NON-GLACIAL FORMATIONS 



653 



found at levels higher than those of the younger subdivisions. In 

 tin- principal valleys, it constitutes broad, mostly rude terraces, 

 which rise up-stream. Up the Potomac, the Susquehanna, the 



Fig. 535- Diagram showing the relations of the three divisions of the Pleisto- 

 cene as seen in valleys. Qc = the high-level Columbia, Qp = the low-level Columbia 

 (or Pensauken), and Qcm, the Cape May formation. 



Delaware, and other valleys, the terraces rise to altitudes notably 

 above those attained by the formation outside the valleys. In the 

 District of Columbia, the second member of the Columbia series 

 (Qp) covers rock terraces 100 feet or so below the oldest member 

 phase of the series (Fig. 535). The relations of the two subdi- 

 visions indicate that extensive erosion followed the deposition of 

 the first, and that the broad valleys then developed were subse- 

 quently aggraded by sediments similar to those of the preceding 

 epoch of deposition. The two deposits are so nearly alike in com- 

 position that their separation is based chiefly on their topographic 

 relations. The third phase of the composite Columbia is found at 

 still lower levels along the streams and coasts. Its disposition is 

 such as to show that the second phase of the Columbia formation 

 had been extensively eroded before the deposition of the third. In 

 the valleys formed during this interval of erosion, and along the 

 coast at accordant levels, the third member of the series finds its 

 chief development. 



The various members of the Columbia series rest unconformably 

 on older formations. On the Atlantic Coast, the oldest division 



Fig. 536. Diagram showing the theoretic relations of the three principal 

 subdivisions of the Pleistocene outside the valleys, along a line normal to the coast. 

 The letters have the same significance as in Fig. 535. 



rests now on the Lafayette formation, and now on terranes from 

 which the Lafayette h^d been eroded before the deposition of the 

 Columbia series (Fig. 537). 



The Columbia series rarely contains fossils; but at a few points 



