A natural clock of another type is provided by Niagara Falls, which are cutting their way back up the gorge at a rate which has been definitely ascertained. Taking into account the varying amounts of water which have passed over the falls at different stages of post-glacial geography, the duration since the region became free of ice has been calculated at about 20,000 years, which agrees closely with the time elapsed since the Scandinavian ice-sheet left the North German coast.
Before leaving North America it is necessary to give a brief account of the phenomena outside the main centres of glaciation, and especially of the history of the Great Basin between the Sierra Nevada and Wasatch Mountains. The lowest levels of this basin are at present occupied by several salt lakes without outflow, of which the largest is the Great Salt Lake, the level of the water being determined by the balance between inflow of the rivers and evaporation from the surface. Twice in the past this balance has been decidedly more favourable, and then the lakes grew to many times their present size. The two greatest of these old lakes have been fully described under the names of Lake Bonneville (of which the Great Salt Lake is a vestige) and Lake Lahontan, further to the west. The investigations have shown that before the Glacial period, and extending back into an unknown past, there was a period of great aridity. To this succeeded a long period of high water, during which, however, neither of the lakes overflowed. This stage was followed by a very long period of great aridity, during which the lakes dried up completely, and all their soluble matter was deposited and buried by alluvial material. This period was followed by a return of moist conditions, during which the water reached a higher level than before, and in the case of Lake Bonneville actually overflowed into the Snake river, cutting a deep gorge. This period, however, was shorter than the preceding moist period. It was followed by an irregular fall interspersed with occasional slight rises, but ultimately both lakes descended below their present level and probably again dried up completely. Both lakes suggest that this low level was followed by a third rise to a height very slightly above the present level, followed by a slow fall in recent years.
The relations of the periods of high water to the glaciations are not clear in these large lakes, but in the Mono Basin, a small basin further west, there is no doubt that the two were almost contemporaneous, high water accompanying the maxima of glaciation and extending some way into the retreat phase. The very long interval between the first and second period of high water, several times that since the second period, agrees with this correlation. We find then that south-west of the main glaciated area there was a district of greater precipitation or less evaporation, or more probably both. This is confirmed by the valley moraines of all this region—Sierra Nevada, Uinta and Wasatch mountains, Medicine Bow Range of northern Colorado, etc., all of which indicate two glaciations, of which the first was the greater, separated by a very long interval. In several ranges the moraines of the second glaciation are double, and some geologists consider that there were three Glacial periods in these regions.
In the extremely arid region of Arizona, on the other hand, which is considerably further south, the evidence of the Gila conglomerates indicates that while frost was very active, the increase of precipitation, though undoubtedly present, was comparatively slight. This shows that the climatic balance was not greatly disturbed, the chief effect being an important lowering of temperature, probably due to cold northerly winds. The Gila conglomerates are double, separated by a period representing present-day conditions.
Summing up the evidences of glacial climate in North America, we find a striking similarity to Europe. In the north elevation and increased land area caused the development of large ice-sheets, which appeared first in the mountainous regions with a heavy snowfall, and later spread over the drier plains and plateaux of the interior. This first glaciation was long and complex. Owing to the anticyclonic conditions which formed over the ice, the rain- and snow-bearing depressions were forced to pass further southward, causing greater snowfall on the mountains and high water-level in the lake basins. This greater snowfall, together with the cold conditions due to the existence of the ice-sheets to the north, caused the development of mountain glaciers south of the main glaciated region. In the east there were cold northerly winds which carried a severe climate as far south as Florida. This Glacial period was followed by subsidence, and a long spell of dry, moderately warm climate lasting perhaps 200,000 years, after which elevation and glacial conditions again set in. These conditions were not so severe as the first, and their duration was much less, while they were broken up by several intervals of temporary recession of the ice, one of which, corresponding to the Riss-Wurm period, lasted for 60,000 years, and perhaps should be considered as an “interglacial.” This period was marked in its early stages by the deposition of the curious æolian deposit known as “loess,” indicating steppe conditions. After the last glaciation there set in a stage of irregular retreat.
BIBLIOGRAPHY
Leverett, F. “Comparison of North American and European glacial deposits.” Zs. f. Gletscherkünde 4, 1910, pp. 280, 323.
Wright, W. B. “The Quaternary Ice Age.” London, 1914, Chs. 8-9.
Attwood, W. W. “The glaciation of the Uinta Mountains.” J. Geol., 15, 1907, p. 790.