REVIEWS 125 



this region, at the beginning of the Champlain epoch, when the gla- 

 ciated area of this continent sank from its previously high elevation to 

 be mostly 300 to 500 feet lower than now. By this depression a tem- 

 perate climate was restored on the border of the continental ice-sheet, 

 which became greatly reduced by its surface melting, so that much of 

 the drift before contained within the ice was at last exposed on the 

 thinned ice-fields, as now on the Malaspina ice-sheet in Alaska. 



The ice-melting and rains probably swelled these great rivers to 

 twice or three times their present average annual volume ; and their 

 supply of silt, brought in abundance by the rills, brooks, and rivers 

 that flowed down from the waning ice-sheet, was very probably fivefold 

 to tenfold more than now. Under these conditions of very abundant 

 silt, rivers swollen to floods throughout the summers, and less current 

 of their sluggish descent to the Gulf, it is estimated that the lowan 

 stage of chief deposition of the valley loess, gradually building up the 

 river flood plains to heights of 150 to 250 feet above the bottom lands 

 of today, may have occupied only about a thousand years. 



During the same time the winds are thought to have blown away 

 much of the loess from the valley flood plains, and from the ice sur- 

 face, spreading it far and wide as the general sheet of upland loess, 

 mostly 10 to 25 feet thick, mantling the high and low lands upon the 

 great areas between the rivers with a surprising uniformity of thickness. 

 It is evident, also, that this silt mantle includes some contribution, 

 most considerable westward, of wind-borne dust from the great west- 

 ern plains, this part not being of glacial origin. 



After the accumulation of the loess, and before the moraine-form- 

 ing Wisconsin stage of the waning and wavering glaciation, this region 

 was uplifted 300 to 500 feet, or perhaps somewhat more, on account 

 of the diminution of the ice weight and pressure, thereby giving to 

 the rivers the same steeper gradients and more powerful currents as 

 now. They therefore eroded the valley loess to depths somewhat 

 below the present bottom lands, and sculptured the valleys in nearly 

 their present forms, with high inclosing bluffs of loess, before the 

 moraines of Wisconsin, Minnesota, and northern Iowa, were amassed 

 along the ice boundary at pauses of its general retreat. 



Again, during this Wisconsin stage much modified drift was borne 

 into the valleys. Its coarser portion of gravel and sand filled the val- 

 leys anew to heights of 100 to 200 feet, or more, near the ice border ; 

 but the strong river currents, with nearly their present slopes, carried 

 the fine silt, corresponding to the former loess deposit, far down the 

 valleys to the lower Mississippi and the Gulf. 



