The ice age in North America and its bearing upon the antiquity of man5th edwith many new maps and illus., enland rewritten to incorporate the facts that bring it up to date, with chapters on Lake Agassiz and the Probable cause of glaciation . areas of loess of great sizeall over northwestern China which indicate deposition bywater. The same is true of the loess in the ]\Iississippi and Mis-souri vallej^s. While it is doul)tless true that in many placesthe accumulations are due to wind, it is equally evident thatthere are many level-topi^ed areas of loess along the bordersof the Missouri which
The ice age in North America and its bearing upon the antiquity of man5th edwith many new maps and illus., enland rewritten to incorporate the facts that bring it up to date, with chapters on Lake Agassiz and the Probable cause of glaciation . areas of loess of great sizeall over northwestern China which indicate deposition bywater. The same is true of the loess in the ]\Iississippi and Mis-souri vallej^s. While it is doul)tless true that in many placesthe accumulations are due to wind, it is equally evident thatthere are many level-topi^ed areas of loess along the bordersof the Missouri which betoken the height reached by theoverflowing floods of the stream when, as already shown, itwas gorged by silt-laden water from the melting ice in theclosing stages of the glacial period. Both wind and waterhad their share in the distribution. The absence of true THE LOESS. 421 water species of shells in the loess is what we should expectin an accumulation which took place in periodical or annualoverflows. Land snails still love the flood-plains of variablestreams. In the vicissitudes of the last stages of the glacialperiod we nave the temporary presence of water provided forto suit the habits of such shells as are found in the Fig. 120.—Deep gulley in the loess at Helena, Arkansas,vey. Photo, by A. F, Crider. Courtesy of U. S. Goo. Sur- CHAPTER XVII. FLIGHT OF PLANTS AND ANIMALS DURING THE GLACIAL AGE. Among the most interesting incidental effects of the Gla-cial period is that of its influence in distributing plants andanimals over the lower latitudes. A glance at a polar pro-jection of the northern hemisphere shows to what a remark-able extent the land is clustered around the north pole, andhow easy it would have been, under favorable conditions ofclimate, for plants and animals to spread from that vicinityalong different meridians, till, in the lower latitudes, theyshould be on opposite sides of the earth. In conformitywith these natural lines of emigration, it l
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