INTERPRETATION OP THE POTOMAC DEPOSITS 207 



landward tilting, also, which appears to find additional support in the 

 very indefiniteness observed at certain points in the Arundel-Patapsco 

 boundary, must therefore be retained until a more satisfactory interpre- 

 tation can be brought forward. 



The well marked unconformity occurring at many points between the 

 Arundel and Patapsco formations, notably in the West Hanover district, 

 indicates emergence and a distinct erosion interval prior to Patapsco 

 deposition. At points somewhat farther seaward and at lower levels, as 

 at Federal hill, the line is less distinct and there is a suggestion of grada- 

 tion. These facts would seem to indicate that the elevation was not an 

 extensive one, bringing only the landward margin of Arundel deposits 

 under the influences of subaerial erosion. In cases where this erosion 

 resulted in a mere shoaling of the waters only a comparatively slight 

 change in lithology and organic remains would be expected. It is cer- 

 tain that the tendency of the more recent investigations on these two 

 formations has been to show that they are more closely allied than was 

 formerly supposed — stratigraphically, lithologically, and paleontolog- 

 ically. 



The highly colored and variegated clays of the Patapsco formation were 

 evidently deposited in the quieter and deeper waters of this epoch, and, 

 like the iron-bearing Arundel clays, bear some relation to the great basic 

 eruptive masses, plentifully iron-bearing, w T hich lie to the north and west 

 of them. This phase of the sedimentation is somewhat more prominent 

 in central Maryland, where the rocks of this character are not only well 

 developed, but nearest the eastern margin of the Piedmont belt. It is 

 also probable that these ferruginous Patapsco clays were also in part 

 redeposited from . the more richly iron-bearing clays of the subjacent 

 Arundel. The Patapsco sands were doubtless derived to a considerable 

 extent from those of the Patuxent terrane. 



The unconformity separating the Raritan from the underlying deposits 

 is likewise more pronounced to the landward and apt to be obscure to 

 the seaward. To the landward also the lithologic break is more clearly 

 defined. That a considerable erosion interval occurred is evidenced b}^ 

 the undulatory character of the Patapsco-Raritan contact and by the 

 marked advance in the grade, variety, and number of the Raritan dicoty- 

 ledons. The source of the Raritan materials was clearly in part the sands 

 and clays of the preceding formations. The common occurrence of 

 white sands, " fuller's earth," and white and generally light-colored clays 

 marks another step in the gradual loss of iron in the progressive redepo- 

 sitions of the more richly ferruginous materials of the preceding deposits. 



That the conditions of deposition, which the heterogenous character 

 of the deposits show to have been highly varied, were, especially toward 



