64 PHYSIOGRAPHY AXD GEOLOGY OF THE COASTAL PLAIN PROVINCE. 



tion of the Capitol, and other public buildings in Washington. The old 

 lightliouse at Cape Henry is built of this Aquia freestone, as are also the 

 local foundations, etc. On the north shore of the Rappahannock River just 

 opposite Fredericksburg there is also a thick bed of Patuxent sandstone 

 but the most extensive deposit of these indurated sands occurs at Point of 

 Rocks on the Appomattox River below Petersburg. Here there is an in- 

 durated arkosic conglomerate, extending from the water's edge to the top 

 of a nearly vertical bluff 80 feet in height. Some layers are very firmly 

 cemented and the material from them has been quarried from colonial 

 days for local use in the construction of walls and buildings. 



Cobbles and boulders are not uncommon in the Patuxent deposits. 

 Sometimes they form distinct beds while at other times they occur irregu- 

 larly distributed throughout strata of finer materials. At the base of the 

 Howlett House bluff on the James River opposite Farrar Island there is a 

 conglomerate bed 30 feet in thickness of large and small cobbles and 

 pebbles in a matrix of coarse arkosic sand, the whole locally indurated. 



Although sands predominate in the Patuxent formation, clays are not 

 altogether absent. The beds of sand occasionally pass into clay deposits 

 while in other places the sand and clay beds are interstratified. The sandy 

 clays frequently contain great quantities of lignitic material. In cutting the 

 Dutch Gap canal on the James River a log of lignite about 60 feet in length 

 and 10 inches in diameter is said to have been taken out. The Patuxent 

 clays are seldom highly colored, so that they can generally be very readily 

 distinguished from the variegated clays of the Patapsco formation. In the 

 vicinity of Dutch Gap there are irregular masses of dark-colored clay em- 

 bedded in the sandy strata. Some of these are several feet in diameter and 

 are decidedly angular, showing that they were only transported a short 

 distance. Many fine plant impressions frequently occur in them. The 

 sands are almost invariably cross-bedded. 



Strike, dip, and thiclcness. — The general strike of the Patuxent formation 

 throughout Virginia is almost due north and south. The dip is to the east 

 at the rate of nearly 50 feet to the mile in the region of the "fall-line", while 

 farther to the eastward it decreases to about 30 feet to the mile. To the 

 eastward of the line where the deposits disappear beneath the later sedi- 

 ments the dip has not been determined, except that at Fortress Monroe, 

 75 miles to the eastward the base of the Potomac deposits is reached at a 

 depth of 2,246 feet indicating a still more gentle dip over this area. The 

 floor of crystalline rocks upon which the Patuxent formation rests is very 

 irregular so that the thickness of the deposits is extremely variable. This is 



