U.S. Department of the Interior / U.S. Geological Survey____ As the Nation's principal conservation agency, the Department of the Interior has responsibility for most of our nationally The Great Ice Age owned public lands and natural and cultural resources. This includes fostering wise use of our land and water resources, protecting our fish and wildlife, preserving the environ­ mental and cultural values of our national parks and histor­ ical places, and providing for the enjoyment of life through outdoor recreation. The Department assesses our energy and mineral resources and works to assure that their development is in the best interests of all our people. The Department also promotes the goals of the Take Pride in America campaign by encouraging stewardship and citizen responsibility for the public lands and promoting citizen participation in their care. The Department also has a major responsibility for American Indian reservation communities and for people who live in Island Territories under U.S. Administration. Blue Glacier, Olympic National Park, Washington. The Great Ice Age land, indicating that conditions somewhat similar to those which produced the Great Ice by Louis L. Ray Age are still operating in polar and subpolar climates. The Great Ice Age, a recent chapter in the Much has been learned about the Great Ice Earth's history, was a period of recurring Age glaciers because evidence of their pres­ widespread glaciations. During the Pleistocene ence is so widespread and because similar Epoch of the geologic time scale, which conditions can be studied today in Greenland, began about a million or more years ago, in Antarctica, and in many mountain ranges mountain glaciers formed on all continents, where glaciers still exist. It is possible, there­ the icecaps of Antarctica and Greenland were fore, to reconstruct in large part the extent more extensive and thicker than today, and and general nature of the glaciers of the past vast glaciers, in places as much as several and to interpret their impact on the physical thousand feet thick, spread across northern and biological environments. North America and Eurasia. So extensive were During the long course of Earth history, the these glaciers that almost a third of the pres­ climate has fluctuated, just as the general ent land surface of the Earth was intermittent­ character of the Earth's surface has changed. ly covered by ice. Even today remnants of the In fact, there is evidence that glaciations oc­ great glaciers cover almost a tenth of the curred long before the Great Ice Age. After a period of warm and equable climate, Prior to the 19th century, observant Swiss a worldwide climatic refrigeration initiated the peasants concluded that the glaciers in the Great Ice Age glaciers. At times during the Alps had formerly been much larger. They Great Ice Age, the climate was cooler and noted that the existing glaciers were slowly wetter and at times warmer and drier than to­ transporting and depositing boulders down- day. Many attempts have been made to ac­ valley and correctly inferred that the boulders count for these climatic fluctuations, but their (erratics) strewn about their pastures had been ultimate cause remains unclear. Although we transported and deposited in the same manner cannot predict a period of climatic cooling, long ago. Also, they observed that the polished another Ice Age in the future is a possibility. and scratched or finely grooved and rounded Although the Great Ice Age began a million bedrock knobs (roches moutonnees) along the or more years ago, the last major ice sheet to valley walls and floors were similar to those spread across the North Central United States emerging from beneath the melting ice of the reached its maximum extent about 20,000 existing glaciers. The similarity suggested that years ago. Waning through a succession of the roches moutonnees had been produced by retreats and relatively minor advances, it the moving glacial ice at a time when the lingered in Canada until about 6,000 years glaciers extended farther down the mountain ago, when it finally melted. Mountain glaciers valleys. are the only remnants of the great glaciers on In Germany and Scandinavia similar features, the mainland of North America. though far removed from existing mountain Land areas formerly covered by glaciers Map of North America showing extent of the Great Ice Age glaciers. Roche moutonnee surface, Alaska. This concept was enlarged and popularized rounded and mantled by glacial debris; and ground moraine in the Midwest in places cover by the eminent Swiss geologist, Louis Agassiz, valleys were choked by sand and gravel and conceal the preglacial hills and valleys. whose arrival in the United States in 1846 deposited by glacial melt waters. All evidence On such plains the position of maximum ad­ marks the beginning of the study of this fasci­ indicates that slowly advancing ice sheets vance of a glacier is commonly well marked nating period of Earth history in North America. plowed up the soil and loose rock, plucked by deposits of till pushed up or dumped as Since that time many significant contributions and gouged boulders from outcrops, and car­ ridges or hummocky belts (moraines). In some have been made to the understanding of the ried this material forward, often for great places narrow, hummocky, sinuous ridges glaciations, as continuing studies, especially in distances. Using the material in transit as an (eskers) may occur, or perhaps swarms of the United States, Greenland, and Antarctica, abrasive, the glaciers smoothed, polished, and rounded, elongate hills (drumlins). reveal the complex history of glaciers and of scratched the rock outcrops, producing a more Where great river valleys such as the past glaciations. subdued landscape. The great glaciers com­ Mississippi and Ohio were overwhelmed by Early observations in the country indicated monly softened the contours by wearing down ice sheets, the rivers were forced into new that large parts of the North Central and North­ the tops of hills and filling the valleys. channels. Where smaller preglacial drainage eastern United States had once been overrid­ As the glaciers melted, unsorted mixtures of systems were completely covered by glacial den by thick ice sheets. Telltale traces of clay, sand, gravel, and boulders (till) were ice, they were commonly obliterated. Glacial glacial erosion and deposition were wide­ generally deposited as an unconsolidated deposits remaining after disappearance of the spread. Rock outcrops were smoothed, pol­ mantle on the countryside (ground moraine). ice were usually poorly drained. Bogs, swamps, ished, and scratched or striated; hills were Extensive level to gently rolling plains of thick and many lakes, typical of parts of Wisconsin and Minnesota, characterize such areas. In mountain areas, valley glaciers modified the landscape in an entirely different manner. Scooping out and widening the valleys through which they moved, the glaciers produced valleys with a U-shaped cross profile, in con­ trast to the normal V-shaped profile produced by stream erosion. Intervalley divides were Shaped, scratched, and finely grooved glacial cobble, sharpened, valley walls were oversteepened, Indiana (actual size). and precipitous cliffs were developed. As a glaciers, were recognized to be of glacial result, glaciated mountain areas are often origin. The idea that such features resulted scenically spectacular, as in Glacier National from an expansion of distant mountain glaciers Park, Mont., and Yosemite, Calif. Arcuate or was gradually modified by the early naturalists. curved moraines loop across the mountain They reached the conclusion that widespread valleys or out onto the plains at the foot of the sheets of moving ice had advanced from the mountains. These moraines mark terminal far north to the plains of northern Germany. positions of valley glaciers that either have This conclusion was supported by the fact that disappeared or survive only as small ice many of the erratics on the north German masses or short tongues moving down valleys plains were of rock types common to Scan­ from the theater-shaped valley heads dinavia. Such an ice sheet, moving from north (cirques). to south, had to be distinct from the expanded Debris of all sizes transported by glaciers mountain glaciers of the Alps. Thus was born was released by the melting ice and was car­ the concept of the Great Ice Age the time ried forward by melt-water streams or deposit­ when vast glaciers spread as ice sheets over ed near the ice margin. Melt-water streams the northern lands and mountain glaciers were were commonly so loaded that they could not transport all the debris supplied to them, and greatly expanded. Drumlins, Canada. valleys became clogged with sand and gravel. 9 U-shaped glaciated valley with moraine in foreground, Glacier National Park, Montana. At times such streams flowed in a shifting In lakes that formed in front of retreating ice maze of shallow channels across the exposed sheets, glacial melt waters deposited well- surface of the valley fill they had deposited sorted sediments. Where only finer sediments (valley train). Winds sweeping across valley- were deposited, alternate layers of silt and train surfaces picked up the finer particles ex­ clay accumulated in the lake basins. It is posed between the shifting channels and generally believed that the lighter silty layer spread them as a dustlike mantle (loess) over was deposited in summer and the dark clayey the countryside. The loess of the last great layer in winter, the two layers forming an an­ glaciation may be as much as 20 feet thick. It nual deposit (varve). By counting the varves is thickest near the great river valleys, such as and matching thicknesses of varves deposited the Mississippi, that served as sluiceways for simultaneously in the many lakes along a glacial melt water. Throughout large parts of retreating glacier margin, scientists can deter­ the Central United States, soils have been mine both the time in years necessary for developed on a mantle of loess whose source varve deposition and the rate of retreat of the areas were the great river valleys.