260 PROCEEDINGS OF THE ACADEMY OF [1880. 



patches of an ancient gravel on high points back of it, the Upland 

 Terrace nevertheless remains as the most important geological 

 feature in sontlieastern Pennsylvania. 



Between the Upland Terrace and the Delaware, claj^s and gravels 

 cover the rocks in a continuous sheet except where eroded away 

 in the neigliborhood of streams. The amount of tlieir erosion is 

 in some respects a measure of the age of the surface formations. 

 It has been noticed that these foi'mations in the vicinit}' of Phila- 

 delphia have undergone very different amounts of erosion, the 

 amount of such erosion increasing as we recede from the Delaware ; 

 and this fact is regarded as offering evidence that the deposits are 

 of different ages ; those lying farthest from the river and highest 

 in elcA^ation being the most ancient, and those which are close to 

 the river, which have undergone but little erosion, being the most 

 modern of our surface formations. Examples of erosion of the 

 Philadelphia gravel may be well seen on the Philadelphia and 

 West Chester Railroad which crosses a number of creeks and runs 

 nearlj'' parallel to the terrace for several miles. As each creek is 

 approached the drift ' disappears and rocks come to the surface. 

 So on the Schuylkill, no gravel is seen on the river drive in the 

 East Park, but upon going back from the river and rising 100 feet 

 above it, as far as the East Park Reservoir, gravel appears abun- 

 dantly. Yet on the same riA^er, nearer the Delaware, a newer 

 gravel, made of different materials, not only forms its banks but 

 underlies it. 



Recent Alluvium. — The most recent of all the surface deposits 

 is the stiff bluish clay which covers the low ground in the southern 

 part of the city. The Richmond meadows and the flats of Moya- 

 mensing, Greenwich and Tinicum are covered by this deposit. It 

 is bounded by a low terrace which may be called " The Floodplain 

 Terrace.'''' This terrace, up to which the river often comes in 

 times of flood, crosses South Broad Street diagonall}^ below Moya- 

 mensing Avenue, and crossing the Delaware extension of the 

 Pennsylvania Railroad near Penrose Ferry Road, winds around 

 Point Breeze Park back towards the Gas Works, and passing 

 below Suffolk Park crosses into Delaware Count3\ This terrace 

 is about ten feet above mean tide. It is the lowest and newest of 

 all the terraces and is formed of the next older formation, the 

 " River gravel." The mud or clay lying between this terrace and 



' The term "drift" here includes all superficial formations of whatever age. 



