GEOLOGIC EXHIBITS IN NATIONAL ZOOLOGICAL PARK — BASSLER 271 



winter and spring floods. Land shells and plants occur as fossils 

 in some of the beds and give further evidence of their origin as 

 fresh-water deposits. They also show the very recent age of the 

 strata, since the same species of shells may be found in the present- 

 day creek and the leaves belong to the trees now growing on the 

 plain. These strata constitute a geologic formation^ since they are 

 a mappable unit formed in the same period of time and hold the 

 same assemblage of species of plants {jlora) and animals {fauna). 

 These layers have a wide enough extent throughout the park to illus- 

 trate correlation of strata from place to place by fossils, by noting 

 that the same stratum throughout the area contains the same 

 assemblage of species. 



A slightly developed ridge along the stream border of this plain 

 illustrates a levee. Similar to those of large size and considerable 

 height along the Mississippi and other rivers, this one was formed 

 by the material dropped when the swiftly flowing current at flood 

 time able to carry much mud, sand, and gravel in suspension, 

 struck the still waters of the flood area which no longer had this 

 power. When tributaries enter Rock Creek, deltas on a small scale 

 may sometimes be formed. Near the edges of the stream are seen 

 miniature sand hars, due to change or slowing down of the current 

 with consequent deposition as in the case of the levees. Occasion- 

 ally, the flooded stream leaves a mud layer on the plain, which, 

 upon drying, contracts, causing sun cracks outlining polygonal areas 

 with upturned edges. These may be broken up and scattered over 

 the plain by wind action, forming still another type of continental 

 deposit similar to loess. From this alluvial or flood plain, built up 

 by stream deposits, the Piedmont Plateau, a plain worn smooth by 

 erosion, can be seen capping the surrounding hills, and not far 

 to the east is the Atlantic Coast Plain, an uplifted sea beach of 

 horizontal marine sediments, these representing the three impor- 

 tant types of flat lands classified under plains. Potholes are oc- 

 casionally seen in the hard rock stream bed where crevices in the 

 granite or other solid rock contained imprisoned pebbles which, 

 whirled round and round by water, wore out circular areas by 

 abrasion. Above, along the stream valley, are terraces of sand and 

 pebbles representing stream deposits of former times accumulated 

 when their waters flowed at higher levels. Near the stream is a 

 lake for the waterfowl (fig. 1, 15) where the deposition in lakes by 

 water is shown to be more active than erosion as the bottom rapidly 

 fills with sediment. 



Upstream, beyond the Zoo grounds. Rock Creek develops falls and 

 rapids as it tumbles over the granite outcrops and boulders. The 

 old bear pits (fig. 1, B, and pi. 3, fig. 1) at the foot of the cliff on 



