THE TERTIARY FORMATIONS OF VIRGINIA. 355 



to the west of these points, the beds of freestone make their appear- 

 ance. 



On the Accakuk, near Mr Brookes's, the eocene and sandstone are 

 seen at very contiguous points, both presenting exposures of conside- 

 rable extent. The marl here consists chiefly of shells, imbedded in a 

 dark olive brown clay, containing a portion of green sand. The shells 

 are chiefly Cytherea? and Ostrea sellseformis, with a few Cardiia 

 planicosta? At Mrs Roll's, on Acquia creek, about two miles below 

 the mouth of Auston's Run, the marl is finely exposed, in an abrupt clifi*. 

 Here fine specimens of Turritella Mortoni, Cytherea ovata (nobis), 

 Crassatella (nobis), Ostrea sellseformis, may be readily procured. The 

 material in which they are imbedded is a friable mixture of sand and 

 cla}^, of a light yellowish brown colour, blended with green sand, in 

 granules of unusually large size. 



In thus drawing an outline of what may be termed the western 

 coast of the eocene formation, it is necessary to remark that the pecu- 

 liar irregularities observed at several points in the line of actual boun- 

 dary, will occasion considerable discrepancies between it and the line 

 above described. 



Besides such flexures as may have originally existed in this line at 

 the period of the deposition of the eocene, great additional irregulari- 

 ties must have been produced by the destructive agencies which sub- 

 sequently operated. The region in which the freestone and eocene 

 formations are brought together, is marked by the effects of violent 

 diluvial action. Coarse gravel, pebbles and boulders bestrew the sur- 

 face, and mingle to considerable depths with the sandy strata usually 

 found upon the heights. Deep and precipitous ravines, connected with 

 the valleys of the creeks leading into the Potomac, attest the energy 

 and extent of the aqueous forces once operating over this region, while 

 the confused mixture of materials, by which the usual upper stratum 

 of the eocene is often seen to be replaced, indicates the power of the 

 denuding and transporting agencies to which that formation must at 

 one time have been exposed. It is thus that many places within the 

 general confines of the eocene, bared of their former covering, now 

 merely expose the underlying beds of freestone, while at other points, 

 VI. — 4 o 



