266 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 19 3 9 



weathering to which they have been subjected. The Zoological Park 

 is located near the fall line marking the eastern edge of this plateau, 

 where the hard, less soluble Piedmont rocks are overlapped by the 

 unconsolidated, easily eroded sands and gravels of the coastal plain 

 to the east, the Atlantic Coast Plain province, made up of practically 

 horizontal sediments deposited mainly by the sea. Slight uplift along 

 the fall line in comparatively recent time, geologically speaking, has 

 sufficiently elevated the area to cause vigorous stream action resulting 

 in deep valleys such as that of Rock Creek, although in its lower 

 reaches the stream descends almost to sea level, the ultimate l)ase 

 level of erosion. As a result of this stream cutting, here are many 

 exposures of the underlying rocks. These, acted upon by the weather 

 and other surface phenomena, illustrate particularly the features of 

 physical geology^ that part of the science dealing with the forces 

 operating on the outer part of the earth and the changes produced 

 by them. Historical geology^ which is concerned more with the sedi- 

 ments deposited by water and wind, and their contained record of 

 life of the past in the form of fossils, may also be studied here but 

 not under such exceptional conditions. 



The logical student, in considering the features of physical geology, 

 commences with the cooled crust of the earth formed of the once 

 molten igneous rocks. He learns that when the earth's temperature 

 became low enough for moisture to condense and fall as rain {mete- 

 oric water), weathering of these igneous rocks ensued by decomposi- 

 tion through solution and chemical means. This was continued 

 through disintegration of the rocks with the aid of physical agents, 

 such as the wind and variations in temperature, both processes result- 

 ing in the surface configuration and the formation of soils. Part of 

 the rain, the run-off, remained at the surface and as running water 

 on its way to lower levels carved out valleys, while the rest sank into 

 the earth as ground water to perform the geologic work of solution, 

 cementation, mineral precipitation, and the like. Ultimately, both 

 the run-off and ground water reached the ocean or other bodies of 

 water where the materials dissolved or eroded from the older rocks 

 were laid down in horizontal parallel layers or strata of sedimentary 

 rocks by the process of stratification. Such rocks are known as ma- 

 rirm deposits if deposited under the sea, or fresh-water deposits if 

 in lakes or streams. In the course of time the sedimentary rocks as 

 well as the igneous rocks sometimes were changed, usually by pres- 

 sure and heat, into another form called metamorphic rocks, for ex- 

 ample, the granites into gneiss, and the clays and shales into slates 

 and schists. Sometimes all three types of rocks become so folded, or 

 fractured, or elevated by the processes classified under diastrophism, 

 that with uplift, plateaus resulted, or with uplift or folding followed 



