QUATERNARY AGE. — GLACIAL PERIOD. 539 



and earth in its under surface; and (2) a tearing and scooping ac- 

 tion, dependent on the pushing action of the ice against weakly co- 

 herent beds, and against ledges of jointed or laminated rocks. With 

 a weight of 450 pounds to the square inch for 1,000 feet of thickness, 

 it pressed down into all depressions, filling the basins of lakes, the 

 trough of Long Island, now 150 to 180 feet deep, and the crevices in 

 the exposed rocks ; and, although the rate of motion could hardly have 

 exceeded a foot a day, and may have been in most parts no more than 

 a foot a week, it worked with great power, planing off and furrowing 

 the harder rocks, excavating deeply the softer, and tearing masses 

 from yielding ledges and rocky summits which lay beneath or pro- 

 jected into it. A broad abrupt hill sometimes saved from abrasion the 

 soft deposits south of it, because of the great arched cavity or notch it 

 made in the ice ; as, near New Haven, Conn., where a ridge of weak 

 sandstone a mile long and 100 to 140 feet high was left under the lee 

 of a trap ridge. The material worn off or loosened was taken into 

 the glacier, and the lower 500 feet of the ice may have contained the 

 most of it. As the glacier moved on, adjusting itself to its uneven 

 bed, the ice-mass was throughout in motion, and the stones were con- 

 sequently ground against one another, their edges rounded, and peb- 

 bles, sand, and earth for the bowlder clay, were made. 



Moreover, since the snows of the commencing Glacial period fell 

 over a continent of great forests, trees were everywhere rooted up 

 or broken off with the first motion of the ice, and afterward partly 

 ground up ; and finally, if not wasted by decomposition, deposited 

 with the Drift, — some portions, perhaps, in beds of vegetable mate- 

 rial, and others as scattered logs, stems, and roots. Land and fresh- 

 water shells also would have been gathered up for transport and dis^ 

 tribution. 



The valleys of the continent owe their depth to erosion by the 

 streams flowing in them. Much of the excavation was done in the 

 Glacial period, partly by the direct action of the glacier, but vastly 

 more by sub-glacial streams, laden with debris from the glacier. This 

 excavation was carried much deeper in very many cases than could 

 have been done with the continent at its present level. Dr. Newberry 

 states that all the river valleys of Ohio are examples of this ; that the 

 valley of Beaver River is excavated to a depth of 150 feet below the 

 present river level; that of Tuscarawas River at Dover, 175 feet; 

 that of the Ohio River, much deeper, 1 00 feet of boring near Cincin- 

 nati not reaching the bottom of the alluvium. Such facts are evidence 

 of erosion at some period when the continent was more elevated than 

 now, and are attributed by many to the agencies of the Glacial era. 

 The remarks on fiords on pages 533 and 540 are in further illustra- 

 tion of this subject. 



