Page 591 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 17 glacial and Periglacial Landscapes 555
Heat
▲Figure 17.23 Permafrost thawing and structure collapse. Building failure due to improper construction as permafrost thaws south of Fairbanks, alaska. [adapted from USgS: photo by Steve McCutcheon; based on U.S. geological Survey pamphlet “Permafrost” by L. L. ray.]
Assiniboine and Saskatchewan rivers—cut downward into their delta sediments. By 7800 years ago, Lake Agassiz, the largest of all North American glacial lakes, disappeared as the ice sheet disintegrated and water drained into Hudson Bay. All that remains of the former greatest lake are lakes Manitoba, Winnipeg, Dauphin, Winnipegosis, and Lake of the Woods.
Sea levels 18000 years ago were approximately 100 m lower than they are today, because so much of Earth’s water was frozen in glaciers. Imagine the coastline of Nova Scotia being southeast of Sable Island about 200 km from the present coastline, Alaska and Russia con- nected by land across the Bering Straits, and England and France joined by a land bridge. In fact, sea ice extended southward into the North Atlantic and Pacific and northward in the Southern Hemi- sphere about 50% farther than it does today.
Glacial Lake Agassiz began to form about 11 700 years ago when water was trapped between the Mani- toba Escarpment and the retreating Laurentide ice sheet (Figure 17.26e). The ice margin determined the locations of outlet channels. Earth scientists have mapped outlets to the south, east, north, and northwest. About 11000 years ago, an outlet opened into the basin of presentday Lake Superior and the level of Lake Agassiz fell. As the lake level fell, the rivers that flowed into the lake—the
▲Figure 17.24 Trans-Alaska oil pipeline. The pipeline is 1.2 m
in diameter, and is supported on racks that average 1.5 to 3.0 m in height above the ground to prevent permafrost thaw. [a96/Zuma Press/ newscom.]
Paleolakes
From 12000 to 30000 years ago, the American West was dotted with large, ancient lakes—paleolakes, or pluvial lakes (Figure 17.27). The term pluvial (from the Latin word for “rain”) describes any extended period of wet conditions, such as occurred during the Pleistocene Epoch. During pluvial periods in arid regions, lake lev- els increase in closed basins with internal drainage. The drier periods between pluvials, interpluvials, are often marked by lacustrine deposits, the name for lake sedi- ments that form terraces, or benches, along former shore- lines. Except for the Great Salt Lake in Utah (a remnant of the former Lake Bonneville; Figure 17.27a) and a few smaller lakes, only dry basins, ancient shorelines, and lake sediments remain today.
Scientists have attempted to correlate pluvial and glacial events, given their coincidence during the Pleis- tocene. However, few sites actually demonstrate a direct relation. For example, in the western United States, the estimated volume of melted ice from glaciers is only a small portion of the actual water volume that was in the paleolakes. Also, these lakes tend to predate glacial times and are correlated instead with periods of wetter climate or periods thought to have had lower evaporation rates.
Paleolakes existed in North and South America, Africa, Asia, and Australia. Today, the Caspian Sea in Kazakhstan and southern Russia has a level 30 m below global mean sea level, but ancient shorelines are visible about 80 m above the present lake level. In North America, the two largest
Permafrost