178 PHYSIOGRAPHY AND GEOLOGY OF THE COASTAL PLAIN PliOVIXCE. 



Stril-e, dip and thichiess. — The strike of the Lafa3^ette formation is in a 

 north and south direction, while there is a slight dip to the east, a dip which 

 seems to be due partly to the general slope of the bottom upon which the 

 strata were deposited and partly to a slight subsequent tilting. The thickness 

 is variable, owing largely to the irregularities of the underlying beds, the 

 greatest thickness being in the vicinity of the larger streams of the period 

 which brought down the materials from the westward and deposited them in 

 the Lafayette sea. The waves and currents spread this debris some distance 

 from the stream mouths along the coast thus forming a continuous deposit 

 over the entire submerged area. The coarser materials were mainly dropped 

 in the vicinity of the streams. The thickness generally does not exceed 30 or 

 35 feet although in some places it is considerably in excess of 50 feet. 



Stratigraphic relations. — A very marked unconformity separates the 

 Lafayette from all the underlying formations. In one place or another 

 within the C^oastal Plain province it ovei-lies almost every older formation 

 represented within the region, and thin remnants are found in many places, 

 resting on the crystalline rocks of the Piedmont Plateau. In Virginia, the 

 Lafayette is chiefly found in contact Avith the Potomac, Eocene, and 

 Miocene formations and so far as known is a surface deposit, although 

 locally it, in all probability, dips beneath deposits of Pleistocene age. 



Faleontologic character. — Fossils are practically lacking in the Lafayette 

 deposits of the Atlantic Coastal Plain, only a few fragments of moHuscan 

 shells which are very imperfect and seem to be re-deposited Miocene fossils 

 have been reported from Mrginia deposits of doubtful Lafayette age. 

 Sandstone and chert pebbles containing Paleozoic fossils are found in the 

 formation at many places throughout the district but ai-o of importance 

 only in showing the source of the materials. In regions to the south of 

 Virginia some fossil plants and animals of Lafayette age have been reported, 

 but very little is known concerning them. 



Areal distribution. — The Lafayette formation covers the divides along 

 the western part of the Coastal Plain and occurs as outliers resting upon the 

 crystalline rocks of the Piedmont Plateau to the southwest of Washington, 

 west of Richmond, and in many other places. Its distribution is co-extensive 

 with the distribution of the Lafayette terrace described in the chapter on 

 physiography. In its wider distribution the Lafa.yette formation has been 

 traced as a nearly continuous mantle over the older members of the Coastal 

 Plain deposits all the way from the Mississippi valley, parallel with the 

 Coastal border, to A'irginia and southern Maryland, north of which the 

 deposits become less extensive and are represented in northern Maryland, 

 Delaware, and Pennsylvania by only a few small remnants. 



