THE COASTAL PLAIN 



831 



to the margin of the old land, the upper thin edges of the last de- 

 posited layer, provided continuous subsidence precedes the elevation 

 of the coastal plain, lap over the earlier layers and rest directly 

 upon the old land. These overlapping edges of the strata are gen- 

 erally the first to be removed again, and in their place will appear a 

 shallow valley running parallel to the upper edge of the coastal plain, 

 the iinier lozvland of the normal coastal plain erosion topography. 

 Dissection of the Coastal Plain. Streams originating upon the 

 coastal plain as the "run-off" of the rain and snow-fall, continue 

 down the slope of the surface to the sea in more or less parallel 

 lines, and with more or less directness, according to the angle of 

 slope. These consequent streams, together with the extended conse- 

 quents, i. e., the old streams of the old land, now extended across 

 the coastal plain to the new sea margin, will incise more or less 



Fig. 205. The emerged coastal plain with the youthful consequent streams 

 bearing a few simple insequents. 



parallel channels across the coastal plain. When these channels are 

 cut down to the level of the "ground water," they will be supplied 

 by springs with a permanent stream, whereupon down-cutting may 

 proceed at an accelerated pace. Along the margin of the main 

 streaius, lateral tributaries will begin, and, cutting headward, wnll 

 soon diversify the original channel of the streams. These lateral 

 branches are the "insequent" streams of the physiographer, since 

 they are not consequent upon a constructional slope. Insequent 

 streams, furthermore, cut their channels backward from the edge of 

 a stream, and only deepen their channels as the channels of the con- 

 sequents to which they are tributary are deepened. Consequent 

 streams, on the other hand, cut their channels downward, headward 

 extension being generally a secondary mode of growth. Near the 

 old land the insequents will remove the feather edges of the coastal 

 plain strata, as indicated above, and here will come into existence a 

 stripped belt, expressed topographically in a broad, shallow valley, 

 the inner lowland, containing the enlarged insequents, now more 



