272 PROCEEDINGS OF THE ACADEMY OP [1880. 



Within the Upland Terrace, resting upon its slope, and extend- 

 ing to the river, is a series of stratified gravels and a boulder- 

 bearing brick-clay. Of these, the oldest is the " Fossiliferous 

 gravel ; " a gravel l^^ing near the terrace and under the brick-clay, 

 and containing pebbles which frequently are fossiliferous. Of more 

 recent age, and at a lower level, is the " Philadelphia red gravel," 

 which is made up of the pebbles of the Fossiliferous gravel 

 mixed with fragments of Triassic red shale and other rocks 

 brought down the Delaware Valley. It is distinctly stratified, 

 rests upon decomposed gneiss, and contains rounded boulders 

 dropped by floating ice. Upon both of these gravels rests the 

 Philadelphia brick-clay, often l^ang unconformably upon them in 

 a series of pot-holes or wave-like forms, and apparently an aque- 

 ous deposit. 



A yet more recent formation, the " River gravel and sand," 

 lies within the others and close to the river, and is made up of 

 flattened pebbles composed of the rocks over which the river flows. 

 Upon this, in the river flats, lies a modern mud, the " Recent Allu- 

 vium." 



Back of the Upland Terrace, isolated patches of two surface 

 deposits, more ancient than any 3ret described, lie upon the hills. 

 These are, the " Branchtown clay," at a height of 250 feet, con- 

 taining boulders of Potsdam rocks, but no traces of Triassic red 

 shale or of fossiliferous pebbles ; and the " Bryn Mawr gravel," 

 which caps hills of a higher elevation, and which, containing 

 boulders and pebbles of identical material with those of the last, 

 is characterized by the presence of a hard iron conglomerate or 

 sandstone. This conglomerate, occurring also in New Jersey, 

 and named the " Mt. Holly Conglomerate," is conjectured to be of 

 Tertiary age. 



In these seven formations is written the geologic history of the 

 Delaware Yalley. 



Much remains to be done before any certain results can be 

 expected. It is hoped that the imperfect examination here 

 recorded may form the basis for a future and more thorough 

 study, which, extending to wider fields, shall make more exact 

 the knowledge of our surface geology. 



