Geoscience Reference
In-Depth Information
to peak values about 10,000 years ago, when the earth's tilt reached its maxi-
mum of 24.5°. However, these astronomical conditions were not enough to
account for the 18°F dif erence between glacial temperatures and Holocene
interglacial temperatures.
Climate feedbacks are required to accentuate these nudges in global con-
ditions. As the glaciers retreated, the white cover of snow and ice, which
rel ects most of the sunlight, was removed from the continents, replaced with
a darker earth surface. h at darker surface absorbed more of the sunlight,
transferring the heat to the surrounding area rather than rel ecting it back
into space. During the early Holocene, once the southern edges of the ice
sheets began to retreat, the earth's rel ectivity (or albedo) decreased, with less
solar radiation rel ected back into space and more of it retained by the earth's
surface, where it warmed the land and melted more ice. Many such feedback
loops across the globe have worked together in leading the earth into the ice
ages and back out again.
enormous meltwater lakes and megafloods
h e melting of enormous ice sheets led to the formation of vast glacial lakes.
Rather than draining directly to the sea, the meltwater from the ice sheets
sometimes became trapped behind ice dams and formed gargantuan lakes.
As these lakes grew, they periodically burst through their dams, forming
some of the largest l oods in the earth's history.
Evidence for these enormous glacial l oods was i rst discovered by a young
i eld geologist named J. Harlan Bretz in the early twentieth century. As described
in a topic by Doug Macdougall, Bretz spent his summers during the 1920s and
1930s documenting and mapping the enigmatic geology of eastern Washington
and Idaho. He found gigantic boulders sitting on the landscape 650 feet higher
than the present-day Columbia River; intertwining channels and canyons cut
into hard basalt bedrock to depths of 1,000 feet; enormous, extinct waterfalls;
and giant gravel bars and potholes. h ese seemingly unrelated features began
to tell an almost unbelievable story: this region of ripped-up earth in eastern
Washington, the so-called “Channeled Scablands” (see i gure 18 in chapter
7), appeared to be the result of a megal ood of unimaginable proportions—
possibly the largest ever to sweep across the face of the earth.
Bretz hypothesized that this megal ood originated in northwestern
Montana, at the southern boundary of the ice sheet two miles thick—now
