28o SURFACE GEOLOGY. 



factory, so that the mutual relations of the three deposits, as displayed 

 in Fig. 59, may be regarded as fixed beyond controversy. The massive 

 beds overlie the lower till, and are covered by the upper till. These 

 facts indicate the deposition, first, of the ferrous glaciated till; second, 

 its submergence to at least 100 feet below the present level; third, the 

 readvance of the ice-sheet so as to cover the Champlain beds ; fourth, 

 the melting of the ice, and the falling down of the debris held in suspen- 

 sion in it. The formation of the upper till does not necessitate a sub- 

 mergence, as I have insisted in previous publications. That these posi- 

 tions may seem well sustained, I will state the most important facts 

 observed about Portland. 



The city is situated upon a promontory, or, practically, an island, with 

 a north-east trend parallel to the coast. The island has two elevations, 

 150 and 160 feet above tide water, and at the remoter ends the extreme 

 edges slope very sharply to the water's edge. The eastern elevation is 

 Munjoy's, and the western Bramhall hill, and between the land sinks to 

 60 feet along the lowest ridge. These two hills consist of the two kinds 

 of till; and each is environed by the Champlain deposits, which cover 

 most of the lower area in the middle. These attain an elevation of about 

 100 feet; and the ocean must have stood a few feet higher, unless the 

 character of the fossils in the lower clays — pelagic forms — necessitated 

 a submergence of 300 feet. In that case, Munjoy's and Bramhall hills 

 would have been deeply buried by the waters. 



Munjoy's hill has been excavated in a multitude of places, showing an 

 upper till, from 3 to 10 feet thick, as clearly defined and distinct from the 

 lower deposit as at Boar's Head. Boulders from 3 to 4 feet in diameter 

 occur upon the surface, and are of the gneissic, granitic, and schistose 

 rocks common from four to ten miles to the north-west. Fig. 59 repre- 

 sents a cutting between North and Washington streets, and may be 

 taken as a sample of excavations on that side of the ridge. Along North 

 street for half a mile is a continuous exposure of the two kinds of till. 

 Upon both sides is a cut 25 feet deep. The larger stones are all under 

 or at the surface, but they are not striated. The lower deposit is com- 

 pact, the stones of small size, and all glaciated and transported a greater 

 distance than those above. I found pebbles of the White Mountain por- 

 phyries and sienites among them, indicating a carriage of 50 miles. The 



