586 HISTORICAL GEOLOGY 



already discussed. Streams debouching from mountainous lands 

 dropped their sediments upon reaching a low gradient, making alluvial 

 fans and plains. On account of the reduction of their volumes 

 through evaporation and seepage, the rivers developed great flood 

 plains. Shallow lakes which existed at that time, formed either by 

 warping or by the choking up of river channels by deposits of sand 

 and gravel, were later filled with sediments. 



Mention should be made of a series of deposits (Lafayette) of Ter- 

 tiary age, the exact status of which is yet in doubt (formerly sup- 

 posed to be Pliocene, but some of which are Oligocene) which have an 

 extensive distribution, occurring in many places on the Atlantic and 

 Gulf coastal plains in the southern portion of the Mississippi Valley 

 up to southern Illinois, and in the valleys west of the Appalachians. 

 This formation (Lafayette or Orange Sand) commonly has a thick- 

 ness of 20 to 30 feet, and is composed of gravel and sand in the lower 

 Mississippi Valley and of clay and silt over large areas of the uplands 

 east of the Mississippi River. It was derived from the insoluble 

 residue of older formations and consists of chert, quartz pebbles, and 

 other insoluble materials. , The color varies, but is often red, orange, 

 or yellow. This deposit was, probably, formed as follows. The pene- 

 planation and subsequent weathering of the land surfaces during the 

 early stages of the Tertiary produced a layer of loose, insoluble mate- 

 rial. In the Oligocene an upwarping alongthe axis of the Appalachians 

 began and increased during the epoch. As a result, the rejuvenated 

 streams carried much detritus and dropped a part of it upon reaching 

 the lower lands. With the continued rise of the mountain belt and 

 adjacent regions, the streams removed the sediments first laid down, 

 and redeposited them farther downstream. The sands and gravel 

 deposited not only filled up the lower portions of the valleys, but also, 

 to some extent, covered the former divides. At present, much of the 

 formation has disappeared in regions of strong erosion and, seaward, 

 is more or less concealed by younger beds. In some places it caps 

 divides but is absent from the valleys. 



The marine deposits have a very limited distribution on the east 

 coast and are of little thickness, being most important in Florida. 



Western Interior. — The Pliocene deposits of the western interior 

 are widely scattered and of limited extent. Beds of this epoch have 

 been recognized in Kansas, Nebraska, Oregon, and the Staked Plain 

 of Texas. As already stated, it is probable that much of the Great 

 Basin and other regions is underlain by Pliocene deposits. 



