326 



SURFACE GEOLOGY, 



until the removal of the eccentricity of the orbit restored the warmer 

 conditions. 



Two suggestions respecting the probable origin of all the inter-glacial 

 deposits found in New Hampshire will not be out of place, i. An ex- 

 planation of Prof. Torell, of Sweden, is adequate to account for the 

 example underneath the lenticular hill in New Ipswich,* and any other 

 stratified beds in similar position. He represents that as the glacier 

 commenced to exist and to move extensively, there must accumulate in 

 favorable localities many stratified glacial deposits, so that a geological 

 section of the edge of the ice would present (a) pre-glacial beds ; (d) 

 stratified glacial deposits ; (c) a ground moraine ; (d) the ice with its 

 terminal moraine. Hence the advancing glacier may often cover strati- 

 fied sandy deposits not of inter-glacial origin. 2. Our sections indicate 

 that the stratified beds commonly occur between the lower and upper 

 tills, as in Figs. 29, 32, 34, and 36. The Champlain fossils occupy the 

 same position in the Portland section (Fig. 59). Assuming the correct- 

 ness of our views regarding the origin of the two tills, all the inter- 

 glacial phenomena are beautifully explained. It is not to be presumed 

 that no variations in the position of the edge of the ice-sheet existed. 

 The outer limit must have varied very much from time to time, just as 

 it does at the present day in existing glaciers. When the ice retreated 

 a few thousand feet, its melting would give rise to currents transport- 

 ing sand. A change in temperature, or other conditions causing a re- 

 advance of the ice-sheet, would cause the stratified beds to be covered 

 again, and the mass might even push a short distance over marine 

 deposits at the ocean border. But no ground moraine made its appear- 

 ance with this readvance. No deposits were left behind except the 

 debris contained in the ice itself, or upon its surface ; and this fell to 

 the ground during the melting process, and now remains as the upper 

 till. This distinction between the tills has been lately recognized by 

 English and Scotch authors. It also appears that the British inter-gla- 

 cial beds occupy the same position with ours between the two kinds of 

 till,! so that probably our explanation of the New Hampshire beds would 



* This is situated just south-west of Jeft's hill. By the side of the road an excavation shows about five feet of 

 sand beneath the glacial drift, probably the lower till. The total thickness of the sand is not seen. In the over- 

 lying drift I observed a boulder of porphyritic gneiss four feet in length. Between this point and New Ipswich 

 village the road cuts through a sand containing boulders, which is also probably underneath the glaciated till. 



t " 'I'he till or boulder-clay that rests above these intercalated beds usually differs from the stony clay which 



