Geoscience Reference
In-Depth Information
Mammals. During this period, parts of the Appalachians were uplifted, including western Newfoundland and
the Cape Breton Highlands; at the same time, the southern Gulf of St. Lawrence emerged above sea level.
The climate cooled as the North American Plate migrated slowly northward and as the tropical Tethys
Ocean, which wrapped around the world at the equator during the Cretaceous, began to close. The last equat-
orial “gateway” was slammed shut with the development of the Isthmus of Panama 3 million years ago. This
obstruction channeled the Gulf Stream farther northward. The warm water, in turn, supplied the necessary
moisture for the production of snow in northwestern Europe and northeastern North America and, critically,
the development of the Arctic ice cap and formation of cold currents around the North Pole. This cooling set
the stage for the onset of glaciations in the Quaternary period, which is divided into the Pleistocene epoch, 2.5
million to 12,000 years ago, and the Holocene epoch, the interglacial in which we are now living.
Glaciers are moving bodies of ice and snow. They form when winter accumulation is greater than summer
melting and the snow begins to build up. As it becomes thicker, the weight of the snow causes ice to form at its
base. When a thickness of about 60 meters (200 feet) is reached, the glacier begins to flow at the rate of a few
meters per year.
Ice Ages, such as the Pleistocene, are characterized by climatic cycles of about 100,000 years each during
which the ice sheets alternately advance and retreat. During these cycles, glacials last 60,000 to 90,000 years
and interglacials 10,000 to 40,000 years. These cycles are known as Milankovitch cycles, for the Serbian
mathematician who first calculated them. The rhythm of these cold periods relates to three planetary cycles
that affect the distribution of solar radiation between the Southern and Northern Hemispheres.
