MARYLAND GEOLOGICAL SURVEY 61 



Patuxent Eiver. It will be remembered in describing the distribution 

 of the terraces in a previous section that the presence of the two lower 

 terraces along the Patuxent Eiver and their absence on the Bay shore was 

 mentioned. This absence of terraces along the Chesapeake shore is doubt- 

 less due to their removal by erosion for the same forces which carried 

 away the terraces have also cut rapidly back into the soft, yielding, 

 unconsolidated material which composes the entire region and have not 

 only produced a straight coast line, but have also cut back so rapidly 

 that the mouthward portions of what used to lie considerable streams 

 have been carried away, leaving only their lieadwaters as weak, short 

 brooks, some of which are unable to sink their valleys down to the level 

 of the beach as fast as the waves can cut back toward their valleys. Others 

 have still enough force to maintain their mouth at sea level by descending 

 rapidly through narrow and steep-walled gorges. The divide at one 

 time probably occupied about the center of the peninsula. Its present 

 position seems to be due to the greater erosive powers of Cliesapeake Bay 

 and the rapid advance of the shore line toward this divide. 



THE STRUCTURE OF THE COASTAL PLAIN. 



The materials of which this region is built consist of clay, loam, sands, 

 gravel, and boulders. These deposits are loose and unconsolidated, ex- 

 cept where local ledges of ironstone have been developed. Although the 

 materials which have built up Calvert County have been deposited at 

 various times and belong to a large number of geological horizons, still 

 they all lie either horizontal or nearly so. Those which have been tilted 

 most, seldom exceed a dip of 12 feet to the mile. The structure of the 

 region, therefore, has not materially influenced the drainage, and the 

 streams flow from its surface as if they were flowing from a country 

 composed of unconsolidated deposits of clays, sands, and gravel hori- 

 zontally bedded throughout. 



Topographic History. 

 A detailed study of the topographic features which have been described 

 above and of the materials out of which the land is composed has re- 

 vealed many of the incidents which have produced the present relief. 



