GLACIAL EROSION AND TRANSPORTATION 241 



that ic is not easy to single out a particular train. Careful 

 attention, however, will doubtless resolve the whole mass of 

 till into confluent trains of bowlders and more finely com- 

 minuted material. 



In New Jersey, according to Professor Cook, the bowl- 

 ders are readily traced all along the morainic margin as be- 

 longing to well-known outcrops of trap, blue limestone, and 

 crystalline rocks to the northwest. Near Drakestown, in 

 Morris county, there is a mass of blue limestone which had 

 been worked for years as a quarry without suspecting that it 

 was but a bowlder. "As exposed it measures thirty-six by 

 thirty feet, and the quarrying has gone twenty feet in depth. 

 Its vertical diameter is unknown. Around it are many 

 gneissic bowlders and other drift materials.'' * This mass is 

 about one thousand feet above the sea-level, and its native 

 place must have been some miles to the northwest. 



In Pennsylvania the distance from which the glacial ma- 

 terial near the border of the glaciated region has been trans- 

 ported becomes at once more evident, because of its unlike- 

 ness to the local rocks. West of the Kittatinny Mountain, 

 there are no crystalline rocks within the State. Nor are 

 there any to the north nearer than the Adirondacks in New 

 York, or the highlands in Canada. Yet granitic, gneissoid. 

 and hornblendic bowlders abound all along the glaciated 

 border, and are an important means of determining the 

 glacial limit. In the valley between Kittatinny and Pocono 

 Molid tains, in Monroe county, and on the summit of the 

 Pocono plateau, 2,000 feet above the sea, granitic bowlders 

 from one to three feet in diameter are abundant, though 

 mingled with great piles of local fragments. The granite 

 must have been transported a distance of 250 miles at least, 

 and carried over the summits of the Alleghanies, intervening 

 toward the northwest, and across the valley of the Mohawk 

 in New York. The northern tributaries of the West Branch 

 of the Susquehanna, likewise, bring down into that stream 



* New Jersey Report for 1880. 



