412 i\T. II. Darton — Magothy Formation of Maryland. 



small amount of clay fleckings and intermixture, derived from 

 the subjacent clay formation, and discontinues streaks of 

 limonite. Towards the western end of the bluffs they include 

 two elongated lenses of tough, laminated, gray clay. The 

 lower mass is near the base of the formation and is about 

 six feet thick ; the upper one is near the top and has a maxi- 

 mum thickness of three feet. These clay streaks merge into 

 the adjoining sands, but the transition is quite abrupt at 

 most points. Overlying the Magothy formation eastward 

 is a thin wedge of weathered Severn sands separated by 

 an east dipping erosion plane similar to the one below. 

 The Severn beds attain a thickness of 15 feet at the eastern 

 end of the bluff's but are cut out westward by an overlap of 

 the overlying Columbia formation onto the Magothy beds. 

 The Severn beds are dark gray, argillaceous, carbonaceous, 

 line, laminated sands with some portions less argillaceous 

 slightly glauconitic, more massive and lighter gray in color 

 with buff mottlings. They are sharply contrasted from the 

 Magothy beds and even without the separating unconformity 

 could never be classed with them. The Columbia capping 

 on these bluffs has a thickness of from four to seven feet. 

 It consists of a basal bed of pebbles, bowlders and slabs with 

 local beds of conglomerate and an upper bed of red-brown 

 to buff, columnar loam with a few scattered pebbles. 



The next notable Magothy exposures southward are in the 

 long high bluffs extending north from Grove Point. Towards 

 the point the bluffs are of Severn beds overlain by Columbia 

 deposits both superbly exposed.* About 1000 yards north of 

 the point the Magothy formation comes up on a low southern 

 dip and soon attains prominence in the bluffs in which it con- 

 tinues to the end. The formation consists of coarse, loosely 

 compacted, cross-bedded gray sands with irregular masses and 

 streaks of brown sandstone and sandy limonite. The erosion 

 plane separating the Severn formation is well exposed and is 

 seen to be less regular in contour than usual. The ferrugina- 

 tion of the Magothy beds is here very irregular and predomi- 

 nates near the summit. Below are many scattered masses con- 

 sisting of reticulated plates and tubes of sandy limonite, some 

 of which are several feet in length and as much as a yard in 

 thickness. 



On the peninsula between Elk River and the head of Chesa- 

 peake Bay there is a high ridge on which there remains an 

 outlier of Magothy with overlying Severn formation constitut- 



* Detailed descriptions of the Columbia formation in these and many other 

 bluffs in this region are given by McGee in a memoir on "The Geology of the 

 head of Chesapeake Bay. U. S. Geol. Survey, Seventh annual report of the 

 Director, pp. 537-646, pis. 1888. 



