LOCATION OF POTOMAC BEDS. 43 



Along- the James from Ridimoml to Deep Bottom, twenty-one miles 

 by river below that city, the Potomac formation shows itself in the base 

 of most of the bluffs and high banks, rising to various heights and capped 

 by Quaternary. Considering the varial)ility of the material, it is here 

 surprisingly like that found in the Fredericksburg and Baltimore areas. 

 On the James its greatest thickness is attained in Drury's liluff, eight 

 miles below Richmond. Here about sixty feet of it can be seen. In 

 Chafin's Bluff, a little below Drury's Bluff, for a long distance it shows 

 a thickness of ten to twenty feet. Tn the vicinity of Dutch Gap it is 

 exposed for several miles, reaching its maximum thickness in Trent's 

 Reach, where forty feet may be seen. 



On the Appomattox it may be found at man}- places up to Peters- 

 burg. Its greatest thickness on this stream is found at Point of Rocks. 

 This is four miles above City Point, where the James and the Appomattox 

 unite. Here fully eightv feet may be seen. It is noteworthy for the 

 coarseness of the sand which makes up most of the formation, and for the 

 large size and abundance of the pebbles and cobble-stones found in it. 

 Some of the latter, composed of Potsdam quartzite, attain the dimensions 

 of ten or twelve inches. This j)oint, although one of the most eastern 

 exposures, shows the formation in the Petersburg area at its maximum 

 of thickness and of coarseness, which, as on tlie Potomac River, indicates 

 that the eastern margin of the formation is still far distant. The eastern 

 limit of exposures in this area may be fixed by a line drawn from a point 

 on the Appomattox three miles above City Point, in a direction a little 

 east of north, to Deep Bottom, on the James. Although exposures do not 

 occur east of this line, there is good reason to think tliat the formation 

 extends far eastward under the Tertiary terrane. 



In 1869 an artesian well was sunk at Fortress Monroe to the depth of 

 907 feet. This well passed through the Miocene and Eocene strata, and 

 at the depth of 835 feet entered a material which differed totally from the 

 Tertiary, but possessed the peculiar fcatiu-es of the Potomac formation. 

 The boring was still in this formation when it was stoppt'd, so that at least 

 seventy-two feet of this may be found here. Professor Rogers had an 

 opportunity to study the material from different depths in this well and he 



