LOCATION OF POTOMAC BEDS. 39 



nous matter. Indian Head, a few niile.s above Intlian Point, sliow.s nnicli 

 the greatest exposure of the Potomac seen on the MaryUmd side. 'Vha 

 rock here has the same irreguhir structure and mixture of materials as is to 

 be seen in the cut at Freestone, but the proportion of Y\g\\t colored .sand is 

 much smaller. It is covered here by Eocene, and the Variegated Clay is 

 wanting. Indeed, as stated before, this group has not been seen anywhere 

 south of Fort Washington. Gunsen's Creek is the estuary of the united 

 creeks Pohick and Accotink. ()n its south and north shores near its 

 mouth are extensive exposures of the Potomac. 



On the south .side, near Gunsen's Landing, fully seventy-five feet of 

 sand and clay appear, having the same general character as in Indian 

 Head. We see here a graduation of the normal sand of the Potomac into 

 a greenish sandy clay, which is sometimes mottled with red. The same 

 thing in a more striking form is seen at Cockpit Point. Some fragments 

 of lignite, but no impressions of plants, were seen at Indian Head and 

 near Gunsen's Landing. 



On the north shore of Gunsen's Creek, White House Point forms a 

 high bluff, called White House Bluff, immediately on the river. Here we 

 may see fully eighty feet of Potomac sand and clay, capped by some ten 

 feet of Quaternary. This material is sufficiently described in the notice 

 of the occurrence of the plants at this place. It is strikingly different 

 from the Potomac shown on the south side of the creek only a short dis- 

 tance away. We find here a good illustration of the- unerpial erosion to 

 which the formation has been subjected. 



At the upper and lower ends of the exposure of the Potomac, points 

 not a half a mile apart, the Quaternary forms the river bank to below 

 water level. It may be seen rising towards the central portion of the bluff 

 until fully eighty feet of Potomac is exposed beneath it. As no Varie- 

 gated Cla}' or Eocene can be detected in this bluff, it would appear that 

 it was not submerged until Quaternar}' times, or else these formations 

 have been removed by erosion. 



Two miles above White House the Potomac again appears in the high 

 shore on which the Mount Vernon mansion is situated. It shows here six 

 to eight feet of the light-colored grit which forms the lower portion of Wiiite 



