North Atlantic Pleistocene Problem — Shattnck. 105 
surface rises gradually from an elevation of 90 feet in southern 
Maryland to about 140 feet behind Philadelphia, yet the distri- 
bution of the terraces does not seem to the author to indicate 
the complicated tilting described by Mr. Dalton, but on the 
contrary points to the following course of events. Miocene 
elevation and erosion ; subsidence and the deposition of the La- 
fayette formation ; elevation and extensive erosion ; subsidence 
and deposition of the Sunderland terrace around the edges of 
the Lafayette ; elevation and erosion ; subsidence and deposition 
of the Wicomico terrace about the margin of the Sunderland ; 
elevation and erosion ; depression and deposition of the Talbot 
about the margin of the Wicomico; elevation and partial ero- 
sion of the Talbot ; subsidence and the deposition of the Recent 
terrace about the edges of the Talbot. 
It is interesting in looking at a map of these various ter- 
races to see how repeatedly the present estuaries have been 
transformed from rivers to fjords and back again to rivers, for 
one may trace the Sunderland, Wicomico and Talbot terraces 
up the valleys of all the principal streams to the Piedmont 
plateau. 
The unconformity which was described by Mr. McGee in the 
Sixteenth street cut, to which reference was made earlier, is 
not regarded by the author as pointing to a period of elevation 
and a subaerial erosion, but simply as indicating a change of 
current or possibly freshet conditions and therefore in no way 
dififering from the unconformities in across-bedded deposit ex- 
cept in the size of the materials moved. Both the restricted 
area in which this unconformity is found, as well as its position 
at the head of the old delta near the mouth of the Potomac 
gorge where the current must have been constantly changing, 
point to this conclusion. 
A comparison of the section proposed by the author with 
that formerly suggested by Mr. Darton is as follows : 
Darfon. Shaftuclc. 
T , /- 1 1 • 1 Talbot formation. ] t^, . • 
Later Columbia >\\t- ■ " ,Columl)ia 
) vVicomico } /-- 
Earlier Columbia .Sunderland " J '' 
Lafayette formation Lafayette " 
After a careful study of the surficial deposits in Maryland 
an attempt was made to correlate them with the formations of 
New Jersey, belonging to the same period of deposition, as 
mapped and described b^• professor Salisburw 
