File:Physical features of the Des Plaines Valley (1909) (14595635419).jpg

Identifier: physicalfeatures00gold (find matches)
Title: Physical features of the Des Plaines Valley
Year: 1909 (1900s)
Authors: Goldthwait, James Walter, 1880-1948
Subjects: Physical geography -- Des Plaines River Valley (Wis. and Ill.) Geology -- Des Plaines River Valley (Wis. and Ill.)
Publisher: Urbana : University of Illinois
Contributing Library: University of Illinois Urbana-Champaign
Digitizing Sponsor: University of Illinois Urbana-Champaign

View Book Page: Book Viewer
About This Book: Catalog Entry
View All Images: All Images From Book
Click here to view book online to see this illustration in context in a browseable online version of this book.

Text Appearing Before Image:
the mountains of northern New York and ofNew England. We may get some idea of its thickness over the DesPlaines basin by measuring the distances out to the border of theglaciated area and assuming a surface slope in this distance, equal tothat of the Greenland ice cap. When the ice had its greatest extent inIllinois (in the Illinoian epoch) its outer edge was more than 300miles south of the Des Plaines district (see Fig. 6). If the averageslope of its surface at that time was thirty feet per mile, i. e., about thesame as that of the interior oi; the Greenland glacier, the thickness jf 28 THE DES PLAINES VALLEY. (BUI.T.. NO. 11 ice over the Dos Plaines basin was some 9,000 feet. It seonis unlikely,liowever, that such a steep slope as that would continue so far from theice maririn. Nansen, in his trip across Greenlanrl, found that on the westside (where the slope was gentler than on the east) the surface of theice cap rose to 6,600 feet in the first seventy-six miles, and after that at
Text Appearing After Image:
Fio. 6. Moraines and other limit of drift in Illinois. After Leverett. U.Geol. Surv.) GOLDTHWAIT.J - SURFACE OF BED ROCK. 29 an average rate (constantly decreasing) of twenty-six feet per mile.If we assume the same rate for Illinois, we get 6,600 feet for the firstseventy-six miles back from the ice border, and 5,850 feet for the 225,miles following, or a total thickness over Joliet and Chicago of over12,000 feet. There is reason to believe, however, that the Greenlandice sheet covers a high mountainous region from which there is a rathersteep descent to the sea; and that the slope of the surface of the icesheet is steeper on that account than it would be over a flat region likeIllinois. So far as this is true it should lead us to reduce the estimateof thickness accordingly. We may reasonably believe, however, thatduring the Illinoian ice invasion the ice sheet was several thousandfeet thick over northeastern Illinois. During the last glacial epoch, how-ever, the ice advanced only a s

Note About Images