1889] MARYLAND ACADEMY OF SCIENCES. 61 



Latiarea, sometimes form drifts of more than a foot in thickness, 

 traceable throughout a distance of many feet. 



Further up the Piscataway other ravines open, wherever 

 streams cut their way down from the highlands. These corres- 

 pond essentially with that just noticed and call for no further 

 description. Along the banks of Tinker's Creek the same marls 

 and fossils also occur. The shell-marl and loamy greeusand of 

 the Eocene keeps on continuously in the ravines of Piscataway 

 Creek, and is discoverable in the banks of all the streams, and 

 in the gullies back to Upper Marlboro ; but the plastic black 

 marl appears to be out of reach, or absent, after going beyond 

 the head of that creek. 



Passing to the north side of the Fort Washington promon- 

 tory, on the shore of Swan Creek, three or four shallow ravines 

 are observed to open out towards the water, but their banks 

 are so deeply covered by sand and loam from above that the 

 strata in place are hid from view. However, at the head of the 

 estuary of this creek, a fine wide brook makes its way out to 

 the broader stream and opens an avenue into the body of the 

 hill. This active little rivulet rises high in the ridge, at a 

 distance of more than half a mile inland, cuts its way deep into 

 the strata, descends by stages, bends at two or three intervals, 

 and rapidly increases its width after having passed about half 

 way down the hill. The ravine cut by this stream has abrupt 

 walls in the upper divisions, but these become much less steep 

 as they descend, until near the mouth they slope at an angle of 

 but little more than 45 degrees, and appear broken into separate 

 rounded hills, by local erosion in the rain-channels. At the 

 mouth, the ravine gapes widely, with reduced sloping sides, 

 and it is choked by a low delta, formed of the clay, sand and 

 loam frequently carried from above by overflows and freshets 

 in the stream. Thus the original wide trough of the mouth is 

 hid by the accumulation of earthy material which composes a 

 sloping marsh extending out to the bed of the creek. The lower 

 part of this ravine, up to about fifty feet above the level of 

 Swan Creek, is composed of the Cretaceous Lower Marl, cov- 

 ered and hid almost entirely by loam and sand washed from the 

 upper parts of the hill. 



