1880.] NATURAL SCIENCES OF PHILADELPHIA. 2G3 



It will be noticed, in the first place, that the clay lies in the 

 form of crests and hollows upon the gravel. This is almost inva- 

 riablj'^ the case. Beautiful examples of wave motion may be seen 

 at Twenty-eighth Street and Columbia Avenue, at Tenth and 

 Tioga, at Fifteenth and Clearfield Streets, and in Fairmount Park. 

 In each of these we have apparently the action of a rushing flood 

 of water upon the gravel. Often the clay lies in a kind of pot-hole 

 in the gravel, and a concentric structure of clay and pebbles can 

 be seen. The following section, at Twent3'-eighth Street and 

 Columbia Avenue, shows six well-marked waves of gravel and 

 clay, the clay always filling the hollows between the crests of 

 gravel. 



Fig. 2. 



s N 



The approximate dimensions of the weaves are given in the 

 diagram. Along the line of contact between clay and gravel there 

 are alternate streaks of fine and coarse gravel. 



A ver}^ beautiful example of water action is exposed at Fifteenth 

 and Clearfield Streets, in a cut about 100 feet in length (Fig. 3). 



The second point to be noticed in the section near the Univer- 

 sity is the stratification of the gravel, and its division into layers 

 of three diflTerent colors, — red, black and yellow. It is instructive 

 to note that this division is by no means a local one, but exists 

 along a line of about equal elevation (60 to 80 feet above ocean level), 

 in widely separated parts of the city. While the colors are of 

 course due simply to different states of oxidation of the iron, the 

 fact that the}^ mark continuous deposits through long distances, 

 indicates a uniformity in the condition of deposition which could 

 be due only to the presence of a large bodj^ of water. 



In the third place, the section (Fig. 1) shows the important fact 

 that the gravel rests, not upon a hard floor of rock, as is usual with 

 the drift in more northern States, butupon a completely decomposed 

 gneiss. This is universall}' the case in every section examined in 

 the vicinity of Philadelphia. In no case does the gravel rest upon 



