holmes] 



DISTRICT OF COLUMBIA GEOLOGY 



33 



I'iver from the liigliland uud its entry into the sea, now the District of 

 Cohxnibia; and as tlie streams draining this shore-line after its eleva- 

 tion fi'om the sea cut down througli the sedimentary formations, these 

 bowlders were exposed, and are now found outcropping iu the sides of 

 the valleys at the base of the sedimentaries and resting (tn the gneisses. 

 Other beds of bowlders are found higher in this section, but noue 

 happen to be so well suited to the use of the primitive implement maker 

 as those representing the work of the waves along the crystalline 

 beach. The surface of the gneisses was somewhat uneven, sloping 

 gently beneath the waves, and the bowlder beds laid down on this sur- 

 face are of uneven thickness and not of uniform character when fol- 

 lowed out horizontally, coarseness decreasing with distance from the 

 river channel. The aboriginal inhabitant, seeking for stone suitable 

 for his use, discovered these outcrops of bowlders along the bluffs of 

 the Potomac and its tributaries, and soon ascertained tluit the deposits 

 were heavier and the quality of the material better and more uniform 

 iu Eock creek valley than in any other section. This discovery led in 

 time to subterranean search ou the more favorable sites and finally to 

 extensive quarrying, the evidences of which arc now brought to light. 



Frn. 2 — Section of the ravine, showing formations ami position of qnarriea. 



Owing to the friable nature of the bowlder beds and of the gravels 

 and sands overlying them, the terrace slojies bordering the streams 

 (save where erosion had recently been particularly a(.'tive) offered no 

 good exposures of the bowlders iu i)lace, but were covered with deijos- 

 its, often many feet in thickness, of gravelly talus derived from the 

 crumbling edges of the strata. The bowlders contained iu this over- 

 placed deposit were the first to be utilized, and the work then extended 

 to the bowlder beds proper, and the refuse of the (piarrying was added 

 to the creeping slope gravels or talus. 



The section given iu figure 2 shows the relation of the gneisses, the 

 bowlder beds, and the superficial deposits of sand and gravel outcrop- 

 ping in the quarry ravine. 



PINY BRANCH QUARRIES 



Location op the Quarries 



In passing out of the city by way of Fourteenth street extended, the 

 bridge over Piny branch of Eock creek is reached at a i)oint 1^ miles 

 1.5 ETH 3 



