CONDITIONS OF SEDIMENTARY DEPOSITION. 487 



The undertow rolls coarser sand and pebbles down the slope 

 of the bottom, and carries out in suspension silt and clay with 

 more or less fine sand. The rolling of coarser sands is promoted 

 by a steep slope. The transportation of finer sanHs depends on 

 the endurance of the undertow of a given initial strength ; and 

 this endurance will be the greater the more gradual the seaward 

 slope and the stronger the tides. The amount of sand thus de- 

 posited is limited only by the supply, and sandy strata may, 

 therefore, attain great thickness and have great extent seaward 

 from a fixed beach line. If the coast be continually maintained 

 by uplift or renewed by volcanic flows the work of the waves 

 may be of like duration and the record will be correspondingly 

 voluminous. Professor Chamberlin mentions the great conglom- 

 erates of Lake Superior in this connection. 



Beach deposits, strictly speaking, are usually of quite coarse 

 sand, clean and characterized by marked and irregular cross- 

 stratification. Sand deposits from undertow graduate from clean 

 to muddy sands, becoming ever finer seaward, and are horizon- 

 tally bedded or massive. 



Therefore the interpretation which may be put on strata, de- 

 posited by the arrest and retreat of waves, are : 



(1) A basal conglomerate is significant of an horizon of 

 wave erosion, due to transgression of the sea and probable sub- 

 sidence of the land. If the basal contact be clean and sharp the 

 waves probably carved a shore cliff in hard rocks. If, between 

 the parent rock and the later sedimentary formation, there be a 

 zone of transition composed of boulders, sand and clay of mixed 

 mineral composition, the waves probably rearranged the cores 

 and finer products of a surface of partial subaerial rock decay. A 

 basal conglomerate of any variety is a definite proof of an un- 

 conformity by erosion ; it is often the only fact by which such 

 an unconformity can be distinguished from an overthrust fault. 



(2) A deposit of clean sands is proof of the former existence, 

 somewhere, of a beach on which they were washed ; but the 

 place of deposit may have been remote from the line of the 

 beach. Coarseness of grain suggests proximity of land and vice 



