The increased precipitation, and probably glacial meltwater, formed large lakes in the closed basins of the Great Basin. There were boreal forests at lower elevations than there are today in comparable latitudes and continuous boreal habitat probably connected many of the isolated mountain ranges of the southwest. That probability is supported by the presence of boreal animals and plants on many of these isolated ranges today. A boreal tree squirrel, such as _Tamiasciurus_, could hardly be suspected of crossing a treeless, intermontane desert valley, miles wide.
[Illustration: FIGS. 6_a_-6_f_. Fig. 6_a_. _Sorex vagrans pacificus_, 1 mi. N Trinidad, Humboldt Co., California, FC 1442. Fig. 6_b_. _S. v. yaquinae_, Newport, Lincoln Co., Oregon, AW 707. Fig. 6_c_. _S. v. yaquinae_ (near _bairdi_), McKenzie Bridge, Lane Co., Oregon, AW 82. Fig. 6_d_. _S. v. setosus_, Reflection Lake, Jefferson Co., Washington, CMNH 4275. Fig. 6_e_. _S. v. obscurus_, 10 mi. SSW Leadore, Lemhi Co., Idaho, FC 1499. Fig. 6_f_. _S. v. vagrans_, Baker Creek, White Pine Co., Nevada, 88042 (after Hall, 1946:113).]
Interglacial ages were characterized by warmth and aridity as compared to the glacial ages. Glaciers retreated or disappeared, boreal forests became montane in much of the United States, and the lakes in the Great Basin were reduced or disappeared. One can envision that during such times boreal mammals were isolated, their geographic ranges were restricted, and Sonoran mammals expanded their ranges.
Evidence is more extensive concerning the number and extent of glacial ages in the eastern than in the western part of North America. This evidence suggests a division of the Pleistocene into four glacial ages and four interglacial ages, the fourth interglacial age corresponding to the present time. More information is available about the Wisconsinan, or last, glacial age, than about the earlier ones, because the last glaciation in many montane areas destroyed evidence of earlier glaciations. The names of currently recognized glacial and interglacial ages of the Pleistocene are listed below. The names of interglacial ages are in Italic type.
Wisconsinan _Sangamonian_ Illinoian _Yarmouthian_ Kansan _Aftonian_ Nebraskan