130 THE GEOLOGY OF PKINCE GEORGE S COUNTY 



broken into the area. Some land barrier to the east of the present 

 shore line probably existed and produced these conditions, but its 

 position and extent can not be determined. 



The period during which the Magothy deposits Avere 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 and Monmouth were laid down. The great variability in 

 the lithologic character of the materials, the coarseness of the sands 

 and gravels, and the cross-bedding all suggest conditions similar to 

 those of the preceding periods. On the other hand, the local pockets 

 of glauconitic sand and the presence of marine invertebrates sug- 

 gest the marine conditions of the late Cretaceous. The probability 

 is that over most of the area where Magothy deposits are now present 

 Potomac conditions prevailed during the greater part of the period 

 and in some places perhaps during the whole of it, but that occa- 

 sionally, 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. 



At the close of Magothy time the region was uplifted and a period 

 of erosion was inaugurated. During this erosion interval compar- 

 atively small amounts of material were removed. In some places it 

 is impossible to establish definitely any stratigraphic break between 

 the Magothy and the Matawan. This may be because the erosion 

 interval was comparatively short or because the elevation of the land 

 above the water was so slight that it did not permit the streams to cut 

 channels in the recently formed deposits. 



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 and Monmouth epochs prob- 

 ably all of the county was beneath the ocean. The streams from the 

 low-lying land evidently carried into the ocean at this time only 

 small amounts of fine sand and mud, which afforded conditions 

 favorable to the production of glauconite and permitted the accumu- 

 lation of the greensand beds that are so characteristic of the Upper 

 Cretaceous deposits along the Atlantic Coast. During this time very 

 slight changes took place along the continental border, although 



