238 PHYSICAL GEOLOGY 



the following paragraphs. A knowledge of these things aids 

 in determining the geographic changes of the past. 



The characteristics of shore lines, and the agents which 

 shape them. The shores of the northern continents are 

 characterized by great projections of the land into the sea, 

 and by great extensions of the sea into the land. Large 

 irregularities like Florida, Lower California, the Iberian 

 Peninsula, and Hudson Bay are due to diastrophism. In a 

 late geological period an upbowing of a part of the marginal 

 sea bottom made an island of Florida which, by continued 

 movement, was attached to the mainland as a peninsula. A 

 geologically recent subsidence let the sea in over the area 

 of Hudson Bay. 



The submergence of a coast land having hills and valleys 

 produces a new shore line which is irregular (Fig. 271). 

 The drowned valley bottoms form bays, while the inter- 

 valley ridges stand 

 forth as headlands. 

 Isolated hills of the 

 old lowlands front 

 the new coast as 

 islands. Chesa- 



FIG. 255. Diagram of a young coastal plain, peake Bay, Dela- 

 with the old land in the background. ware Bav and 



many other smaller bays along the eastern coast of the United 

 States are drowned valleys or valley systems. On the other 

 hand, the emergence of a coastal strip tends to produce an 

 even, regular shore line, for the edge of the sea rests against 

 the gently sloping former bottom (Fig. 255). (What should 

 be the general height of new coasts due to (1) submergence, 

 and (2) emergence?) 



In addition to the features formed by diastrophism, many 

 coastal irregularities are due to the work of gradational 

 agents. Under normal conditions rivers erode but little at 

 their mouths, but may build deltas into the sea (p. 185). 

 Glaciers descending into the sea help to develop fiords 



