46 



GEOMORPHOLOGY: 



land forms whose features are often sculptured by 

 stream erosion. Whether a particular form is destruc- 

 tional or constructional is dependent upon how it 

 originally came into being, not upon its later erosion. 



COASTAL PLAINS 



A coastal plain is any portion of the land that was 

 once ocean floor. Included are recently emerged 

 shorelines and areas further removed from shores. 

 Here, only the more remote parts of coastal plains 

 will be discussed; shorelines of emergence are treated 

 elsewhere. 



The profiles of coastal plains range from land 

 gently sloping into the sea to rather rough shoreline 

 features, primarily vertical clifTs and horizontal 

 benches but also old stacks, clefts, and caves. In 

 further discussion, rough features will be ignored. 

 Such land forms erode so rapidly that they barely 

 modify the generalized life history of a coastal plain. 



Life History. Coastal plains actually start in the 

 sea. There, sediments in the form of muds, sands, 

 and gravels accumulate; they then compress as a 

 result of overlying sediment pressure; finally, they 

 consolidate into various sedimentary rocks. When 

 such areas emerge and form a coastal plain, the less 

 resistant rocks immediately start to erode. Also, pre- 

 existing streams extend their path to reach the sea, 

 and new streams may form. In this initial stage, such 

 things as old islands, stacks, and shorelines are least 

 modified. 



In the youth of coastal plains, streams running into 

 the ocean gradually wear away the layers of sedimen- 

 tary rocks, the less resistant rocks eroding faster than 

 the harder ones. In areas of less resistant rocks, tribu- 

 tary streams form perpendicular to the streams run- 

 ning into the ocean (Figure 4.12). 



Maturity occurs when the tributary streams de- 

 velop lowlands in softer rocks. Lowlands near the 

 site of the shoreline before elevation are called inner 

 lowlands; those now closest to the sea (but not includ- 

 ing the shore) are called outer lowlands. Ridges or 

 elevations between the two lowlands are formed by 

 the tributary streams and are called cuestas. Each 

 cuesta has a steep slope facing inland and toward the 

 old shoreline and a gradual backfacing slope toward 

 the present shoreline. The ridge of the cuesta is 

 parallel to the present shoreline (Figure 4.12). 



In the early maturity of coastal plains, adjacent 

 headwaters of two opposite-running tributary streams 



YOUTH 



old land 

 fall line 



coastal 

 plain 



Figure 4.12 Cuesta formotion. Erosion of o coastal plain elevated 

 as part of a shoreline of emergence. Note the development of streams 

 and stream capture. 



may cause little degradation of the old ocean floor 

 between the headwaters. Later, these headwaters will 

 cut toward one another on a line parallel to the shore 

 and just landward from the cuesta. If this occurs, 

 there is a finger of land extending inland from the 

 infacing slope of the cuesta toward the old shoreline. 

 This extension, a cuesta bridge, gives the appearance 

 of a mesa joined to the cuesta and is of about the same 

 altitude as the cuesta summit. 



Full maturity occurs when one seaward flowing 

 stream, via its tributaries, captures many others and 

 becomes the local source of drainage into the ocean. 

 Therefore, the lower parts of captured seaward flow- 

 ing streams no longer exit as streams. However, the 

 dried paths of the captured streams may be indi- 

 cated for some time; the last remaining indication is a 

 notch, or wind gap, through the cuesta. It should not 

 be assumed that the presence of a wind gap is always 

 indicative of coastal plains and cuestas. Wind gaps 



