DEFINITION OF THE POTOMAC FORMATION. 



437 



Kegarding this formation I have to report adversely, for it is found to 

 consist of certain local sand beds occurring at several horizons in the Poto- 

 mac clay series. In Uhler's typical section along the banks of the Severn 

 river above Round bay, the "Albirupean " sands graduate above and later- 

 ally into typical iron-bearing clays of the Potomac formation. Other 

 "Albirupeau " sands were found to be included in the clays along the Pa- 

 tapsco and elsewhere in the Baltimore region. Usually these sands are 

 sharply demarked from clays below by sharp unconformities, but above and 

 laterally they become argillaceous and gradually merge into pure clays. 

 There can be no doubt in regard to the wide vertical distribution of these 

 sand streaks in the formation, and they cannot consistently be grouped as 

 a separate formation. 



Shore Deposits. — The shore deposits at the western edge of the Potomac 

 formation often lack the distinctive features of the beds further eastward, and 

 some errors have been made in their identification. In Maryland the pure 

 clays sometimes lie directly on the crystalline rocks, but usually they become 

 sandy and gravelly, and in some cases are represented solely by coarse 

 materials. In Virginia the arenaceous beds are usually overlapped by later 

 formations, but a few basal contacts are exposed in which the sands, with 

 clay pebbles and quartz pebbles, lie directly on the crystalline schists. Both 

 in Virginia and Maryland there are regions in which the western margin of 

 the formation is represented by fringes and outliers of coarser shore deposits, 

 usually capped by a protecting mantle of the less destructible Appomattox 

 gravels. The Appomattox formation in these regions consists largely of re- 

 arranged Potomac gravels, and this has led them formerly to be mistaken 

 for undisturbed Potomac sediments, but it is almost always possible to dis- 

 criminate the two formations. In the terraces west of Alexandria, Wash- 

 ington, and Baltimore, notably at Tennallytown and Catonsville, this joint 

 occurrence is especially notable. 



In the Sassafras river region I find that McGee * has included a portion 

 of the dark Cretaceous (Severn) beds in the Potomac, and in Maulden's 

 mountain excluded as Cretaceous some gray and brown sands really belong- 

 ing to the Potomac. 



Stratlgraphic Position. — The Potomac formation, as originally defined by 

 McGee, is a stratigraphic unit lying between the Newark formation and the 

 New Jersey Cretaceous greensand series and separated from both by structural 

 breaks representing long time gaps. Its upper part or entire thickness in the 

 north may extend more or less above the horizon of the beds in the James 

 and Potomac regions, but the formation represents continuous deposition 

 throughout. 



* Geology of Head of Chesapeake Bay; U. S. Geol, Survey, Reoort of Director, 1885-'86, pp. 585, 

 587, 590, G13. 



