90 THE GEOLOGY OF PKINCE GEOKGE's COUNTY 



The Patapsco Formation. 



The Patapsco formation received its name from Patapsco River, 

 in whose valley it is typically exposed. 



Areal Distribution.- — The Patapsco has a more extended develop- 

 ment within this county than either of the two preceding formations. 

 Its outcrop is confined to the northwestern portion of the county, 

 extending from Anacostia northeastward to the boundary and occur- 

 ring in narrow bands near the headwaters of a few of the streams 

 south of Anacostia. To the southeast of this outcrop it is supposed 

 to extend over the entire county, underlying all the formations of 

 later age. In its wider distribution the Patapsco formation has been 

 recognized in discontinuous outcrops from the valley of Schuylkill 

 River, near Philadelphia, to the valley of the Rappahannock, in 

 Virginia. 



Character of Materials. — The Patapsco formation is composed 

 chiefly of highly colored and variegated clays, interbedded with 

 sandy clays, sands, and gravels, the materials of different kinds 

 grading into each other both horizontally and vertically. In many, 

 places the arenaceous material in the vicinity of clay beds is indu- 

 rated to a conglomerate or rough, irregular, pipelike concretionary 

 mass called "pipe ore." The variegated clays exhibit a great variety 

 of rich and delicate tints in irregular patterns. In places they grade 

 downward or horizontally into massive clays of chocolate, drab, and 

 black tones, locally carrying lignite and pyrite and in some places 

 containing iron ore and leaf impressions. The sands, which are 

 very commonly cross-bedded, here and there carry decomposed grains 

 of feldspar and pellets of white clay. A red ocher that is locally 

 known as "paint rock" or "paint stone" is not uncommon, and 

 limonite with botryoidal surfaces is found at various horizons. 



Paleontologic Character. — The Patapsco formation contains a 

 rich flora of ferns, cycads, conifers, monocotyledons, and dicotyle- 

 dons. The dicotyledonous plants still constitute a minor element as 

 compared with the other types of vegetation represented. The fauna 

 of the Patapsco consists of a few molluscan shells and a single dino- 



