Page:The Journal of geology (IA journalofgeology11893univers).pdf/61

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 sands 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 deposited 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 conglomerates 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 horizontally bedded or massive.

Therefore the interpretation which may be put on strata, deposited 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 subsidence 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 subaërial rock decay. A basal conglomerate of any variety is a definite proof of an unconformity 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