARA FALLS.
Due to rearrangement of the drainage by the ice of the Glacial Period.
states were produced.
The Merrimac furnishes a
fine example of water power due to glaciation. The great manufacturing cities of Lowell， Lawrence and Haverhill would not exist had not the river been displaced from its previous channel by the glacial ice， and in developing its new valley come upon ledges which it is now trying to reduce to grade. The Niagara is another notable example of vast water power due to the displacement of drainage by the ice. It is probable that in pre-glacial time there was a river which carried off the drainage of the area now drained by the Niagara， but it did not flow where the Niagara now flows.
173.Glacial Period. -Evidences of an ancient ice covering areseen in North America， even as far south as the Ohio River and ex- tending over a vast region which now enjoys a temperate climate.
AREA COVERED BY THE ICE OF THE GLACIAL PERIOD.
The greatest ice invasion during this period extended from northern Canada across New England into the sea， across the basins of the Great Lakes and the upper Mississippi valley and across a part of the Mis- souri valley. It wrapped in its icy mantle almost the entire region be- tween the Ohio and Missouri rivers and the Atlantic Ocean.
Another great ice invasion spread out from the highlands of Scandinavia. As in later days the Norsemen， so at that time the glacial ice overspread northern Europe， carrying Scandinavian bowlders across the Baltic and what is now the basin of the North Sea， forerunners of the Scandinavian sword which in later ages carried devastation to these regions.
The thickness of the ice over these central areas was very great， probably approaching a mile. The pressure on the ground below must have been tremendous and the scouring and erosive effect vast indeed. The soil which previously covered the surface was swept away and borne toward the ice margin， leaving the rocks smoothed and bare.
Prehistoric man probably saw the great ice mantle； he may even have been driven from his hunting grounds by its slow encroachment. His rude stone implements are found mingled with the glacial gravels. But like the spreading ice he has left no record from which the time or cause of the Glacial Period can be determined.
174.Effect of the Glacial Period upon Plants and Animals. -Allplants and animals were forced either to migrate before the slowly advancing ice or to suffer extermination. Individual plants， of course， could not move， but as the ice spread toward the south with extreme slowness and with many halts， the plants of colder latitudes found con- ditions suitable for their growth ever opening toward the south. They were thus induced to spread in that direction， so that at the time of the greatest extension of the ice the plants suitable to a cold climate had penetrated far to the south of their former habitat.
As the ice receded， these cold-loving plants were forced to follow its retreat or to climb the mountains in order to obtain the climate they needed. They did both， so that in areas covered by the ice， plants similar to those of far northern regions are found on the tops of the mountains in middle latitudes. What was true of the plants was true also of the animals. Thus the conditions at the time of the Glacial Period explain some of the most difficult problems in Botany and Zo？logy.
175.Man and the Glacial Period. -Although the Glacial Periodoccurred thousands of years ago， probably before man was widely spread over the earth"s surface， yet its influence upon him has been most marked. His manufacturing depends largely for its power upon the falls and rapids due to the rearrangement which the glaciers made in the drainage. Some of the most fertile soil of middle latitudes is dueELECTRIC PLANT AT NIAGARA.
Man"s use of the power which the glaciers arranged for him.
to the pulverized rock left un- exhausted by plant life as the glacier retreated. Since the soil was largely brought from the inhospitable northern re- gions where man cannot eas- ily exist， it has increased the extent of arable land suitable for his cultivation.
By the mingling of unweathered yet valuable soil-producing rocks over the surface， the permanence of the soil"s fertility has been increased， although the difficulty of tillage is greater. The surface has been beautified by innumerable lakes which furnish man excellent water supplies and restrain the rivers from excessive floods. Glacial lake beds of great productiveness have been formed for his cultivation. Hardy plants from the north have been brought to cover the mountain sides in middle latitudes. In fact， man"s whole condition in these latitudes has been modified by the ancient ice invasion.