38 THE POTOMAC OR YOUNGER MESOZOIC FLORA.. 



composed of Quaternary material. In some places this is wholly com- 

 posed of the ruins of the Potomac beds, depo.sited witli but little sorting 

 and modification In these places it is sometimes difficult to distinguish 

 the Quaternary from the Potomac. This is the case in High Point, near 

 the mouth of Occoquan River, where for a long distance the banks of the 

 Potomac River are formed of Quaternary. Professor Rogers, in his annual 

 reports, makes this material Potomac. 



The exposures of Potomac beds on the Marjdand shore are much 

 fewer than on the Virginia side. This shore, lying farther east in the direc- 

 tion of the declination of the beds, contains them at lower level. The 

 formation may be seen barely exposed at Fort Washington, while farther 

 down the river, at Indian Head, diagonally above High Point and on the 

 opposite side of the river, a thickness of fully eighty feet is shown. 



Along the Potomac River on both sides, as everywhere else, the for- 

 mation has lost greatly by erosion, and the amount thus lost is very 

 unequal in different places. It is impossible to say when this erosion took 

 place, but it is clear that much of it occurred prior to the deposition of the 

 Eocene. The result has been to leave the formation with an exceedingly 

 irregular surface, and to remove it entirely from cousideral)le areas. 



Perhaps the greatest thickness of Potomac exposed at a single locality 

 is to be found on Acquia Creek, about two miles above tlie point where the 

 Atlantic Coast Line Railroad crosses it. Here a high hill shows tlie forma- 

 tion rising to the height of one hundred and forty feet above the water. 

 The character of the material is well exposed by quarries It is mostly a 

 coarse gritty sand of light color, which is generally consolidated to a pretty 

 firm sandstone. In some places we find a good many pebbles, and some- 

 times a curious mixture of these and clay balls vs'ith coarse sand. Little 

 intercalated clay is found here. From Acquia Creek to near Quantico the 

 Potomac, althougli occuri'ing a little back from tlie river, is mostly want- 

 ing in its banks, Quaternary taking its place. At Quantico and Shipping 

 Point only about twenty feet is to be seen. Cockpit Point, a.bout two 

 miles north of this last, shows fully sixty feet of very varying material. 



At Freestone Point there is a long deep railroad cut, which shows that 

 here the Potomac is over one hundred feet thick, composed of heteroge- 



