450 GEOLOGY. 



Even in such cases, however, there is abundant evidence that the 

 series is not a unit in origin. Locally at least, deposition probably 

 alternated repeatedly with erosion, in the course of the history of the 

 Columbia series. Even where the topographic distinction between 

 the two most marked divisions of the series is not pronounced, there 

 is evidence of one interval of erosion more important than others, 

 and this may well correspond with the time of pronounced erosion 

 between the high- and low-level members of the series in the type area. 



The third phase of the composite Columbia is found at still lower 

 altitudes, 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 develop- 

 ment. Its relations, as shown along the valleys, are diagrammatically 

 represented by Fig. 533. 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. Fig. 534 shows, dia- 

 grammatically, the supposed relations of the three phases along an 

 interfluvial tract, from the coast inland. This figure represents the 

 seaward margin of the oldest subdivision, as buried by the next mem- 

 ber of the series, and the seaward margin of the latter, as covered, in 

 turn, by the youngest subdivision. It should be understood that this 

 relation is diagrammatic, since no section showing the three subdivi- 

 sions in such superposition has been seen. Since the deposition of 

 the third phase of the formation but little erosion has taken place. 

 It should also be understood that the three subdivisions are probably 

 not sharply separable from one another, because of the manner in which 

 the deposition took place (see p. 452). 



The threefold division of what was originally called the Columbia 

 formation calls for a change in nomenclature. It is convenient to 

 have a name for this coastal series as a whole. If the name Columbia 

 be used in this way, its several subdivisions should have separate names. 

 In New Jersey, the name Bridgeton has been applied to what is prob- 

 ably the equivalent of the High-level Columbia, and the name Sunder- 

 land was later applied to the High-level Columbia of Maryland. The 

 name Pensauken 1 has been applied in New Jersey to what is prob- 



1 Report of the State Geologist of N. J. for 1894, p. 105. 



