264 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1916. 



I'he hiimmocky, moiitonnees surfaces left by the Pleistocene gla- 

 ciers on Archean rocks which have disordered structures and vary in 

 durability are very characteristic and were once looked on as the di- 

 rect handiwork of the ice sheets themselves. The clean and polished 

 surfaces of fresh rock, generally well stwated and often deeply 

 scored, are eloquent of the stripping and grinding of the glacier, but 

 the original surface forms have not been greatly changed, as will be 

 shown later. 



Most of the great Pleistocene ice sheets gathered on comparatively 

 low ground and reached sealevel, often occupying large areas of shal- 

 low sea-bottom as well as the land. Few of them began in mountain 

 regions, and the flow of those on level ground was caused by the 

 slope of the upper surface of the ice mass and not by the inclination 

 of the floor beneath. They could even move uphill for thousands of 

 feet, when the ice sheet was thick enough in the center, and their 

 flow took place outward in all directions. 



Doubtless conditions were similar in earlier glaciations, and it is 

 not necessary to assume great mountain ranges to account for them, 

 as some geologists have done. 



PERMOCARBONIFEROUS ICE AGE. 



The first undoubted proofs of ancient glaciation seem to have been 

 found by the Blandfords in India, and the first memoir of the Indian 

 survey (1859) contains a brief account of the Talchir tillite in central 

 India, illustrated b}'^ a rough sketch. Soon after South African and 

 Australian tillites of the same age were described. There was at first 

 a good deal of skepticism expressed by European and American 

 geologists as to the reality of the discoveries. Ramsay's interpreta- 

 tion of certain English bowlder conglomerates as glacial a few years 

 before had been disputed, which cast doubt on the new reports from 

 the far east and south. Was not the Carboniferous a tropical time, 

 even in the Arctic regions? Glaciers and the steamy coal swamps 

 did not mix well together. 



Since then, however, many northern geologists, including expert 

 glacialists, have studied these marvelous deposits, and for a number 

 of years no one has doubted their glacial origin, in spite of the fact 

 that most of the localities are in what are now warm, temperate, or 

 even tropical regions. All the evidences for the ice action on a large 

 scale found in our Pleistocene are repeated, with the difference that 

 the Pleistocene till ceases about 38° from the Equator, while the Tal- 

 chir tillite in India reaches well within the Tropics (18° North) and 

 Permocarboniferous tillite in West Australia touches the Tropics. 

 In South Africa the Dwyka tillite reaches 24° 30', or even 22°,^ and 



1 For literature see Glacial periods and their bearing on geological theori*, by the 

 writer. Bull. Geol. Soc. Am., vol. 19, pp. 347-366 ; and Schuchert : Climates of geologic 

 time. Carnegie Inst., Pub. No. 192, pp. 263-298. 



