Historical Geology
meltwater formed in a bedrock depression carved out by glaciers, sat at the edge
of the retreating ice sheet in southern Manitoba, Canada.
When the North American ice sheet retreated, its meltwaters flowed
down the Mississippi River into the Gulf of Mexico. After the ice sheet sub-
sided beyond the Great Lakes, the meltwaters took an alternate route down
the St. Lawrence River. The cold waters entered the North Atlantic Ocean.
Simultaneously, the Niagara River Falls began cutting its gorge and has tra-
versed more than 5 miles northward since the ice sheet melted.
The rapid melting of the glaciers culminated in the extinction of micro-
scopic organisms called foraminifera (Fig. 196). They met their demise when a
torrent of meltwater and icebergs spilled into the North Atlantic. The massive
floods formed a cold freshwater lid on the ocean that significantly changed the
salinity of the seawater. The cold waters also blocked poleward flowing warm
currents from the tropics, causing land temperatures to fall to near ice age levels.
The ice sheets appeared to have paused in midstride between 13,000 and
11,500 years ago.That period is called the Younger Dryas, named for an Arc-
tic wildflower, a cold-tolerant plant that grew in Europe.The climate returned
to near-glacial conditions midway through the transition from Ice Age to
interglacial. Afterward, the warm currents returned. The warming remained
permanently, prompting a second episode of melting that led to the present
volume of ice by about 6,000 years ago. Following the receding ice sheets,
plants and animals began to return to the northern latitudes.
When the ice sheets melted,massive floods raged across the land as water
gushed from trapped reservoirs below the glaciers. While flowing under the
ice, water surged in vast turbulent sheets that scoured deep grooves in the
crust, forming steep ridges carved out of solid bedrock. Each flood continued
until the weight of the ice sheet shut off the outlet of the reservoir. When
water pressures built up again, another massive surge of meltwater spouted
from beneath the glacier and rushed toward the sea. Huge torrents of melt-Figure 196Foraminifera
were important limestone
builders.