GLACIAL DRIFT, 



283 



He will shortly present the results of this examination, and color upon 

 the map of Surface Geology their geographical positions. No feature 

 of our drift moraines is so striking as this, and it is singular that pre- 

 vious authors have almost universally overlooked or misunderstood it. 



The question has been put to us, As New Hampshire has not been 

 submerged since the Helderberg period, and there may have been other 

 periods of cold besides the one called par excellence the glacial drift, why 

 do we not find moraine accumulations of the earlier ones .? I think we 

 have abundant evidence of a Triassic glacier in Massachusetts, formed 

 of materials partly derived from New Hampshire. The stones of the 

 Mt. Mettawampe conglomerate are too coarse to have been moved by 

 water alone, and the stones have a glaciated appearance. As there 

 seem to be no rocks in our state analogous to the Triassic conglomer- 

 ates, we may say, with assurance, that if any glaciation occurred previous 

 to the post-tertiary, it could not have antedated the New Red Sandstone. 

 It seems probable that Tertiary glaciated beds would be characterized by 

 features quickly discernible, and not easily confounded with anything 

 else earlier or later. 



But certain beds are brought to our notice, which seem to antedate 

 the lower till. The best known is represented in Fig. 60. A railroad 

 cut in South Lyndeborough, 

 two miles west of the station, 

 exhibits three layers in the till. 

 The top is the familiar loose 

 ferruginous earth, such as uni- 

 versally covers the ground- 

 moraine. Next, b, is a good 



example of the lower till, full of glaciated pebbles, porphyritic and granitic 

 gneisses, mica schist, etc., 5 feet, and in one case 6 feet long. The lam- 

 inated appearance arising from compression is clearly defined. Beneath 

 this is a coarser mass, reaching to the bottom of the cut, so very com- 

 pact that a pick had no effect when struck into it by the workmen ; only 

 gunpowder or a stronger explosive could excavate it, and it was neces- 

 sary that the holes should be bored horizontally near the surface to be- 

 come effectual in removing the earth. There is nothing visible in the 

 earth itself different from the lower till above it, save that the compo- 







Fig. 60. — Section in Till, Lyndeborough. 



a. Upper till; b. Lower till; c. Hardpan. 



