SEDIMENTS TRANSPORTED BY GLACIAL STREAMS. 87 



this fact much more plainly than the existing maps do. Where these gravel 

 ridges cross a level and swampy region, they often form a remarkable fea- 

 ture of the landscape. In many cases they form natural roadways across 

 the swamps and have been utilized for this purpose by both Indians and 

 whites. When an ex^Dlorer has followed one of these great embankments 

 for 50 or 100 miles, crossing rivers and valleys, climbing over hills, now 

 skirting hillsides far above the valleys, now meandering across a plain where 

 nothing now exists to cause meanderings, and bending abruptly in order to 

 penetrate some low pass — by the time he has seen all this and noted how, 

 within certain hmits, these gravel systems disregard the surface features of 

 the land, he will be ready to admit the utter impossibility of accounting for 

 the existence of water-rolled gravels in such situations by any form of 

 fluviatile, marine, or lacustrine agency, or by any known means except by 

 streams confined between walls of ice that have now disappeared. 



Sizes and lengths. — The uan'ow two-slded ridges are sometimes barely 3 

 feet high and three or four times as broad, and all sizes exist up to 100 or 

 more feet high, with corresponding breadth. The broader ridges or plains 

 vary in height to a maximum of about 150 feet. The deepest kettlehole 

 measured was about 100 feet in depth. Many of the ridges are barely 

 wide enough for a road on the top, while massive plain-like ridges are found 

 which are from one-eighth mile to more than a mile wide. The plains of 

 reticulated ridges are sometimes 3 or 4 miles wide, and the marine delta- 

 plains are still broader. Where the gravels, when mapped, are plainly 

 seen to be arraug-ed in lines along routes that do not cross very high hills, 

 they are assigned to the same system. The gravels of a single system are 

 supposed to have been deposited by a single glacial river. The gravel is 

 not continuous throughout the course of a system. Sometimes the gaps 

 are due to erosion of the gravel, bi;t more often they are due to failure of 

 the glacial river to deposit gravel throughout its whole course. The gaps 

 are usually less than one-half mile across, but in some cases 2 or 3 miles. 

 When gravel deposits are separated by such long gaps, I have never assigned 

 them to one system without special proof according to the principles laid 

 down here and elsewhere. Several of the systems are 100 or more miles 

 in length. 



Branchings. — Tlie brauclies of the longer gravel systems ma}?- be classified 

 as follows: (1) Tributary branches. The map shows that many of the 



