Glaciers

Majorie Glacier Alaska

In high mountain areas small cirque glaciers form in protected hollows on mountain sides and are found at high altitudes all over the world, even within a few degrees of latitude from the Equator. Major mountain ranges in moderate and high latitudes also contain valley glaciers, bodies of ice that are confined within the valley sides. In high latitudes valley glaciers may be fed by larger bodies of ice at higher altitudes, which are ice caps that wholly or partially blanket the higher parts of the mountains. The lower slopes of a mountain range may be the site of formation of a piedmont glacier, where valley glaciers may merge and spread out as a body of ice hundreds of metres thick.

Erosional glacial features

The geomorphological features associated with the glaciations of the past few hundred thousand years are largely found in upland areas and therefore will not be preserved in the geological record: cirques, U-shaped valleys and hanging valleys are evidence of past glaciation, which, in the framework of geological time, are ephemeral, lasting only until they are themselves eroded away. Smaller scale evidence such as glacial striae produced by ice movement over bedrock may be seen on exposed surfaces, including roche moutone´e. Pieces of bedrock incorporated into a glacier by plucking may retain striae, and contact between clasts within the ice also results in scratch marks on the surfaces of sand and gravel transported and deposited by ice. These clast surface features are important criteria for the recognition of pre-Quaternary glacial deposits.

Transport by continental glaciers

Debris is incorporated into a moving ice mass by two main mechanisms: supraglacial debris, which accumulates on the surface of a glacier as a result of detritus falling down the sides of the glacial valley, and basal debris, which is entrained by processes of abrasion and plucking from bedrock by moving ice. Supraglacial debris is dominantly coarser-grained material with a low proportion of fine-grained sediment. Basal debris has a wider range of grain sizes, including fine-grained rock flour produced by abrasion processes. This basal debris of very fine to coarse material tends to be most abundant in polythermal glaciers because the alternation of pressure melting and freezing of the ice in contact with the bedrock exerts a strong freeze–thaw weathering effect. Melt water between the glacier and the bedrock forms a lubrication zone allowing the ice to move more freely and there is less erosion by the ice. Cold glaciers move only by internal deformation and hence do not erode bedrock. Cold and temperate glaciers therefore carry mainly coarser-grained supraglacial debris. During movement of a glacier the ice mass undergoes deformation, internal folding and thrust faulting that can mix some of the basal and supraglacial debris into the main body of the glacier. In addition, the merging of two or more glaciers brings detritus from the margins of each into the centre of the combined glacier. Some modification of the debris occurs where it is carried along in the basal layer, with abrasion and fracturing of clasts occurring: water in channels within and at the base of the ice (englacial channels and subglacial channels) may also sort sediment carried in temperate glaciers. Supraglacial detritus is usually unmodified during transport and retains the poorly sorted, angular character of rockfall deposits.

Deposition by continental glaciers

The general term for all deposits directly deposited by iceistill if it is unconsolidated or tillite if it is lithified. These terms are genetic, that is, they imply a process of deposition and should therefore not be used as purely descriptive terms: for example, a bed may be described as a matrix-supported conglomerate, but because a deposit of this description could be formed by a number of different mechanisms in different environments (e.g. on alluvial fans and associated with submarine slumps), the beds may or may not be interpreted as a tillite. To overcome this problem, the terms diamicton and diamictite are commonly used to describe unlithified and lithified deposits of poorly sorted material in an objective way, without necessarily implying that the deposits are glacial in origin. (It is noteworthy, however, that these terms, along with diamict for both unlithified and lithifed material, are rarely used by sedimentologists for deposits of pre-Quaternary age, and hence their use tends to be associated with glacial facies.) Tills can be divided into a number of different types depending on their origin. Meltout tills are deposited by melting ice as accumulations of material at a glacier front. Lodgement tills are formed by the plastering of debris at the base of a moving glacier, and the shearing process during the ice movement may result in a flow-parallel clast orientation fabric. Collectively meltout and lodgement tills are sometimes called basal tills. Flow tills are accumulations of glacial sediment reworked by gravity flows.

Characteristics of glacially transported material

Glacial erosion processes result in a wide range of sizes of detrital particles. As the ice movement is a laminar flow there is no opportunity for different parts of the ice body to mix and hence no sorting of material carried by the glacier will take place. Glacially transported debris is therefore typically very poorly sorted. Fragments plucked by the ice will be angular and debris carried within ice will not undergo any further abrasion, and only material on the top of an ice body will be subject to weathering processes. In addition to the poor sorting, debris carried by glaciers is very angular and the overall texture is therefore very immature. The constituents of tills and tillites are the products of weathering in cold environments, where physical weathering processes break up the rock but chemical weathering does not play an important role. For this reason, the mineral composition of the deposit may be very similar to that of the bedrock and unaltered lithic fragments are common. Clay minerals are often rather uncommon even in the fine-grained fraction of a till because clays form principally by the chemical weathering of minerals and in glacial environments this breakdown process is suppressed. The fine-grained rock flour formed by glacial abrasion is different in composition to similar grade sediment produced by other mechanisms of weathering and erosion. Rock flour consists of very small fragments of many different minerals. In contrast the same sized material produced by chemical weathering typically consists of clay mineral sand fine-grained quartz. Unlike clay minerals the fine particles in rock flour do not flocculate and tend to remain in suspension for much longer periods of time. This high proportion of suspended sediment gives the characteristic green to white colour to lakes fed by glacial melt waters. Material carried by a glacier is not necessarily all the result of glacial erosion. Valley sides in cold regions are subject to extensive freeze–thaw weathering, the products of which fall down the valley sides onto the top surface of the glacier. In more temperate regions detritus may also be washed down the valley sides by overland flow and by streams, which are active during the summer thaw. Streams may also form on the surface of a glacier or ice sheet during warmer periods and their action may contribute to the transport of debris.