REVIEWS 385 
Michigan, on the west and southwest generally by the Valparaiso 
moraine which loops around the southern end of Lake Michigan, 
through northern Indiana into Michigan. The plain topography is 
varied by three prominent “islands:” Stony Island, a drift-covered, 
dome-shaped hill of Niagara limestone with quaquaversal dip; Blue 
Island, a single morainic ridge about six miles long and fifty feet above 
its surroundings; and Mt. Forest Island, a portion of the Valparaiso 
moraine about 120 feet higher than the plain, separated from the rest of 
this moraine by the Chicago outlet. The plain is continued through the 
Valparaiso moraine southwest of Chicago by the Chicago outlet, which 
is divided by Mt. Forest Island into the Sag outlet and the Des Plaines 
outlet. The Des Plaines outlet is now followed by the Chicago Drain- 
age Canal. Several less conspicuous gravel and sand beach ridges 
converging toward the Chicago outlet from the northeast and south- 
east help to break the monotony of the plain. On these ridges are 
oak groves, which have apparently suggested the names for the towns 
Oak Park, Oak Lawn, Englewood and others. The eastern third of 
the plain is largely made of gravel and sand. With the exception of 
the beach ridges, the western two thirds is largely of till. These deposits 
vary in depth from o to 130 feet. The country rock is Niagara lime- 
stone which has an elevation varying from 124 feet below the lake level 
to about 20 feet above it in Stony Island, and roo to rro feet above it 
under the Valparaiso moraine. The southeastern edge of the plain is 
occupied by a series of small lakes, the basins of which are in large 
part made by enclosing beach ridges. At the south end of Lake 
Michigan there are sand dunes with a maximum height of 100 to 200 
feet. Other smaller dune areas exist nearer the city. 
The main recorded events in the geographical history of the region 
since Devonian times are: (1) Withdrawal of the sea and destruction 
of formations younger than the Niagara with the exception of some 
fossiliferous Devonian material preserved in joints of the Niagara 
formation; (2) invasion of the ice in the glacial period, rounding off 
the angularities of the rock surface and probably diminishing the relief 
of the region by deposits of drift. At a late stage of the ice invasion the 
Valparaiso moraine was made. (3) As the ice edge retreated from the 
Valparaiso moraine a lake accumulated in the depression between the 
ice front and the moraine until the water stood at an elevation sixty feet 
above the present Lake Michigan when it overflowed to the west through 
the valley of the Des Plaines River and through the Sag outlet. To 
