A glacier is a large, slow-moving mass of land ice that moves under its own weight. It is formed by the accumulation, compaction and recrystallization of snow. For a glacier to form, more snow must accumulate than is melted. Two types of glaciation are recognized and both have affected Idaho. Alpine glaciation of smaller aerial extent is found in mountainous regions; whereas continental glaciation has covered a large part of the continent with a huge ice sheet. Both types of glaciation have dramatically changed the landscape.

The Great Ice Age was a period of recurring glaciations that affected northern Idaho. This ice age began about a million years ago and marked the beginning of a long period of colder climate. Mountain glaciers formed in all the high country. These glaciers were so extensive that almost a third of the present land surface of the earth was covered with ice. About 20,000 years ago the last great ice sheet retreated from the northern United States. During the later stages it went through a succession of retreats and minor advances. The part of the ice sheet that affected northern Idaho lingered in Canada until about 6,000 years ago when it finally melted. Evidence of this ice age is now widespread throughout the high country of Idaho. Traces of glacial erosion and deposition can be found in most of the mountainous areas.

Slow-moving glaciers plowed up soil and loose rock and plucked and gouged boulders from outcrops. This material, caught in the glacier, was used as an abrasive to grind down, polish and scratch the exposed outcrops in its downward path. In this way glaciers soften landscapes by wearing down hilltops and filling in valleys. In mountainous areas, glaciers confined to valleys (valley glaciers) scooped out and widened the valleys leaving a U-shaped cross profile. Stream erosion normally leaves a V-shaped valley so that the presence of a U-shaped valley is strong evidence that the valley was shaped by a glacier.

A glacial cirque is a steep-sided, rounded, bowl-shaped feature carved into a mountain at the head of a glacial valley. In the cirque, snow accumulates and eventually converts to glacier ice before heading down the glacial valley. A horn is the sharp peak that remains after cirques have cut back into a mountain on several sides Sharp ridges called aretes separate adjacent glacially-carved valleys. The Sawtooth Mountains of Idaho offer exceptional examples of glacial erosional features such as U-shaped valleys, cirques, horns and aretes as well as smaller features such as polished and striated bedrock.

As glaciers move down valley, rock fragments are scraped and plucked from the underlying bedrock and the canyon walls. Most of these rock fragments are angular. When the material picked up and transported by the glacier is deposited, it is called till. Glacial till consists of unsorted fragments ranging from clay size to boulder size, all mixed together with no layering. Glaciers can easily carry any size rock fragment including boulders as large as a house. An erratic is a huge, ice-transported boulder that is not related to bedrock in the vicinity.

A moraine is a body of till deposited by a glacier. Ridge-like piles of ice left at the sides of a glacier are called lateral moraines. Medial moraines are developed where two glaciers come together and their lateral moraines merge and continue downglacier as a single long ridge of till. An end moraine is piled up along the front edge of ice at the downslope terminus of a glacier. Valley glaciers tend to leave an end moraine with the shape of a crescent ridge.

There are two types of end moraines: a terminal moraine is an end moraine marking the farthest advance of a glacier- a recessional moraine is an end moraine developed while the terminus of a receding glacier is temporarily stationary. Redfish Lake near Stanley is a glacial lake. The lake occupies a U-shaped, glacially-carved valley and the water is contained on the sides by lateral moraines and on the north end near the lodge by a terminal moraine.
As the ice of a glacier melts, ground moraine is deposited at its base. This blanket of till extends over larger areas that were covered by an ice sheet.

Oval-shaped hills consisting of thick ground moraine deposits are called drumlins. Drumlins have their long axis parallel to the direction of ice movement.

Streams that drain glaciers are heavily loaded with sediment, especially during the summer months. These outwash streams form a braided pattern and their deposits are layered as are all stream deposits. Thus, outwash deposits can be distinguished from till which is unsorted.

Eskers are long, sinuous ridges of water-deposited material up to 30 feet in height. They are deposited in tunnels either within or under glaciers where the meltwater loaded with sediment flows under and out of the ice.

Large blocks of ice are commonly buried within the thick deposits of outwash in front of a retreating glacier. When the ice block melts, a depression called a kettle is formed. These depressions may be later filled with water and become permanent lakes.

Glaciation began in the northern hemisphere more than two million years ago. Ice sheets that formed in southern British Columbia probably moved southward following south-trending valleys repeatedly between 2.5 m.y. ago and 100,000 years ago. However, the two most recent events occured in the last 100,000 years. These recent events left most of the physical evidence we see today.

The topography of Central and northern Idaho are a result of both continental ice sheets and mountain glaciers. Evidence of Pleistocene glaciation can be seen in mountainous areas at elevations as low as 5,000 feet above sea level.

In northern Idaho, the continental ice sheet moved from the Canadian ice fields towards the south into northern Idaho. This ice sheet probably extended no further south than the north end of Coeur d'Alene Lake. As climatic changes affected the sources of ice in the Canadian ice fields, the ice sheet may have receded and again moved north through several episodes. The continental ice sheet, originating in the Canadian ice fields, invaded northern Idaho repeatedly. Slow advances were followed by retreat as the climate warmed or cooled. During the melting phases, deposits of sand and gravel accumulated at the margins of the ice lobe. These deposits are commonly called recessional moraines. The grinding of the moving ice sheet left scratched, grooved and polished surfaces on much of the bedrock in northern Idaho.

During maximum glaciation, the ice was thick enough to pass over the highest peaks of the Selkirk and Cabinent Ranges at elevations of more than 6,000 feet. This required an ice sheet to be more than 4,500 feet thick in the vicinity of Sandpoint. The ice may have been more than 2,000 feet thick at the southern end of Lake Pend Oreille during maximum glaciation.

From 7,000 to 25,000 years ago, alpine glaciation was widespread in the higher elevations of the state. At least two periods of major glaciation are evident in Idaho. The last stage of alpine glaciation occurred about 4,000 years ago. This glacial action was relatively minor as glaciers existed only in the highest mountains of the State.

Large quantities of glacial meltwater had a dramatic effect on the landscape. Much rock debris was transported by water and deposited in valleys. Many floods were caused by glacial ice impounding water and then bursting. Huge catastrophic floods were caused in such a manner and drastically eroded the landscape.

During and immediately following the ice age, the streams of Idaho carried much more water than they do now. Larger streams and rivers could transport a much greater sediment load, mostly of glacial debris. At this time, the abundance of water caused large lakes to form in closed basins. One of the largest of these lakes was ancient Lake Bonneville, once covering more than 20,000 square miles with a maximum depth of more than 1,000 feet. The Great Salt Lake is a remnant of this lake. Lake Bonneville rose until the water broke through Red Rock Pass in southeastern Idaho. This huge flood swept over the pass, down the Portneuf River to the Snake River. The flood waters roared across the Snake River Plain, and for the most part, followed the Snake River into Oregon.

(The remarkable depositonal and erosional features caused by this flood are discussed in the Lake Bonneville Flood module.)

Ancient glacial Lake Missoula was created by an ice lobe forming a dam near the Idaho-Montana border. By melting or erosion this dam was suddenly removed and great floods were released throughout the northwest. Glacial debris left by the retreat of the great glaciers dammed streams and formed many modern lakes in Northern Idaho, including Hayden, Spirit and Twin Lakes. Pend Oreille Lake was formed in a similar fashion by glaciers eroding the lake basin and glacial debris damming the south end.

Floods from glacial Lake Missoula passed through the northern part of Pend Oreille Lake. Ice dams forming glacial Lake Missoula failed many times causing floods to move across northern Idaho. These catastrophic floods flowed south and southwest scouring great channels in the Columbia Plateau in the eastern and central parts of the State of Washington. This area is commonly called the channeled scablands. Three terraces along the Clark Fork and Pend Oreille River valleys were formed by the three floods from glacial Lake Missoula.

Supplemental Material

Rocks, Rails and Trails: pp. 18, 24, 25, 27 and 32

The field guides below are a part of the Guidebook to the Geology of Eastern Idaho :

Also Visit

The Lake Bonneville Flood

This module was created by Digital Atlas staff members.