Geological Papers. 87 



gravel, and boulders — which has been deposited over portions 

 of the earth's surface, mostly in the higher latitudes." 



Gumbo. — This formation is the stratified portion of the 

 lower till of the Mississippi valley. It is a granular, adhesive 

 clay, often several feet in thickness. It is not such a con- 

 tinuous deposit as the overlying loess, there being many places 

 where the loess rests directly upon typical till. Like the loess, 

 though, it seems to be independent of contour lines in its dis- 

 tribution. Its color varies from light gray or ash to nearly 

 black. The black portions are heavily charged with humus 

 and in places present the appearance of swamp muck. It is 

 from this clay that the black soil so often seen at the base 

 of the loess is usually developed. This gumbo clay contains 

 a few pebbles, much less than the typical till or loess. Ac- 

 cording to Mr. Leverett, these deposits may be of aqueous 

 origin; but such an hypothesis cannot be confidently put for- 

 ward as a solution. 



Drumlins. — A drumlin is a smoothly rounded hill. Mounds 

 or hills of this sort are found all over the glaciated region, 

 from Maine to the Rocky Mountains. Their origin is in doubt. 

 Prof. G. F. Wright' and others believe that during the final 

 melting of the ice the surface would melt unequally, since 

 the large boulders and deeper masses of till would partially 

 protect the ice beneath them from melting, and that, conse- 

 quently, there would be much lateral sliding of till into the de- 

 pressions thus formed on the ice surface, which, when the 

 glaciers disappeared, would remain as drumlins. But it is 

 difficult to conceive how smoothly rounded hills in such large 

 numbers and such great size could result from this process. 

 Moreover, some of the masses of thoroughly glaciated matter 

 are long ridges parallel with the glaciation, masses still more 

 difficult of explanation as being due to accumulation in surface 

 hollows of the ice. 



Prof. N. S. Shaler believes that these hills are the remains 

 of a former sheet of till of which the greater part has been 

 eroded by the sea waves, but to this opinion there are many 

 objections: First, these deep masses of till are sometimes one 

 mile or more from any similar mass. The amount of erosion 

 required is enormous. Second, had a great mass of till been 

 eroded, most of the larger stones would now remain as broad 



1. Wright, Ice Age of North America. 



2. Bull. Geol. Soc. Am., vol. 10; also Shaler and Davis, Glaciers. 



