THE PLEISTOCENE OR GLACIAL PERIOD 901 



Columbia; but the materials formerly grouped under this name 

 represent at least three somewhat distinct stages of deposition. 1 



The oldest subdivision of the Columbia series is found at levels 

 higher than those of the younger subdivisions. In the principal 

 valleys, -it constitutes broad but often rude terraces, which rise 

 up-stream. Up the Potomac, the Susquehanna, the Delaware, 

 and other valleys, the terraces rise to altitudes notably above those 

 attained by the formation outside the valleys. In New Jersey this 

 member of the Columbia series is called the Bridgeton formation. 



In the District of Columbia, the second member of the Columbia 

 series covers rock terraces 100 feet or so below the oldest member 

 phase of the series (Fig. 593). The relations of the two subdi- 



Fig. 593. Diagram showing the relations of the three divisions of the Pleis- 

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

 low-level Columbia (or Pensauken), and Qcm, the Cape May formation. 



visions indicate that extensive erosion followed the deposition of 

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

 sequently 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. This member of the Columbia series is called, in New 

 Jersey, the Pensauken formation. 



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 somewhat 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. Outside the valleys, the landward 

 edge of this member of the series is as ill-defined as the landward 

 edge of the older members in the inter-stream areas. In New 



1 Reports of the State Geologist of New Jersey, 1897-1900: also Phila- 

 delphia folio U. S. Geol. Surv. 



