Annals of the Association of American Geographers Volume 12 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1922 Excerpt: ...Sporobolus heterolepis, Cristatella Jamesii, and Callirrhoe triangulaia can be adequately explained only by assuming a former period of climate sufficiently drier than the present to permit their continuous migration. This, with the existence of relic prairie colonies at the east and the prevailingly xerophytic nature of the relic boreal colonies at the west, seems to be sufficient evidence of the actuality of a post-Wisconsin xerothermic period. - Jennings, Otto E. A botanical survey of Presque Isle, Erie County, Pennsylvania. Annals Carnegie Mus. 5: 289-421. 1909. The eastward advance of the prairies was either accompanied or followed by some specific evolution among the species participating, as indicated by such plants as Phymosia remota," Synthyris Bvilii, and TetmwRuris herbacea. These plants are now confined to the eastern arm of the Prairie Province, but in each case have their nearest related species much farther west. There is no reason to suppose that the xerothermic period came to a sudden close, which would require an equally sudden geological event in explanation. In fact, it may have been most pronounced during the Wisconsin glaciation and diminished in intensity ever since, but not sufficiently to check the rapid advance of the prairies behind the retreating coniferous forests. Neither can the time of maximum advance of the prairies be correlated at present with the postglacial stages of the Great Lakes. Early Migrations Of The Deciduous Forests.--At some time an amelioration of the climate began. This change was probably less connected with temperature, which has doubtless changed but little in our area since the Wisconsin glaciation, than with rainfall and atmospheric humidity. The total rainfall probably increased considerably, ...