408 N. H. Darton — Magothy Formation of Maryland. 



Geological Survey. Finally, I will describe the formation 

 more fully in a Monograph on the Geology of the Chesapeake 

 Bay region now in preparation. 



General Relations. 



The coastal plain of eastern Maryland is underlain by a 

 series of later Mesozoic to Pleistocene deposits lying on an 

 east-sloping floor of crystalline rocks. These deposits are 

 widely extended sheets of sands, clays, and marls inclined and 

 thickening to the southeast and separated into formations by 

 erosion breaks. In the following table there is given a list of 

 these formations and breaks, an explanation of their age and 

 a brief description of their general characters : 



Formation. Characters. Paleontologic position. 



g {Columbia, Loams, sands, and gravels. On 



8 I • terraces. 



to j Erosion interval. Development of 



•2 [ outlines of present topography 



Lafayette Gravels, sands, and loams Pliocene? 



Erosion interval. Base levelling 



over Coastal plain and westward 



j Chesapeake Sands, clays, infusorial earths, 



(^ and marls Miocene. 



Erosion interval, planing of sur- 

 faces of preceding deposits 



Pamunkey Glauconitic marls and sands . . Eocene. 



Erosion interval, planing of surface 

 of Severn and Potomac formations 



Severn Black, argillaceous sands, mainly.. Cretaceous. 



Erosion interval, planing of surface 



of Magothy and Potomac formations Cretaceous. 



\ Magothy White sands and brown sand- 

 stones, also gravels Cretaceous. 



Erosion interval, planing of surface 



of Potomac formation Cretaceous. 



Potomac Clays and sands, also gravels and Early 



sandstones .. Cretaceous. 



Great erosional and stratigraphic break 

 following Jura-Trias deposition. 



The Potomac formation lies directly on the crystalline rocks 

 and outcrops over a wide belt eastward. Its western border 

 extends westward on the ridges and the crystalline rocks 

 extend eastward in the intervening depressions. It is suc- 

 ceeded eastward by the younger formations outcropping in 

 succession in irregular northeast and southwest belts which are 

 deflected westward of the ridges and eastward down the de- 



