34 GLACIAL GRAVELS OF MAINE. 



position and shapes beneath the ice as a ground moraine proper, while the 

 upper part of the till, of less com23act structure, less marked glaciation, 

 and containing the largest bowlders, is composed of matter which was 

 distributed throughout the lower portion of the ice. The classification of 

 the till into a lower and an upper member, early adopted by Professor 

 Hitchcock in the New Hampshire geological reports (substantially that 

 proposed by Torell), seems to have a basis in fact. At one time I thought 

 it possible to distinguish in the field between the ground moraine and the 

 upper till, but subsequent observations have shown many places where this 

 is difficult, if not impossible. Indeed, it appears probable that the two 

 formations often blend with each other, so that there is no sharp line of 

 demarcation between them. 



It is well known that in the Mississippi Valley there are two or more 

 layers of till separated by strata containing peat and other traces of a 

 warm interglacial period. No such signs of two general glaciations have 

 yet been fotmd in Maine. The few facts that indistinctly point that way 

 seem as yet to be capable of other interpretations, although during the 

 final melting there may have been alternate reti'eat and advance near the 

 ice m-argin. 



SEDIMENTS TRANSPORTED BY GLACIAL STREAMS. 



These deposits of water-assorted drift have attracted attention all over 

 the world wherever they are found. Their most obvious characteristics are 

 the following: 



External forms of deposits. — Tlic siuiplcst fomi is that of a coiie, dome, or 

 hummock, and we find all transitions between these forms and the elon- 

 gated, two-sided ridge. When enlarged on all sides, the dome becomes a 

 rather round plain with flattish top. The single ridge may fork into two 

 ridges, which soon come together again, thus inclosing a basin or kettle- 

 hole, which not infrequently contains a lakelet; or it may divide into a 

 large number of branches which are themselves connected by transverse 

 ridges, the whole forming a plexus of ridges inclosing depressions of all 

 shapes. ISuch networks have been called reticulated ridges by Prof. N. S. 

 Shaler. The depressions inclosed between these ridges are of various 

 shapes and have received many names, such as basins, sinks, funnels, kettle- 

 holes, punch bowls, hoppers, Roman theaters. 



