THE ELEMENTS OF A PALEOGEOGRAPHIC PROBLEM 15 



from a basal conglomerate. Such regressive or negative movements of the 

 sea, caused by uplifts of the land or over-deepening of the ocean basin, 

 is generally more rapid than positive movements and a quickening of the 

 streams frequently affords much information concerning the adjacent lands. 

 If the land were still unsubdued, the rivers would run over hard rocks and 

 coarse deposits would be brought to the sea, while if the land were low and 

 covered with a mantle of residual soil the river-borne sediments would be 

 finer and result in clay beds, or insufficient in amount to prevent the growth 

 of organisms which in favorable localities secreted CaCO 3 in large quantities, 

 resulting in beds of limestone. In either case the change of sediments would 

 not be conspicuous if the change from advance to retreat of the sea were 

 actually or relatively sudden, but a change from a static strand-line to a 

 retreat would result in a radical change of the deposits. 



Such soft deposits as would follow a retreating strand under the circum- 

 stances cited above would be especially liable to destruction in any reverse 

 movement of the strand-line; the loose material, unless cemented with 

 exceptional rapidity, would be easily torn up and redistributed by the waves 

 and the coarser material would be sorted out as a basal conglomerate. 

 However, the advance of the strand over such a flat would be quite rapid 

 and the pebbles would be little worn by the waves and would be more likely 

 to retain the character of river pebbles than to assume that of beach pebbles. 



Such unconsolidated material also would be subject to rapid subaerial 

 erosion; the inner limits would soon disappear and the connection of the 

 unit with the edge of the old land would be effaced and its original continua- 

 tion recorded, if at all, in occasional outliers, as on the Atlantic and Gulf 

 Coastal Plains. 



If the elevation of the land were of such a character as to form mountains 

 or steep slopes close to the most distant line of retreat of the strand, such a 

 cycle of events as suggested above would not occur, for the material from 

 the land would be carried out into deep water, or at least into the zone of 

 wave action, where it would at once be distributed in the form of pure 

 marine sediments. 



c. The seaward limits of deposits. The outer limits of any marine unit 

 should coincide with the limits of deposition for any position of the strand- 

 line. In the open sea the approach to such an outward limit would be 

 indicated by the occurrence of progressively finer material until the clastic 

 debris was replaced by organic or chemical deposits. In a typical case the 

 littoral, benthal, and abyssal zones would be marked by the fossils and the 

 nature of the material. Such a typical series would extend over so broad an 

 area that some portion would be almost certainly concealed by later deposits 

 and only a very broad elevation of the land and only exceptional conditions 

 of erosion or structural changes would expose the whole history. The 

 determination of the outer limits of a marine deposit would be one of the 



