NO. 23.] CENTRAL CONNECTICUT IN THE GEOLOGIC PAST. 23 



unbroken desert of ice mantling the northern half of the continent, 

 similar to the present ice caps of Greenland and Antarctica, and 

 attaining a thickness of at least two miles in its central portions ; 

 a desert whose icy floor was in slow but perpetual motion toward 

 its margin, while its surface snows, like the dusts of tropical 

 deserts, were hurtled outward more rapidly than the solid ice 

 below by the freezing winds which at short intervals blew from 

 the interior. The ice removed the original soil and ground off a 

 certain amount of rock, but did not remodel the landscape; it 

 left the landscape in all its larger features essentially as it found 

 it, a surface shaped by subaerial decay and running water. Upon 

 the final retreat, however, a disordered mantle of glacial waste 

 was left upon the rock floor. The hollows were marked by lakes 

 and swamps ; the river valleys were choked with sand and gravel 

 deposited by the streams flowing from the receding glacial mar- 

 gin. Such features cannot be expressed upon the section but 

 constitute the evidence from which the appearance of the ice cap 

 is restored. 



The Close of the Tertiary Period, Figure 4. During the 

 Tertiary period several movements of regional uplift of the 

 Appalachian province took place, and at each halt the rivers 

 carved down to near the new and, with respect to the rocks, the 

 lower level of the sea ; their tributaries sapped the hills and a new 

 cycle of erosion with respect to a new base-level became initiated. 

 In the latest Tertiary the land again stood still for a considerable 

 time, and the peneplain of the Central Lowland became developed 

 at sea level. The time, however, was too short for the harder 

 rocks of the Highlands to suffer much destruction, and the differ- 

 ence in level between the general highland surface and the lowland 

 measures the amount of the several Tertiary movements. At 

 about the close of the Tertiary the temporary crustal quiescence 

 was destroyed. A marked uplift of the lands, especially in higher 

 latitudes, preceded the gathering of the ice sheets and character- 

 ized the earlier portion of the Glacial period. It occurred in 

 several stages and was marked by oscillatory reversals, but the 

 aggregate effect was to initiate a new cycle of erosion, during 

 which Chesapeake and Delaware Bays and Long Island Sound 

 were carved as river valleys in the soft deposits of the Coastal 

 Plain, and gorges and narrow valleys were cut by the larger 



