Glaciers, Deserts, and Wind begins by examining the types, the movement, and the formation of glaciers. Also presented are the processes of glacial erosion and deposition, as well as the features associated with valley glaciers and ice sheets. The Pleistocene epoch and some indirect effects of Ice Age glaciers are also discussed. The section closes by examining some of the theories that attempt to explain the causes of glacial ages.
The study of deserts begins with a review of the roles of weathering and water in arid climates. A discussion of the evolution of much of the Basin and Range region of the United States provides insight into the processes that shape desert landscapes. The chapter ends with an investigation of wind erosion and deposition, including the various types of sand dunes.
Learning Objectives
After reading, studying, and discussing this chapter, you should be able to:
•Describe the types and locations of glaciers.
•Discuss glacial movement.
•List the types of glacial drift.
•Describe the features produced by glacial erosion and deposition.
•Define the Ice Age and the Pleistocene epoch.
•List the theories for the causes of glacial ages.
•Discuss the roles of weathering and Water in arid climates.
•Describe the geologic evolution of the Basin and Range region.
•List the types of wind deposits and describe the features of wind deposition.
Chapter Summary
•A glacier is a thick mass of ice originating on the land as a result of the compaction and recrystallization of snow, and shows evidence of past or present flow. Today, valley or alpine glaciers are found in mountain areas where they usually follow valleys that were originally occupied by streams. Ice sheets exist on a much larger scale, covering most of Greenland and Antarctica.
•On the surface of a glacier, ice is brittle. However, below about 50 meters, pressure is great, causing ice to flow like a plastic material. A second important mechanism of glacial movement consists of the whole ice mass slipping along the ground.
•Glaciers erode land by plucking (lifting pieces of bedrock out of place) and abrasion (grinding and scraping of a rock surface). Erosional features produced by valley glaciers include glacial troughs, hanging valleys, cirques, arêtes, horns, and fjords.
•Any sediment of glacial origin is called drift. The two distinct types of glacial drift are 1) till, which is material deposited directly by the ice; and 2) stratified drift, which is sediment laid down by meltwater from a glacier.
•The most widespread features created by glacial deposition are layers or ridges of till, called moraines. Associated with valley glaciers are lateral moraines, formed along the sides of the valley, and medial moraines, formed between two valley glaciers that have joined. End moraines, which mark the former position of the front of a glacier, and ground moraine, an undulating layer of till deposited as the ice front retreats, are common to both valley glaciers and ice sheets.
•Perhaps the most convincing evidence for the occurrence of several glacial advances during the Ice Age is the widespread existence of multiple layers of drift and an uninterrupted record of climate cycles preserved in sea-floor sediments. In addition to massive erosional and depositional work, other effects of Ice Age glaciers included the forced migration of animals, changes in stream and river courses, adjustment of the crust by rebounding after the removal of the immense load of ice, and climate changes caused by the existence of the glaciers themselves. In the sea, the most far-reaching effect of the Ice Age was the worldwide change in sea level that accompanied each advance and retreat of the ice sheets.
•Any theory that attempts to explain the causes of glacial ages must answer the two basic questions: 1) What causes the onset of glacial conditions? and 2) What caused the alternating glacial and inter-glacial stages that have been documented for the Pleistocene epoch? Two of the many hypotheses for the cause of glacial ages involve 1) plate tectonics and 2) variations in Earth’s orbit.
•The same geologic processes that operate in humid regions also operate in deserts, but under contrasting climatic conditions. In dry lands rock weathering of any type is greatly reduced because of the lack of moisture and the scarcity of organic acids from decaying plants. Practically all desert streams are dry most of the time and are said to be ephemeral. Nevertheless, running water is responsible for most of the erosional work in a desert. Although wind erosion is more significant in dry areas than elsewhere, the main role of wind in a desert is in the transportation and deposition of sediment.
•Many of the landscapes of the Basin and Range region of the western and southwestern United States are the result of streams eroding uplifted mountain blocks and depositing the sediment in interior basins. Alluvial fans, playas, and playa lakes are features often associated with these landscapes.
•In order for wind erosion to be effective, dryness and scant vegetation are essential. Deflation, the lifting and removal of loose material, often produces shallow depressions called blowouts and can also lower the surface by removing sand and silt, leaving behind a stony veneer, called desert pavement. Abrasion, the “sandblasting” effect of wind, is often given too much credit for producing desert features. However, abrasion does cut and polish rock near the surface.
•Wind deposits are of two distinct types: 1) extensive blankets of silt, called loess, that is carried by wind in suspension; and 2) mounds and ridges of sand, called dunes, which are formed from sediment that is carried as part of the wind’s bed load. The types of sand dunes include 1) barchan dunes, solitary dunes shaped like crescents with their tips pointing downwind, 2) transverse dunes, which form a series of long ridges orientated at right angles to the prevailing wind, 3) longitudinal dunes, long ridges that are more or less parallel to the prevailing wind, 4) parabolic dunes, similar in shape to barchans except that their tips point into the wind, and 5) star dunes, isolated hills of sand that exhibit a complex form.