Maryland Geological Survey 57 



The Lower Cretaceous deposits are more highly differentiated in Mary- 

 land than elsewhere along the Atlantic border, as is seen in both 

 their lithologic and paleontologic diversity. The several formations 

 present certain common features which need to be taken into considera- 

 tion in any discussion of the strata. 



The deposits, which are largely sands and clays of varying strati- 

 graphic and lithologic characteristics, are, for the most part, uncon- 

 solidated, although certain marked exceptions to this are to be seen 

 in the locally developed sandstone beds in the lower part of the series. 

 The deposits in general dip at progressively lower angles in passing 

 upward in the series, although the Arundel formation affords some 

 striking exceptions to this general rule. Again, the deposits thicken 

 down the dip within the limits of the area of outcrop, although they 

 apparently thin farther to the seaward, as shown by the well borings in 

 which Lower Cretaceous strata are encountered. The stratigraphic re- 

 lations show that after the deposition of the Patuxent and Arundel for- 

 mations they were gradually transgressed toward the northward by the 

 Patapsco formation before the close of Lower Cretaceous time. A study 

 of the organic remains reveals a gradual progress in the types of plant 

 life from the Patuxent through the Arundel and Patapsco, especially in 

 the gradual advent of dicotyledonous types of plant life. These various 

 features will be fully discussed in the descriptions of the several for- 

 mations. 



The Potomac Group 



The Potomac Group, originally named by McGee for the deposits thus 

 characterized in this report, was divided by Clark and Bibbins into the 

 Patuxent, Arundel, and Patapsco formations, although they and others 

 included within the Potomac certain higher deposits of somewhat sim- 

 ilar character (Raritan formation) which are now recognized as of 

 Upper Cretaceous age. A sufficient lithologic and paleontologic differ- 

 ence occurs in these higher deposits to warrant the restriction of the 

 term Potomac to those formations characteristic of the Potomac River 

 region where they were first described by McGee under the name of the 

 " Potomac formation." 



