THE REIGN OF ICE. 273 



on the unglaciated area. This resulted partly from the south- 

 ward growth of the glacier, and partly from its southward 

 motion. In its motion, it prostrated the standing forest, and 

 the ruins were mingled with the rock-ruin which the ice-mass 

 stirred and transported. The march of the glacier's southern 

 border continued until it reached New York, Louisville, St. 

 Louis, and Topeka. It passed over Long Island Sound and 

 reached the ocean shore. Westward, over the Great Plains, 

 its footmarks are not traced. Perhaps the precipitation there, 

 as now, was insufficient to enable the snow to outlast the sum- 

 mer heat. Further west, the glaciation seems to have been 

 restricted to mountain ranges. But glaciation was far from 

 unknown, even to the Pacific slope. Northward, the extent 

 of the glaciation increased, as far as Alaska. 



Now the geologic winter was marked by an "'open" spell. 

 It probably lasted for centuries. The ice dissolved, and the 

 border of the glacier retreated perhaps, to the latitude of 

 Marquette. Over the uncovered area, it was a new spring- 

 time. Vegetation sprang into existence, and a fresh soil 

 accumulated. Then came a recurrence of cold. The old 

 glacier resumed its southward movement. In saying it was a 

 "continental" glacier, it is not meant that an ice-field conti- 

 nent-wide moved with a consentaneous movement. The ice- 

 sheet felt the influence of the underlying topography. Its 

 motion tended everywhere to the lower level and warmer 

 situation. Seldom was that direction precisely south. In the 

 valley of the Connecticut, the movement was south. In the 

 valley of the Mohawk it was eastward. Through the valley 

 now occupied by Lakes Ontario and Erie, it was south-west 

 as far as Indianapolis, and south as far as Columbus. Another 

 glacier stream flowed through the valley destined to be the 

 basins of Lake Huron and Saginaw Bay. Its southern border 

 joined the Lake Erie glacier ; and the long broken chain of 

 sand and bowlder hills passing through Ann Arbor, shows 

 where the joint rubbish of the two glaciers was left. Another 

 glacier stream passed along the valley of Lake Michigan; 

 another down the valley of Green Bay and its continuation to 



