540 STUDIES FOR STUDENTS 



Nature of the geological record. — Flood-plains differ from playas 

 and the shores of interior lakes in important particulars. Playas 

 are formed in local basins or as the ends of desert rivers, usually of 

 limited length. Playas do not fill up valleys from which the rivers 

 escape, but are entirely phenomena of interior drainage. They 

 are not built out against the sea and the deposits are at least 

 brackish from the inclosed salts. 



The flood-plain of an aggrading river covers a wide area, in the 

 case of the larger rivers measured by thousands of square miles. It is 

 all periodically subject to inundations which may last for a few days 

 or weeks, but leave the greater part of the surface exposed to the 

 air during much of the year. The invading flood waters frequently 

 sweep sand with them, but after the flood is at its height the waters 

 drain away quietly and much of the fine clay is deposited from 

 suspension. Thus on river flood-plains there is peculiar liability to 

 form well-interstratified deposits of sand and clay: to fill up the 

 last-formed mud-cracks with coarser material, and hence permanently 

 to record them through the varying composition and structure of the 

 formation. Such successive strata of the same nature may be indefi- 

 nitely accumulated. 



Such valleys even in desert regions commonly support considerable 

 life. Drifting vegetation is liable to become buried and animals 

 crossing the half-dried flats in search of fresh water may leave through 

 their foot-prints a record of their visits. The periods of desiccation 

 are seasonal and sufficient to harden this record of cracks, rain-prints, 

 and foot-prints to such an extent that the next invasion of water does 

 not wash it out, but by depositing upon it a new layer of sediment per- 

 manently preserves it. As the main streams or their distributaries 

 wander over the plain from century to century they form a network 

 of channels-which cut through the preceding fine-grained layers of the 

 flood-plains, and the channels become filled with sand or even gravel, 

 as they are finally abandoned for new courses. They may be dis- 

 tinguished from the beach sands and gravels of lakes from their 

 linear, treelike arrangement, their occasional cutting-down into the 

 finer-graind layers, and their occurrence far from the margins of 

 the basin. 



The mud-cracked strata of flood-plains not only stand excellent 



