North American Interglacidl Deposits. — Uiihain. 279 
drift and bearing a luxuriant growth of coniferous forest 
trees up to three feet in diameter, dense thickets of shrubs, 
and many herbaceous flowering phints, the occurrence of such 
temperate species of plants and animals in interglacial beds 
was commonly accepted as proof of the retreat of the ice- 
sheet tea great distance and an ensuing long reiidvance. It 
is now seen, however, that ixny climatic change producing a 
reiidvance of the Malaspina ice-sheet, though onl}' enduring 
through a few years or decades and sending the border for- 
ward to any small extent sutiicient to cover portions of its 
marginal flora and fauna, would be recorded by temperate 
species enclosed between deposits of glacial drift.* Many 
of the features of the Pleistocene drift formations in our 
northern states, especiall}^ of the eskers and stratified valle}" 
drift, indicate that the recession of this part of the North 
American ice-sheet, under the warm Late Glacial or Cham- 
plain climate, was more rapid than the present retreat of the 
Malaspina and Muir glaciers, the contrast being probabl}^ as 
great as between the present mean temperature of the Alas- 
kan coast region and that of the upper Mississippi and the 
Lauren tian lakes. 
3Iinnesota mid Iowa. — Remnants of an interglacial surface 
soil, trunks and limbs of trees, deposits of peat, and stratified 
sand and silt containing fresh-water molluscan shells, at one 
or several definite horizons, between sheets of till or unmodi- 
fied glacial drift, are shown by wells and by borings for oil 
and gas upon an extensive area which stretches from Ohio 
westward through Indiana and Illinois to Iowa, and thence 
northwestward through southern Minnesota into South and 
North Dakota. 
The most elaborate stud}^ given to an}' part of this area is 
by McGee in northeastern Iowa, where the forest bed, from 
one to five feet in thickness, is found to be practicall}^ con- 
tiniu)us on tracts ten to twenty miles or more in extent, oc- 
curring commonly at the junction of distinct sheets of lower 
*More extended comparison of the iVlaskan, Greenland, and Antarc- 
tic ice-slioets with those of the Glacial period in North America and 
P^urope is presented in the Bulletin of the Gcol. Societv of America, 
vol. IV, 189:j. pp. HJl-:>04. 
