190 CLARK AND BIBBINS — GEOLOGY OF THE POTOMAC GROUP 



pressions as to their character as a whole. Again, in central Maryland 

 the Potomac beds reach their maximum breadth of outcrop, their 

 greatest thickness, as well as their greatest lithologic and paleontologic 

 diversity. With these facts clearly in view, the authors have attempted 

 the interpretation of the Maryland area, and have brought to bear on 

 its problems the results of a large amount of systematic field work in 

 this region, as well as in the areas both to the north and south. 



The constituent formations of the Potomac group, with the exception 

 of the Arundel formation in part, dip at progressively lower angles from 

 below upward, and in general gradually thicken down the dip within 

 the limits of the area of outcrop, although they gradually thin farther 

 to the seaward. Their stratigraphic relation is that of progressive trans- 

 gression landward from the southwestward, the younger formations ex- 

 tending farther and farther toward the Coastal plain margin northward, 

 until they successively come to rest on the Piedmont plateau. The gen- 

 eral relations for the Maryland deposits are shown in the accompanying 

 vertical and columnar sections. 



Since the publication by the authors in the Journal of Geology in 1897 

 of "The Stratigraphy of the Potomac Group in Maryland " field investiga- 

 tions have been steadily in progress during the summer months under the 

 joint auspices of the Maryland Geological Survey, the Woman's College 

 of Baltimore, and the U. S. Geological Survey. Although the position 

 taken by the authors in that paper was based on a moderate field knowl- 

 edge of the Potomac beds, subsequent work has, in the main, confirmed 

 the conclusions there stated. A great number of comparative sections 

 have been made and, collections of fossils obtained from many stations 

 heretofore unknown. Detailed mapping, on the base of the U. S. Geo- 

 logical Survey topographic atlas sheets, is nearing completion in Mary- 

 land and the District of Columbia, and much work has also been done 

 in Virginia, Delaware, Pennsylvania, and New Jersey. The lines for the 

 Maryland area are shown on the accompanying map, reduced from those 

 on the atlas sheets to a scale of 12 miles to the inch. 



Descriptions of the constituent Formations of the Potomac Group 



in Maryland 



the formations and their relations 



In the Potomac deposits of the middle Atlantic slope four formations 

 are recognizable, named in order of age the Patuxent, the Arundel, the 

 Patapsco, and the Raritan. Their relations to one another and to their 

 immediately subjacent and superjacent terranes are shown in the follow- 

 ing table : 



