602 THE PLIOCENE PERIOD 



It is composed of gravel (and occasionally bowlders), sand, 

 silt, and clay, variously related to one another. It may be said to 

 be both heterogeneous and homogeneous; that is, there is consider- 

 able variation in composition in short distances, and but little more 

 in great ones. In the lower Mississippi basin, whence the name is 

 derived (Lafayette County, Miss.) it is of sand and gravel chiefly, 

 having in many places the distinctive characteristics of fluvial sand 

 and gravel. Over a broad tract of the uplands east of the Missis- 

 sippi and away from valleys generally, it is composed largely of silt 

 and clay. Its constituents are chiefly the insoluble residues of 

 older formations farther up the continental slope on which it lies, 

 chert and quartz pebbles making up its gravels, and other insoluble 

 matter its fine constituents. These constituents replace one 

 another at short intervals and in various ways, and no systematic 

 succession is observable. Irregular stratification is the rule, but 

 some portions are not bedded. Certain lenses of sand suggest an 

 eolian origin, and pebbly-earths that find their analogue in subaerial 

 and flood-plain deposits are common. The color of the formation 

 ranges from brick-red through various pinks, purples, oranges, and 

 yellows, to white. The color is more irregular than the composition, 

 bands, blotches, and mottlings diversifying the structural units. 

 Fossils are rare. In its representative parts they are all of land 

 plants and animals (except, of course, the fossils derived from earlier 

 formations). 



Origin. The preferred interpretation of the Lafayette formation 

 is as follows: At the opening of the Pliocene, the Appalachian tract 

 is supposed to have been affected by broad, flat, intermontane val- 

 leys, mantled by a deep residual soil and subsoil. The Piedmont 

 tract to the east is supposed to have been a peneplain near sea-level. 

 It is assumed that the upward bowing was felt first in a relatively 

 narrow belt along the axis of the mountain system, that the rise 

 was gradual, and that the rising arch increased in width as time 

 advanced. The first up-bowing rejuvenated the head waters of 

 the streams from the mountain tract, and the surface, with its 

 heavy mantle of residual earth, readily furnished load to the streams. 

 When they reached that portion of the peneplain not yet affected, 

 or less affected, by the bowing, they dropped part of their load 

 (at b, Fig. 501). With continued rise, the zone of deposition is sup- 

 posed to have been shifted seaward, and the deposits already made 

 were eroded and the eroded material was redeposited farther from 



