Paleoclimatology 101 - The Great Meltdown.

Figure One - South of the Ice. 

In my first article on the ice age, I discussed the orbital theory (Milankovitch Theory) behind the creation of Pleistocene ice ages. In my second article, I explored North America during the height of the Wisconsin Ice Age, at the Last Glacial Maximum (LGM) around 18,000 years ago. In this article, I discuss what happened in North America when the ice sheets began to melt.

The Cordilleran and Laurentide ice sheets altered both land and sea geography and profoundly affected the climate and environment in North America. 

Figure Two - A map depicted the area in North America covered 
by the Cordilleran and Laurentide ice sheets 
and the new coastlines at the LGM.  

With four hundred feet of water from the oceans locked up in ice sheets on land and sea, there was more coastline for humans and animals to explore and inhabit around North America. Unfortunately, there wasn't much interior land in Canada or the northern US that wasn't buried under ice. Figure two depicts North America and where scientists believe the ice sheets were located at the height of the LGM around 18,000 years ago. Note the borderline of the United States now and how far land extended out onto the continental shelf during the LGM.   

In my second article, I explored the Last Glacial Maximum (LGM), or what scientists believe was the height of the Wisconsin Ice Age, around 18,000 years ago. It was not long after the LGM, perhaps around 17,000 years ago, that the Cordilleran and Laurentide ice sheets started to melt. It was a slow melt and scientists believe it took around twelve thousand years to melt most of the two ice sheets. After all, these were not your average, normal-sized, run-of-the-mill glaciers. The two ice sheets covered over a million square miles! Scientists estimate the thickness of the ice sheets from around a half a mile thick along the southern edges to over two miles thick in the middle and northern part of the ice sheets. The ice sheets were so heavy that they compacted the Earth's crust down around one thousand feet! 

Figure Three - My first book and my latest book. The Second Edition 

of my most popular book Shadows on the Trail, the introduction to my 

Paleoindian book series.  CLICK to ORDER BOOK 

According to the Milankovitch or orbital theory, the melting began because of the orbital cycle that allowed higher volumes of summer insolation (radiation from the sun) in the northern hemisphere. In my first article, I explained that for ice sheets to grow, they require less summer insolation which allowed ice from previous winters to survive and not melt during summer heat. By adding more ice year after year, decade after decade, century after century; the ICE SHEETS CAN GREW!    

By 13,000 years ago, the Laurentide ice sheet had decreased in  area by at least twenty-five percent and by volume by at least fifty percent. During this time, most of the meltwater from the Laurentide ice sheet was flowing down the Mississippi River drainage system causing flooding along river banks and eroding sediment as a massive flow of water advanced toward the ocean. Once the sediment-laden water reached the calmer sea, sediment settled out and formed a massive delta along the coastline. At the same time sea levels were rising, flooding river valleys with water and more sediment. Once the weight of the ice sheets was removed from the coastlines of Alaska and British Columbia, the land decompressed and rose in elevation.

One question often asked; how fast did sea levels rise? One estimate stated that sea levels across the globe rose 110 meters (361 feet), or 1.1 meter per century (3.6 feet per century) between 16,000 to 6,000 years ago. This was at a slow enough rate for humans and animals to adapt to the new environment. As a comparison, sea levels today are estimated to be rising 10 to 20 centimeters (.33 to .66 feet) per century. 

During the great meltdown, water was everywhere, including the parched desert basins of the American west. In the Great Basin of Nevada, California, Idaho, Oregon, Utah, and parts of Wyoming, there were over 100 freshwater lakes. Diversion of the jet stream to the south of the ice sheets in Canada, caused heavy rainfall from the Pacific on the deserts. Add in the meltwater from the ice sheets and lakes formed all over the Great Basin. With no outlets and low evaporation rates, some of the lakes grew gigantic enormous. 

One ice age lake example was Lake Bonneville which covered a good portion of Utah. Lake Bonneville was 19,000 square

Figure Four - Lake Bonneville compared to
size of Utah. 

miles and had a depth of over 1000 feet (figure four). Death Valley even carried water during the ice age meltdown. Further east, wind squalls roaring off the ice sheets, sandblasting the land. Since cold air is denser than warm air, the squalls blew down the slopes of the ice sheets creating gusts of wind up to 100 miles per hour. During the winter when the soil near the ice sheets was dry, the winds carried sand and silt in great dust storms across the great plains, creating 35,000 square miles of sand hills in north central Nebraska and South Dakota. Today, it is estimated that silt from the ice ages covers thirty per cent of the United States, now mostly hidden under forests and grasslands.

One of the great floods of all time happened during the meltdown of the ice sheets. A lobe of ice from the Cordilleran ice sheet served as an ice dam for Lake Missoula, a 3000 square mile ice age lake in what is now the Clark Fork Valley in Idaho (figure five). When the Cordilleran ice sheet melted, so did the lobe of ice that held the waters back at Lake Missoula. When the rising level of meltwater breached the weakening ice dam, it failed and a wall of water and ice, as much as 600 feet high, flooded Idaho and eastern Washington on its way down the Columbia River to the Pacific Ocean. The flood lasted for a month and created the scarred Channeled Scablands in eastern Washington. On the other side of Canada, Lake McConnell and Lake Agassiz raised their own havoc.    

Figure Five - a depiction of what the ice dam at Lake Missoula might
have looked like.  

Even after the great meltdown, the ice age had one more gasp. Around 12,900 years ago in a period called the Younger Dryas, a sudden and dramatic drop in temperature occurred in North America, returning us to the climactic conditions of the Last Glacial Maximum. The great plains returned to the cool, wet glacial conditions, reminiscent of the Wisconsin Ice Age. This cold period lasted for over one thousand years! 

The Younger Dryas is important for another reason; it marks the end of what some archaeologists believe to be the Clovis Paleoindian culture in North America. Some investigators believe that the Clovis culture was wiped out by an extraterrestrial event which triggered the Younger Dryas while others believe the Clovis culture ended in a more traditional manner. I will explore this in my next article. 

Figure Six - Author John Bradford Branney. See all of his
books at John Bradford Branney Books

If you missed my first two paleoclimatology articles, here are the links.  
Hey, and check out my books, OK?  
Milankovitch Ice Age Theory
Last Glacial Maximum