408 J. BARRELL RECOGNITION OF ANCIENT DELTA DEl>OSlTS 



"Potomac history.— The earliest of the knowii uncousolidated deposits lyiii;; 

 upon the floor of ci'j'stalline rocks belong to the Patuxent formation of the 

 Potomac group. ... It indicates a submergence of the Coastal Plain in 

 this region of sufficient extent to cover the whole area with shallow water. The 

 cross-bedded sands and gravels furnish evidence of shifting currents, as do 

 also the abrupt changes in the character of the materials, both horizontally 

 and vertically. The presence of numerous land plants in the laminated clays 

 shows the proximity of the land." 



The Arundel formation was deposited within stream valleys which had 

 become eroded into the Patuxent. The Patapsco and Earitan are, how- 

 ever, like the Patuxent, widely developed formations and were deposited 

 uiider similar conditions. These four formations constitute the Potomac 

 group. 



"The widespread development of shallow-water deposits, everywhere cross- 

 bedded and extremely variable in lithologic character, and the presence through- 

 out these deposits of land plants furnish some evidence that the Potomac sedi- 

 mentation took place not in open ocean waters, but in brackish- or fresh-water 

 estuaries and marshes that were directly connected with the ocean, which may 

 have at times locally broken into the area. Some land barrier to the east of 

 the present shoreline probably existed and produced these conditions, but its 

 position and extent can not be determined. 



"The period during which the Magothy deposits were formed was one of 

 transition from the estuarine or fresh-water conditions of Patapsco and 

 Raritan time to the marine conditions under which the Matawan. Monmouth, 

 and Rancocas were laid down. . . . The probability is that over most of 

 the area where Magothy deposits are now present Potomac conditions i)re 

 vailed during the greater part of the period, and in some places perhaps during 

 the whole of it, but that occasionally, through the breaking down of the land 

 barriers which had kept out the ocean, there were incursions of sea water, 

 bringing in marine forms of life. Thus far there is no evidence that such 

 incursions took place anywhere except in New Jersey. 



"Later Cretaceous history. — Not until late Cretaceous time did a downward 

 movement occur of sufficient extent to permit the ocean waters to transgress 

 widely over this region. During the Matawan, Monmouth, and Rancocas 

 epochs probably all of the quadrangle was depressed beneath the ocean waters." 



From these statements it is seen that an estuarine origin is invoked as 

 far as possible, though marsh conditions are granted for the Arundel 

 formation, which was discontinuous in its original development and de- 

 posited apparently in river valleys cut through the older deposits. That 

 the estuarine interpretation for these formations is dominant in geologic 

 thought is illustrated also by quotations from the Philadelphia (Pennsyl- 

 vania, New Jersey, Delaware) folio, page 18, under date of 1909. Here 

 the possibility of marsh conditions is not noted, and it is merely stated 

 that 



