Glacial Geology Supplementary Materials

GLACIAL GEOLOGY Much of the landscape in northern latitudes owes its unusual geography to the large glaciers washed out of the polar regions during the last ice age. Glaciers excavate some of the most monumental landforms. Outwash streams of glacial meltwater carve out many bizarre sights. The strength of glacial erosion is well demonstrated by the deepsided valleys carved out of mountainsides by thick layers of flowing ice (Fig. 199) a mile or more thick. Glacial erosion radically modifies the shape of river valleys occupied by glaciers. This process is most active near the glacier head, where the ice deepens and flattens the gradient of the valley. Small hills or knobs in valleys overwritten by glaciers are rounded and smoothed by abrasion. Glaciers flowing down the top of the mountain gouge out large holes called cirques, from the French word for "circle." They are semicircular basins or indentations with steep walls high on the mountainside at the head of the valley. The expansion of adjacent cirques by glacial erosion creates arêtes, horns, and cols. Arête, from French meaning "fishbone," is a sharp, jagged, or bladed crested ridge that separates the heads of bordering cirques. It also forms a dividing ridge between two parallel valley glaciers. Col is a sharp-edged or saddle-shaped track in the mountains formed by erosion toward the head where cirques meet or intercept each other. When three or more cirques erode towards the same point, they form triangular peaks called horns, like the famous Matterhorn in the Swiss Alps. The glacier extends far down the valley, grinding rock at the valley floor as the ice advances and recedes. As a result, rivers of solid ice embedded with rock move along the valley floor, grinding it like giant files as glaciers flow back and forth over it. The advancing glaciers leave parallel grooves called glacial striae at the valley floor as they slice through the mountainside. Miles of existing glaciers are large areas of polished and highly wrinkled rock, and large rock piles mark the vastness of the former glacier. Much of the northern landmass is dotted with glacial lakes developed from deep holes dug by itinerant glaciers. Most evidence of extensive glaciation is found in moraines, tillites, and other glacial rocks. Moraines (Fig. 200) consist of rocks and material carried by glaciers and deposited in a regular linear pattern. They are named according to their position in relation to glaciers. Soil moraine is an irregular carpet consisting mostly of clay, silt, and sand deposited beneath glaciers. This is the most common type of continental glacial deposit. Terminal moraine is a ridge of erosional debris deposited by the margins of glaciers melting forward. It is a mass of ridge-like glacial debris formed by a prominent glacial snout and deposited on the outermost edge of glacial progress. Much of the upper midwestern and northeastern United States is eroded into granite bedrock. Debris was left in a large pile. Glacial sediments were lowered covering much of the landscape, burying older rocks under deep layers up to. Glacial till is a nonstratified or mixed material consisting of clay and boulders deposited directly by glacial ice. The basalt to the bottom of the glacier is usually placed underneath. Ablation up to or near the surface of glaciers deposited when ice melts. Tillites are a consolidated mixture of boulders and gravel in a clay matrix. They were deposited by glacial ice and are known to exist on every continent. In some areas, older deposits are buried under thick glacial deposits, forming elongated hills parallel in the same direction called drumlins (Fig. 201). The drumlin is high and narrow at the upper end of the glacier and slopes to a low and wide tail. Hills appear in concentrated fields in North America, Scandinavia, England, and other areas that were once covered in ice. The drumlin field may contain as many as 10,000 knolls, looking like rows of eggs lying on their side. Drumlin is the least understood of all glacial landforms. How they achieved their characteristic oval shape remains a mystery. Perhaps the vast drumlin fields of North America were created during the cataclysmic flooding process during the melting of vast ice sheets. Similar to drumlin is roche moutonnée, from French meaning "soft stone." The term is applied to glacial outcrops because they resemble a sheep's back, thus driving the name of the sheep's back rock. Roche moutonnée is a glacial bedrock surface with asymmetrical mounds of various shapes. The side of the upper glacier has been glacially eroded and exfoliated smoothly. The lower glacial side has steeper, jagged slopes, resulting from glacial plucking. The ridge dividing the two sides of the roche moutonnée is perpendicular to the general flow of the ice sheet. On the periphery of the ice sheet is a rough periglacial area. These features develop along the ice tip and are directly controlled by glaciers. Cold winds blowing from permafrost affect the climate of glacial margins and help create periglacial conditions. The zone is dominated by processes such as frost, ice separation, and sorting, which create large rock fields of what was once solid bedrock. Large erratic rocks (Fig. 202) are transported glacial rocks that are glacially embedded until or exposed on the surface. They indicate the direction of glacial flow. They range in size from pebbles to boulders and have traveled as far as 500 miles or more. Erratic consists of typical rock types that can be traced to their place of origin. Indicators of erratic rocks of known origin are used to locate the source area and the distance traveled for any particular glacial up-to-date. Their identifying features include a distinctive appearance, a unique collection of minerals, or a distinctive fossil content. Eratik is often arranged in stone trains, forming a line or series of rocks that originate from the same bedrock source and extend in the direction of glacial movement. A stone fan is a cone-shaped deposit containing a distinctive eratic that comes from an outcrop at the top of a fan. Glacial shifts include all rock material deposited by glaciers or streams and glacier lakes, where the greatest thickness is achieved in buried valleys. Drift is divided into two types of materials. One of them was deposited directly by glacial ice and showed little or no sorting or stratification (layering) as if it were disposed of carelessly. The other is stratified drift, which is a well-sorted, layered material that is transported and deposited by glacial meltwater. Flows flowing from melting glaciers rework parts of glacial material, some of which is carried to puddles, forming banded deposits called glacial varves. Long, sinuous sand deposits called esker (Fig. 203) are formed from glacial debris from outwash flows. They are meandering steep-walled mountains that can extend up to 500 miles in length but rarely exceed more than 1,000 to 2,000 feet wide and 150 feet high. Esker appears to have been created by streams flowing through tunnels beneath the ice sheet. When the ice melts, old river deposits are left standing as ridges. Esker appears to have been deposited in channels beneath or within slow-moving or stagnant glacial ice. Their general orientation runs at right angles to glacial edges. On the shores of glacial lakes, they probably form river deltas. Kames are mounds consisting mainly of stratified sand and gravel that form on or near the snout of ice sheets or deposited on the edges of melting glaciers. They are found in areas where large amounts of coarse material are available during the slow melting of glacial ice. Melt water is present in sufficient quantities to redistribute debris and deposit sediment in the margins of decaying ice masses. Most kames are low, irregular conical mounds of glacial sand and coarsely layered gravel that often occur in clusters. They accompany the moraine region of terminal valleys and glaciers of the continent. They appear to represent sediment replenishment from openings in stagnant ice. Many kames may form when streams flow from the top of glaciers onto bare ground, dropping sediment into thick piles. The Ice Age is still with us, it's just that we're lucky to live in warmer periods between glaciations. Perhaps in a few thousand more years, huge ice sheets will again go berserk, wiping out everything in their path. Debris from destroyed northern cities would be bulldozed hundreds of miles to the south, gripped in the frozen jaws of advancing ice. Global temperatures will plummet as plants and animals, including humans, scramble for warmth in the tropics.