506 H. E. Gregory — Formation and Distribution of 



and their ancient representatives, the conglomerates, are in 

 general much thicker and many times greater in extent than 

 are those of marine origin. To a greater degree than streams, 

 the tendency of marine agents during a cycle with constant sea 

 level is to destroy rather than to accumulate and preserve 

 gravel. 



Distribution and Preservation of Marine Gravels during 

 Interrupted Cycles. 



Effect of Coastal Downwarp. — Coastal downwarp or its 

 equivalent, the rise of sea level, introduces a new physiographic 

 cycle. The work of the waves and currents is put out of 

 adjustment with the coastal contour and the shore profile. 

 Unless the crustal warping were abnormally localized it includes 

 wide subaerial areas and increasing amounts of land are brought 

 within range of the destructive work of the sea. Conditions 

 are favorable for marine planation at rates depending upon 

 the nature of the initial coastal belt and the efficiency of waves 

 in eroding the cliffs. Because of the amount of material to be 

 removed planation is favored by an initial coast which has 

 attained old age and consequently possesses slight relief ; 

 while a rugged coast of mature topography is unfavorable for 

 coastal migration of the shore line. 



Since the depression of a coast does not materially modify 

 the force of waves, the conditions affecting deposition are 

 directly related to the degree of adjustment of the former shore 

 profile to the supply and transportation of coastal material at 

 the beginning of the cycle initiated by downwarp. If the 

 profile is over-steep it must be reduced by filling ; if over-flat, 

 it is cut by waves and bars are developed. With a coastward 

 migrating shore line the section of the compound curve of 

 shore maturity most directly concerned is that part between 

 the outer edge of the concave portion and the shore line ; for 

 this is the part of the underwater slope over which gravel is 

 normally deposited. The depth at which the concave curve 

 merges with the plane of deposition is, as shown by Fenneman, 

 10 or 12 fathoms on the Atlantic shores and 20 to 30 fathoms 

 on the Pacific shores of the United States. 



So far as the thickness of gravel deposits is concerned 

 the conditions associated with downwarp are essentially like 

 those in a cycle marked by an initial over-steep shore profile. 

 As in other cases, the work of the waves at any stage 

 of downwarping is between the breaker line and the shore 

 line ; but waves during subsidence are effective at con- 

 tinuously higher levels and the gravel of successive submerged 

 beaches may be preserved as a mantle covering the continu- 



