what glaciers covered North America during the Pleistocene?

Answer 1

The last glaciation ended about 10,000 years ago. But the period between 10,000 years ago and 3 my ago (Pleistocene epoch) was a time of many glacial and interglacial ages. During this period sea level fluctuated because:

during glaciations the continental land masses were depressed by weight of ice.


during glacial periods much sea water was tied up in glaciers so sea level was lower.


during interglacial periods sea level was higher due to melting of the ice.


during interglacial periods land that were covered with ice during a glaciation are uplifted due to removal of the weight of the ice.


Based on evidence from glacial deposits and glacial erosion features geologists have been able to document at least 4 glaciations during the Pleistocene. But recent studies of deep-sea sediments and dating of these deposits suggest that there were at least 30 glaciations that occurred during the Pleistocene. This evidence comes from studies of fossils found in deep-sea sediment cores, and what they tell us about ocean surface temperatures in the past. The results come from studies of the isotopes of oxygen.

Oxygen has two major isotopes, 18O, which is considered heavy, and 16O, which is considered light. Both of these isotopes are stable and non-radiogenic, so their ratio is constant through time.


Because 16O is lighter, it is preferentially evaporated with sea water from the oceans, and thus gets concentrated in the water that eventually falls on the continents as rain or snow. Because of this, 18O gets concentrated in ocean water.


During constant climatic conditions the 16O lost to evaporation returns to the oceans by rain and streams, so that the ratio of 18O to 16O (18O / 16O) is constant.


But, during a glaciation, some of the 16O gets tied up in glacial ice and does not return to the oceans. Thus during glaciations the 18O / 16O ratio of sea water increases.


During an interglaciation, on the other hand, the 16O that was tied up in glacial ice returns to the oceans causing a decrease in the 18O / 16O ratio of seawater.

Thus, we expect that during glaciations the 18O / 16O ratio in seawater will be high, and during interglaciations the 18O / 16O ratio in seawater will be low.

Since organisms that live in the oceans extract Oxygen from seawater to form their carbonate (CO3-2) shells, measuring the 18O / 16O ratio in the shells of dead organisms gives a record of past ocean temperatures. The record for the past two million years is shown here and in figure 16.30 on page 434 of your text. This suggests about 30 glaciations separated by interglaciations during the past 2 million years.



During the last 1 million years it appears that each glacial - interglacial cycle has lasted about 100,000 years, but earlier cycles were about 40,000 years long.

Other periods of glaciation are known from the geologic record, mainly from preserved glacial striations and tillites (consolidated till). The earliest recognized glaciation occurred about 2.3 billion years ago, but at least 50 other glaciations are recognized to have occurred during the Paleozoic era.

Answer 2

At their peak extent, glaciation covered Yellowstone Park and much of the upper midwest, but regions such as Arizona, Texas and the Deep South never had glaciation.

Answer 3

If they have a name, I am not aware of what it is. There was one on the east side and one on the west side and passage inbetween them.