GEOLOGIC EXHIBITS IN NATIONAL ZOOLOGICAL PARK — BASSLER 273 



igneous material, cooling slowly into pegmatite dikes in which 

 quartz, mica, and feldspar, with crystals larger than in ordinary 

 granite, are the prevailing minerals. 



Along the base of this cliff quite different phenomena are in 

 evidence. Temperature changes resulting from the alternate heating 

 and cooling of the rocks on the face of the cliff due to the hot sun 

 of the day and the cold of the night cause fragments of rocks to be 

 splintered off as the first stages of clastic material and to fall to the 

 bottom, where they accumulate as a talus slope at a definite angle, 

 the angle of rest. In the winter, frost actioii in the form of freezing 

 and thawing of water percolating through the rocks gives the same 

 results. In a ravine the fragments may slide to the base by gravity, 

 forming a rock glacier. This slope, furthermore, often shows creep 

 of soil, a geologic phenomenon of considerable importance when 

 developed on a large scale. In this process, it will be remembered 

 that water seeping into the soil freezes and the resulting expanded 

 ice crystals raise rock particles up on edge. Then, with melting, 

 these particles drop again but slide a little farther down the slope. 

 Still another example of weathering may be noted along such a 

 slope where pebbles or rock fragments cap a series of pillars of softer 

 material giving a castellated effect. In such a case the capstone 

 protects the soil particles underneath it from being washed away by 

 the rain, imitating, on a small scale, the 'badlands scenery of the 

 west. Here, also, on the face of the cliff the action of life on rocks 

 in the process of weathering is shown by the roots of trees forcing 

 their way between the layers and following the joint planes at high 

 angles. However, water is the more important factor here because 

 in percolating along the joint planes it disintegrates the rocks by 

 solution. 



Turning now to nearby Rock Creek, the natural exposures 

 illustrate particularly weathering by erosion. The creek bed itself 

 is sometimes covered with large angular blocks of granite torn from 

 their native ledges by the force of the water and borne some distance 

 with the volume and strength of the current. Wlien these frag- 

 ments drag along the stream bottom, they wear the underlying rocks 

 by actual scratching or abrasion, specifically called corrasion- (pi. 4, 

 fig. 2). Here occurs still another type of boulders, usually much 

 smaller, which show from their composition that they have been 

 derived in other ways or from far-away sources. Most of those in 

 the stream bed today were washed out from the nearby gravels of 

 the Potomac formation or from the more recently deposited Pliocene 

 and Pleistocene gravels, but all in turn were previously derived from 

 older formations outcropping to the west and deposited by streams of 

 ancient times. Pebbles of smoky quartz, amethyst, milky quartz, 



