Subglacial till

Subglacial till has a compact, fine-grained, unsorted matrix of clay, silt and sand in which pebbles and boulders of varying size ‘float’. Stratification is absent. The roundness of pebbles and boulders is usually moderate at best. The stones are often aligned in fabrics with one or more preferred orientations. Subglacial till forms at the base of a glacier. Striae formed by mutual scratching of stones during transportation and lodging are typical. Occasional thin layers of sorted clay, silt and sand – often with intra-bed deformation structures – are indicative of subglacial meltwater flow during the formation. Bodies of pre-existing sediments may have been incorporated in the till.

Subglacial tills are very common in Vorarlberg and adjacent areas, either as a mantle covering the substratum or as drumlins. The occurrence is not an indicator of a former ice-margin position. Thick packages of subglacial till are often, though not typically, found in association with ablation tills, which do form in an ice-marginal position.

Recently exposed ablation tills south of Grächen (Wallis, Switzerland). Lateral moraines consisting of ablation till cover the bedrock of the left and right flanks of the glacial valley, and are being degraded by mass movement to form talus. The veneer of blocks in the valley floor is a valley train.

Ablation till

Ablation till is a poorly sorted sediment generally consisting of a mixture of clay, silt, sand and larger fragments. Compared with subglacial till, ablation till usually contains less clayey and silty material, and it is also less compact. It may be in part clast-supported and show a crude stratification. Relatively sorted sandy and gravelly layers are common, due to reworking by meltwater. Intercalations of unsorted mud-flow or debris-flow layers may also be present. Striae on stones are not normal, fabric trends are variable.

Ablation till forms at the margin of the glacier below the snowline, or better: below the equilibrium line altitude. It forms ridges (walls) and hummocks which mark the (former) ice-margin position. Ablation till also occurs in the form of ‘carpets’ of erratic blocks, called valley trains, which are not linked to a stationary position of the glacier snout.

Diamictons with characteristics intermediate between subglacial till and ablation till occur. Their genetic interpretation relies on other criteria than lithological. The association of sediment facies and/or the geomorphological setting guide the interpretation.

Ablation tills and rock-glacier deposits are very much alike. In case of fossil deposits, diagnostic criteria may come from shape analysis and, more importantly, from the association of landforms or the geomorphological setting.

Debris-flow deposits Diamictons similar to tills are often found interstratified with sorted and bedded waterlaid deposits of the ice-marginal environment. Such diamictons are interpreted as the product of gravitational flows and are called (ice-marginal) debris-flow deposits. The term till or flow till has been used for these deposits; we prefer not to use these terms. Distinguishing between subglacial till and muddy debris-flow deposits may be troublesome, especially in small outcrops. Unlike subglacial till, muddy debris-flow deposits often show textural grading, little intra-bed deformation, high variability of clast orientiation and high grain-size variability.

Waterlaid deposits

A strict separation between fluvial and glaciofluvial (i.e. glacier meltwater) deposits is often difficult to make in the Quaternary deposits of Vorarlberg and adjacent areas. Environmental reconstructions have shown that the running water in many cases was a mixture of glacier meltwater and of discharge from rivers developing in the ice-free terrain in front of the glacier or from streams of ice-free tributary valleys. Therefore, we use the generic term waterlaid deposits, with the qualifier ice-marginal in those cases where a substantial contribution by glacier meltwater can be demonstrated on the basis of sediment characteristics and/or facies associations and/or geomorphological setting. The term ice-marginal is also used for sediments which have been deposited in close proximity to the glacier, as deduced for instance from the presence of tensional deformation features resulting from the melting of buried dead or stagnant ice.

Waterlaid deposits range from poorly sorted sandy conglomerates to well-sorted and stratified sands and gravels. The nature of the waterlaids sediment can vary markedly over short distances in one sediment body. Many ice-marginal ridges in the Alpine foreland are underlain in their proximal parts by poorly sorted conglomerates – often alternating with debris-flow deposits – which grade into bedded sands and gravels distally over a few hundreds of meters only.

Deltaic deposits

Deltaic deposits are a special type of waterlaid deposits. Gilbert-type deltas with the typical configuration of sands and gravels in foreset and topset beds occur throughout the area – witnesses of large late glacial ice-dammed lakes.

Lacustrine deposits

Fine-grained lacustrine deposits are usually found in association with deltaic deposits in Vorarlberg and the Alpine foreland. Varve-type layering is frequently observed, suggesting seasonal/annual  variation in sediment supply. The presence of  dropstones – isolated rock fragments embedded in the fine-grained sediments  – is evidence for the proximity of a glacier: ice rafts, from which the rock fragments dropped, were present on the lake.