212 PHYSIOGRAPHY AND GEOLOGY OF THE COASTAL PLAIN PROVINCE. 



shown in a number of instances. The occurrence of cla_Y balls of Patuxent 

 age within the Patuxent sand beds along the James River has already been 

 noted. During the Patuxent submergence the region was probably 

 beneath the water for some distance to the west of where we find the most 

 westerly deposits now, but these deposits must have been very thin and 

 they have since been removed by erosion. 



The deposition of the Patuxent formation was ended by an uplift which 

 brought the region above the water and inaugurated an erosion period which 

 persisted long enough to permit the removal of a vast amount of material. 

 This was followed by a subsidence in which many of the stream valleys but 

 lately eroded were occupied for a portion of their courses by bogs and 

 swamps. In these marshes there was an extensive development of plant life, 

 and in them also were deposited the iron ores which in Maryland have been 

 so extensively Avorked since Colonial days. These deposits constitute the 

 Arundel formation. 



After another elevation and erosion interval the land was again depressed 

 beneath estuarine waters. Deposits similar to those which had formed 

 during the Patuxent submergence were laid down throughout the northern 

 Atlantic Coastal Plain. In Maryland these are very well exposed but in 

 Virginia they appear at the surface chiefly along the Potomac River. They 

 constitute the Patapsco torination. The greater abundance of red clay 

 found in the Patapsco in(]icates that the source of some of the streams was 

 in a region where the decomposition of the igneous rocks and the oxidation 

 of the residual material had been going on for a long time. A marked 

 change in the flora as compared with that of Patuxent and Arundel time 

 indicates that a considerable period probably elapsed before the deposition of 

 the Patapsco formation. 



Following the deposition of the Patapsco formation the region again 

 became land through an upward movement which drained all of the pre- 

 viously existing estuaries and marshes. Erosion at once became active and 

 the Patapsco surface was dissected. 



Late Cretaceous history. — A very long period of erosion apparently 

 elapsed before the deposition of the Upper Cretaceous formations in 

 Virginia. The land barrier that had kept out the ocean waters during the 

 Potomac epoch had been broken down. The streams from the low- lying 

 land evidently carried to 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 accumulation of the greensand beds which are 

 so characteristic of the Upper Cretaceous formations along the Atlantic 



