24 CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [Bull. 



streams farther inland. The structure section shows the gorge 

 of the Connecticut River trenched in the peneplain of the Central 

 Lowland. A subsidence which began in the latter part of the 

 Glacial period has, however, brought the Central Lowland part 

 way back toward its original level. This lower attitude of the 

 lands, as compared to the elevation attained in the latter part of 

 the Tertiary and early part of the Glacial period, has resulted in 

 the development of Long Island Sound and the partial silting up 

 of the channel of the Connecticut River. 



In the Cretaceous Period, Figure 5. During the Jurassic 

 period erosion had progressed over the Appalachian region until, 

 along the region which is now the coastal plain from Connecticut 

 southwestward, wherever the evidence has been preserved, the 

 country had been reduced to a peneplain. Hills several hundred 

 feet in height still diversified its surface, however, and, farther 

 inland, mountain groups doubtless continued to exist.' Then, at 

 the beginning of the Comanche period a new movement was 

 manifested. Upwarping became pronounced along the entire 

 Appalachian system, but the upward movement was restricted 

 to that northwestern belt where the present highest elevations are 

 still found, extending from the White Mountains to the Southern 

 Appalachians. Southeast of the Mountains, on the contrary, a 

 downwarping took place, carrying this belt below base-level, and 

 obscuring the fact that in earlier periods this too was a part of 

 the mountain system. These movements corresponded to a warp- 

 ing or tilting about a horizontal hinge-line or axis which followed 

 in a general way the line of the present great Atlantic seaboard 

 cities, Boston, New York, Philadelphia, Baltimore, Washington, 

 and Richmond. Northwest of this line the uplift increased with 

 distance from the hinge-line. On the southeast, subsidence in- 

 creased with the distance. Erosion was reawakened in the up- 

 lifted region and numerous rivers carried the land waste south- 

 eastward to that region which was sinking. The rivers here 

 could no longer erode but began to build by depositing their 

 sediment. For a time this was more than enough in volume to 

 compensate for the subsidence, so the sea waters were kept back 

 and a delta plain was built out along the entire Atlantic shore. 

 The fresh-water deposits of what is known as the Potomac for- 

 mation were thus laid down in unconformable relation upon the 



