MARYLAIN^D GEOLOGICAL SURVEY 113 



formation and the ancient crystalline rocks. The Lafayette plain 

 is in reality the oldest and highest of a series of five plains which 

 were developed at snccessively lower levels dnring various epochs 

 ranging- in time from Pliocence (?) to Eecent. It extends almost 

 uninterruptedly from the District of Columbia line to the southern 

 boundary of the county and forms one of the most striking topo- 

 graphic features in this area, l^ortheast of the District of Columbia 

 the Lafayette is represented by a few outliers, the largest of which 

 occurs in the vicinity of Laurel. It covers the higher portions of the 

 divide between the PatAixent and Potomac river systems and thus 

 becomes the most conspicuous formation of the region. 



Character of Materials. — The Lafayette forination is composed of 

 gravel, sand, and loam. These materials were so imperfectly sorted 

 by the waves of the Lafayette sea that they are now found inter- 

 mingled in varying proportions. Although there is a rough bipartite 

 division in the deposit as a whole, the gravel occurring in greater 

 abundance at the base and the sand and loam at the top of the forma- 

 tion, yet these elements are mixed together in a confusing manner. 

 No particular kind of material is confined to any definite stratum, 

 but all kinds may occur anywhere throughout the section. Irregular 

 beds or lenses of loam, sand, or gravel also occur and are exposed 

 in many places throughout the county. The gravels are consider- 

 ably decayed and are usually rather small, but in the vicinity of 

 Washington they become very coarse and are embedded in a coarse, 

 compact sand or very stiff, clayey loam. The appearance of the 

 gravels also changes from place to place ; near Washington they are 

 almost invariably covered with a dark-brown ferruginous coating, 

 but farther south the amount of iron decreases considerably and the 

 coating of iron oxide is practically absent. The heterogeneous char- 

 acter of the material furnishes evidence of the varied sources from 

 which it has been obtained. Pebbles of quartz and crystalline rocks 

 indicate the Piedmont as the source; broken iron crusts were derived 

 from the Paleozoic formations farther west ; and finally, decayed 

 blocks of l^ewark sandstone are occasionally observed. While all 

 of these various materials are present, the gravels are composed prin- 

 cipally of quartz. 



