OUTWASH FROM THE OLDEST DRIFT. 247 



From near Allegheny cemetery the gravel passes to the west side of 

 the river at "Mount Troy" in the east part of Allegheny, where a narrow 

 remnant of the old gradation plain is preserved. It also covered "Monu- 

 ment Hill" in Allegheny, an island-like remnant of the gradation plain, but 

 nearly all the gravel has now been removed from this hill. 



In closing this discussion of the Allegheny Valley a few paragraphs 

 are selected from Jillson's paper which contain levels taken in the vicinity 

 of Pittsburg.-^ The altitudes were in all cases obtained by Locke hand level 

 from the nearest railway track. ' The base is the city datum, 698.43 feet 

 above mean tide at Sandy Hook. 



In describing the terraces belonging to the Pittsburg group we will begin with 

 "Monument Hill." This hill stands on the north side of the Allegheny River, and 

 is a typical "hog's back," 1,500 feet long and exactly 200 feet high. On its top is a 

 thin layer of gravel, in which several pieces of granite have been found, one 2 inches in 

 diameter. "Mount Troy" rises abruptly from the north bank of the Allegheny, It 

 is 209 feet in height, a mile and a half in length, and throughout is as level as a floor. 

 Its top is covered with foreign gravel, and at the west end the water basin was exca- 

 vated from a mass of coarse gravel and cobblestones. In the Allegheny cemetery 

 three terraces can still be seen, though much changed by necessary improvements. 

 Entering the cemetery from Butler street by the old gate and passing up the stone 

 steps to the right, we find a terrace 120 feet above our base line. This terrace can be 

 easily followed along the east side of Butler street through the United States Arsenal 

 grounds to Penn avenue. At the corner of Davison and Forty-sixth streets its 

 height is 134 feet; in the arsenal grounds it is 125 feet, and near the junction of 

 Forty -second and Sherman streets, 158 feet. 



In the upper part of Allegheny cemetery is an immense mass of sand and 

 gravel, the highest point of which, near the monument erected to the memory of 

 those killed b}^ the arsenal explosion in 1S62, is 250 feet. The character of this 

 deposit is well shown just back of the receiving vault, where the sand has been 

 removed, exposing a perpendicular section 25 or 30 feet in height and some 75 feet 

 in length. The base of this section is 213 feet above our base line. No outci*op 

 of rock is seen in the immediate vicinitj', but not a great distance oil a layer of shale 

 was found in situ 22 feet below it. Whether this shale is at the top of the rocky 

 bed I have at present no method of determining. At Geneva and Main streets this 

 shelf is 200 feet in height; at Liberty avenue and Fortieth streets, 206 feet. The 

 bed of sand and gravel extends from the cemeteiy across Penn avenue into the East 

 Liberty Valley at Bloomfield, and, like other beds found at this height, consists of 

 stratified sand, gravel, cobblestones, bowlders, and angular fi-agments, many pieces 

 being of material not properly belonging to the rocks of this vicinity. The top of 

 this bed at Pearl street, where it crosses Penn avenue, is 259 feet; at Forty-fifth 

 street, 256 feet; at Rebecca street, 274 feet. At Thirty-third street, just above the 



'Op. cit, pp. 6-10. 



