176 PHYSIOGRAPHY AND GEOLOGY OF THE COASTAL PLAIN PROVINCE. 



The Lafayette Formation. 



Name. — The name Lafayette Avas propo-ed by Hilgard" in 1S!)1 frcnn 

 Lafayette County, Mississippi, to replace tlie terms "Orange Sand" liitherto 

 employed in Tennessee and Mississippi and for "Appomattox" which was 

 used in Virginia, District of ('olnmbia, and jMaryland, for these deposits. 



Since Berry'' has shown that the so-called Lafayette of the type section 

 in Lafayette County, Miss., is of Eocene age, the question of the proper 

 nomenclature for these surficial deposits of the middle Atlantic slope 

 will have to be reconsidered. 



LitlioJogic rJiaracter.- — The formation is composed of gravels, sands, and 

 loams, usually unconsolidated, although locally cemented by iron oxide. 

 The l)eds in most cases show approximately horizontal stratification 

 although occasionally stratification lines are hot noticeable; at other times 

 the deposits show marked cross-bedding. The materials of the various beds 

 were imperfectly sorted by the waves of the Lafayette sea, so that tlie gravels, 

 sands, and loam are frequently found intermingled. There is a rough bi- 

 partite division in the deposit, as a whole, whereby the gravel occurs in 

 greater abundance at the base and the sand and loam at the top of the for- 

 mation. In certain places, however, irregular lenses of loam may occur in 

 the lower gravelly portion and l)eds of gravel and sand in the upper loamy 

 portion. 



The gravels of the Lafayette furnish the most characteristic feature of 

 the formation, as they constitute the greater portion of the deposits, and 

 are seen in almost every exposure of the formation They are of various 

 sizes, ranging from coarse sand to cobbles several inches in diameter gen- 

 erally imbedded in a matrix of sand or sandy loam. Near the crystalline 

 rocks the gravels are commonly coated with a thin layer of ferruginous 

 material which increases their value for road-making purposes. The 

 gravels are almost invariably well-rounded. They are mainly of quartz 

 although pebbles of gabbro and other igneous rocks, usually much decom- 

 posed, occur. Pebbles of fine-grained and quartzitic sandstones from the 

 Appalachian region with impressions of Paleozoic fossils are found in places 

 and also pebbles of Newark sandstone. A large part of the gravel is doubt- 

 less reworked Potomac material, derived from nearby regions. The hetero- 

 geneous character of the pebbles thus furnishes evidence of the varied 

 sources from which the gravels have been obtained. 



oAmer. Geol. a'oI. iii. pp. 129-131. 



''Jour, of Geol., vol. xix. pp. 249-256. 1911. 



