GEOLOGIC EXHIBITS IN NATIONAL ZOOLOGICAL PARK — BASSLER 275 



contracting with the cold of night fail to occupy the same space as 

 before and tend to exfoliate or split off in thin sheets. Boulders of 

 this type rarely occur under the moist conditions of the park. 



In the northern part of the Zoo occur narrow bands of a darker 

 colored crystalline rock which may be recognized at the surface by the 

 slippery soil into which it weathers. This is dionfe, an igneous rock 

 composed of more basic minerals (feldspar and hornblende) intruded 

 into the surrounding rocks and weathering at the surface into soap- 

 stone. This basic rock with its weathered soil is quite in contrast 

 with the light-colored acid granite so prevalent throughout the park. 

 A still more basic type, gahhro, is one of the igneous intrusive rocks 

 elsewhere in the park. One band of diorite continues to the north- 

 east, where outcrops in the vicinity of Albemarle Street and Connec- 

 ticut Avenue outside of the park supplied the impure soapstone blocks 

 which the Indians in former days carved into pots and kettles. 

 Traces of the old Indian quarries are still extant, and it is not improb- 

 able that the park itself contained such quarries. The boulder beds 

 overlying the ancient granites and other rocks of the park have also 

 been the site of Indian quarries, notably along Piney Branch near 

 Sixteenth Street, where incomplete hatchets and arrowheads 

 {artifacts) are found, particularly in the streams draining from 

 this region. 



ADAMS MILL ROAD GEOLOGY 



Coming back to the bear pits and following the path to the Adams 

 Mill Road entrance at the top of the hill, one first notes several faults 

 and folds in the schists (fig. 1, C) as indicated by their various angles 

 of tilting. Another feature is the evidence of recent erosion by 

 human agency in the worn stone steps cut in the original rock. The 

 pillars at the Adams Mill Road gates again give opportunity for the 

 study of rock classification (pi. 6) and the effect of the weather upon 

 them just as at the Harvard Street entrance. As we leave the park, 

 good exposures a short distance to the right along Adams Mill Road 

 (fig. 1, O) show the normal occurrence of the Potomac formation 

 pebble beds over the pre-Cambrian schist, but a few feet to the left 

 of the gates (fig. 1, F) this succession is seen to be reversed by fault- 

 ing. This particular fault although of small magnitude is of such 

 interest that a protection against the weather and the small boy has 

 been built around it. Here, along an oblique fracture in the earth's 

 crust, pressure from the southeast has thrust the rocks on one side 

 forward over the other, producing a thrust fault. After present-day 

 weathering and removal of the upper part of the rocks, the normally 

 underlying schists are now seen to overlie the Lower Cretaceous sands 

 and gravels. The movement along this fault line is plainly registered 

 on the boulders and schists at the point of contact by scratched sur- 



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