GEOLOGICAL IMPORTANCE OF SEDIMENTATION 445 



however, by the incurving of the spit-bars thrown partly across the 

 mouth of the estuary. Illustrations of this nature are well exhibited 

 along the Atlantic coast of the United States. As seen in cross- 

 section, the bottom line of Fig. 85 represents the center line of the 

 estuary bottom, while the depth of the marine planation is deter- 

 mined by the average slope of the land. 



In the sedimentary record made, therefore, upon a subsiding land, 

 only a fragmentary littoral record should be preserved at the base, 

 and upon tracing the formation laterally there should be frequent 

 places where the marine off-shore sands, gravels, or conglomerates 

 should rest directly upon the old land surface. This contact of true 



Fig. 10. — Ideal cross-section of a delta, showing geological relations of f3uvial, 

 littoral, and off-shore deposits. Vertical scale much exaggerated. 



off-shore marine deposits with the old land surface seems to be the 

 common rule observed upon that most striking of American trans- 

 gressions, that of the middle and upper Cambrian, across the greater 

 portions of the North American continent, though Dana^ states that 

 the beds "are in part beach made and wind made sandstones, .... 

 and the layers often bear ripple-marks, shrinkage cracks, worm bur- 

 rows, and, in some places, tracks of animals." The shrinkage cracks 

 may doubtless be taken as good evidence in this instance of a littoral 

 origin. 



To sum up, it is to be expected that littoral deposits should be 

 found dividing former land surfaces from sea bottoms, commonly 

 present where the land surface is represented by subaerial delta 



I Manual of Geology, p. 464. 



