No. 23.] CENTRAL CONNECTICUT IN THE GEOLOGIC PAST. 39 



Mammals. In the west the present ranges of the Rocky Moun- 

 tains become outlined, but in the Appalachian region a broad warp- 

 ing independent of structure raises the old peneplain into a 

 plateau above which rise the few remaining mountains. The 

 movement of uplift is intermittent, and at several times during 

 the Tertiary the sea returns over southern Connecticut and prob- 

 ably reaches as far as the line of our geologic section. Follow- 

 ing each retreat of the sea, the rivers flow in southeasterly courses 

 across the bared sea floors. From these channels gained in the 

 last emergence the Connecticut and Housatonic have never been 

 deflected. At each halt in the oscillatory uplift of the land the 

 rivers establish a new base-level of ,erosion and begin to widen 

 out the valleys in the softer rocks. At last a halt in the latest 

 Tertiary permits the soft rocks of the Central Lowland to be 

 widely eroded to near the level of the sea, while in the same 

 period of time narrow valleys only are cut in the harder rocks of 

 the Highlands. 



Now begin those broad oscillations of the continents connected 

 with the crustal and climatic revolution which closes the Tertiary 

 and marks the beginning of the Quaternary period. The move- 

 ment is dominantly one of uplift, and the rivers cut their valleys 

 deeper in obedience to the law that they shall seek the level of the 

 sea, but even in the softer rocks the new work of erosion is 

 hardly begun when it is interrupted by the coming of the Glacial 

 catastrophe. The winter snowfall begins to exceed the summer 

 melting. Slowly gathering ice fields form, deepen, and creep 

 toward the south, driving all life before the advance of the frozen 

 desert. Warmer intervals come, marked by the retreat of the 

 ice, but the glacier each time recovers its lost ground and ad- 

 vances farther into more temperate latitudes until it reaches to 

 Long Island, and the Allegheny, Ohio, and Missouri Rivers. 

 The northern half of the continent is given over to a reign of 

 ice. The ice margin advances and recedes, and upon each retreat 

 leaves behind it belts of moraines, soil mantles of stony till, rock 

 ledges polished and scored. The elephant and mastodon and 

 others of the race of mammals warm blooded, clothed with 

 hair, and adaptable to changing conditions follow quickly 

 northward each recession of the ice. 



