178 SURFACE GEOLOGY. 



have been written by several geologists upon these subjects, which can 

 be consulted by those desiring further information. It is obvious that 

 our first duty is, to state whatever facts have been observed, and then 

 discuss the general bearings of the subject, if an opportunity is pre- 

 sented. It may be premised, however, that the glacial theory of the 

 origin of the cold and of the dispersion of the materials seems to explain 

 all the phenomena better than the older view of the agency of icebergs. 

 Any complete discussion of the phenomena must relate partly to the 

 effects produced upon the ledges by the ice-movement, and partly to a 

 description of the materials transported, their position, shape, size, 

 amount, etc. We will first speak of the action upon the ledges. The 

 ledges have been broken, rounded, or embossed, planed down, smoothed, 

 and striated. The formation of pot-holes took place after the ice began 

 to melt. 



Fractured Ledges. 



Since the Helderberg period, the rocks of New England had been 

 subjected to sub-aerial decomposition, whereby they were softened and 

 rendered friable to great depths, often as low as the water level, or as 

 much as a hundred feet. The change was mostly chemical, consisting 

 of the removal of the alkalies, and the disappearance of lime and magne- 

 sia, by solution, the residue being clayey. Hence the ledges were in 

 excellent condition for removal by the ice-sheet. Enormous quantities 

 would be easily rubbed off, and then assorted by water. Besides the 

 decomposed rocks, the ice removed perhaps as great a mass of the solid 

 portions, which now constitute boulders and the pebbles of the till. Still, 

 this ice action does not represent all the erosion that has taken place in 

 our state, as may be proved by calculations of the mass that has been 

 removed from the ledges to fill up gaps in the strata. It has been gen- 

 erally estimated that this amounts to as much as the average height of 

 the land above the sea, or 1,200 feet in New Hampshire. The condition 

 of the surface in the Southern states, in Brazil, and in certain sheltered 

 spots in Minnesota and Massachusetts, as described by Hunt, Hartt, 

 White, and others, illustrates the nature of the land surface with us, be- 

 fore the ice acted upon it. 



In the earlier New England reports, several cases of ledges fractured 



