32 



STONF. IMPLEMENTS 



[ETIl. AXN. 15 



rolatioiisliip, siuce, (lie association isiioccssaiil\- ])in('ly adveTititioiis. It 

 is I'm tlier ascertained tliattlie other scdinu^ntary rocks of tlie snrround- 

 ing region are all older tliaii those witli which the works of man are 

 known to be contemporaneously associated. Tlie deposits with which 

 remains of human handiwork are directly associated are mainly talus 

 accumulations, the formation and modltic^ation of which have been 

 going on for a long period and are still in jjrogress. 



The broad plateau bordering the city on the north is cut by Eock 

 creek and Anacostia river and their tributaries. It is capped with 

 sedimentary formations which extend fiir eastward and southward, 

 covering the tidewater country ; these are underlain by crystalline rocks, 

 gneisses, granites, schists, etc (figure 1), well exposed by the deep scor-^ 

 ing of llock creek and its branches. On the western side of that 

 stream the latter rocks rise to and form the surface of the country. 

 The sedimentary rocks were laid down along the crystalline shore, which 

 sloped gently eastward, in api)roximately horizontal strata, two forma- 

 tions in Mesozoic time and the Cretaceous period, known as the Potomac 



^E 



Fig. 1 — General section across RocTc creek and Piny branch valleys, sliowinff frneissic formations 

 and their relation to the overlying beds of Potomac ;;ravel8. 



and Severn formations; two in the Eocene period, named in order of 

 deposition the Pamunkey and the Chesapeake; one in the Neocene 

 period, known as the Lafayette foinnation ; and one in the Pleistocene, 

 named after the Federal District the Columbia formation. 



The Potomac formation rests on the uneven surface of the gneissic 

 rocks exposed in Rock creek valley, and is composed to a great extent 

 of coarse sediment and fiaginental rocks, brought down mainly by the 

 great streams that drained tlie highland. The lower members of this 

 formation are usually of very coarse materials, and in the Rock creek 

 region they consist largely of pebbles and bowlders of quartz and 

 quartzite, well rounded by water action. The Lafayette formation, 

 resting on the npper surface of the Potomac series in this region, is 

 not to any extent concerned in the jjresent study, although in some 

 sections of the Potomac valley the heavy bowlder deposits included in 

 it were utilized by the aborigines. 



Especially heavy accnmulations of bowlders occur along that por- 

 tion of the old shore-line bordering the exit of the ancient Potomac 



