32 THE PHYSICAL FEATURES OF PRINCE GEORGE S COUNTY 



But while the Miocene strata of Prince George's County contain 

 few shell beds thej are especially rich in microscopic fossils. Dia- 

 tomaceous earth from the vicinity of Piscataway and Nottingham 

 has been studied by many investigators and many species have been 

 described. Ehrcnbcrg in 184-i, and Bailey in 18-i-i-5 and 1849, 

 described microscopic organisms from Piscataway and specimens 

 from diatomaceous earth near jSTottingham have been described by 

 Johnston, !N'orman, Tyson, and Woolman. Heilprin in 1884, and 

 Dall and Harris in 1892, summed up all the information then avail- 

 able concerning the distribution, age, and contained fossils of the 

 Miocene. In the Washington Folio Darton gave detailed informa- 

 tion of the Miocene of the Washington quadrangle. The latest and 

 most complete work on the Miocene of this section is contained in 

 the Miocene volume of the Maryland Geological Survey published 

 in 1904. In this volume Clark and Shattuck describe the deposits 

 and their classification. Dall discusses their correlation, while sev- 

 eral specialists in various groups of animals and plants describe 

 the fossils contained in the strata. 



Tlie Pliocene (?) and Pleistocene. — The Pliocene ( ?) and Pleis- 

 tocene deposits until comparatively^ recently have not been clearly 

 differentiated hence the necessity of including them in the same 

 section in discussing the literature. 



In his report for 1834, Ducat el mentions the occurrence of gravel 

 on the top of the Fort Washington hill, which, so far as known, is 

 the first reference to the surficial deposits of Prince George's County. 



In an article published in 1852 Desor speaks of the boulders found 

 about Washington and says they have been carried there by the 

 Potomac River. Their origin is traced to regions west of the Blue 

 Ridge as is shown by the fossils which they contain. In 1877, 

 Rogers described the gravels from the same vicinity. He states the 

 materials to be frequently stratified and that some of the gravels 

 bear Potsdam fossils. The origin of this material is due either to 

 valley glaciers during the Ice Age or to swollen streams. He dis- 

 cusses both hypotheses without reaching any definite conclusion. 



Chester in 1885, advanced the idea that the gravel areas found 

 about Washington were continuous from Viro'inia to Xew Jersey, 



