1.Introduction

Till is a type of unsorted and unstratified sediment that is deposited directly by glaciers. It is composed of a mixture of different-sized particles, including rocks, boulders, pebbles, sand, silt, and clay, that are eroded and transported by glaciers as they move across the landscape. Till can vary widely in its physical properties and composition, depending on the type of bedrock, the direction and speed of glacier movement, and other factors.

Till deposits are typically found in areas that were covered by glaciers during past glacial periods, including much of North America, Europe, and parts of Asia. These deposits may be several meters thick in some areas, and can cover large areas of the landscape. Till deposits can be found in a variety of landforms, including moraines, drumlins, eskers, and kames, which are all formed by the movement of glaciers and the deposition of sediment.

Till is an important geological formation, and can provide valuable insights into past environmental conditions, including climate, landscape, and glaciation history. Studies of till deposits can help to reconstruct past ice movements, and to understand how glaciers have influenced the formation of landscapes and ecosystems over time. Tills are also important for agriculture, as they can provide fertile soils that are rich in minerals and nutrients.

Till is unsorted and unstratified glacial drift deposited directly by and undemeath glaciers. Its soil grains are usually angular and all size fractions are normally present (Legget, 1962 and 1979; Legget and Karrow, 1983). Basal till (consolidated under the full weight of the glacier) is normally very dense, whereas ablation till (deposited from the glacier during ablation) may not be dense. Till is generally a good foundation material, but problems have arisen with the presence of soft layers and large boulders. Till may be difficult to excavate. Fine-grained till is generally susceptible to frost. Till or glacial till is unsorted glacial sediment. Till is derived from the erosion and entrainment of material by the moving ice of a glacier. It is deposited some distance down-ice to form terminal, lateral, medial and ground moraines. Till is classified into primary deposits, laid down directly by glaciers, and secondary deposits, reworked by fluvial transport and other processes. Till is a form of glacial drift, which is rock material transported by a glacier and deposited directly from the ice or from running water emerging from the ice.[1] It is distinguished from other forms of drift in that it is deposited directly by glaciers without being reworked by meltwater. Till is characteristically unsorted and unstratified, and is not usually consolidated. Most till consists predominantly of clay, silt, and sand, but with pebbles, cobbles, and boulders scattered through the till. The abundance of clay demonstrates lack of reworking by turbulent flow, which otherwise would winnow the clay.[6] Typically, the distribution of particle sizes shows two peaks (it is bimodal) with pebbles predominating in the coarser peak.

The larger clasts (rock fragments) in till typically show a diverse composition, often including rock types from outcrops hundreds of kilometers away. Some clasts may be rounded, and these are thought to be stream pebbles entrained by the glacier. Many of the clasts are faceted, striated, or polished, all signs of glacial abrasion. The sand and silt grains are typically angular to subangular rather than rounded.

It has been known since the careful statistic work by geologist Chauncey D. Holmes in 1941 that elongated clasts in tills tend to align with the direction of ice flow. Clasts in till may also show slight imbrication, with the clasts dipping upstream.

Though till is generally unstratified, till high in clay may show lamination due to compaction under the weight of overlying ice. Till may also contain lenses of sand or gravel, indicating minor and local reworking by water transitional to non-till glacial drift.

The term till comes from an old Scottish name for coarse, rocky soil. It was first used to describe primary glacial deposits by Archibald Geikie in 1863.[8] Early researchers tended to prefer the term boulder clay for the same kind of sediments, but this has fallen into disfavor.[9] Where it is unclear whether a poorly sorted, unconsolidated glacial deposit was deposited directly from glaciers, it is described as diamict or (when lithified) as diamictite. Tillite is a sedimentary rock formed by lithification of till.

2.Identification

Till can be identified in the field based on its physical characteristics, location, and geological context. Some common features of till that can help with identification include:

1. Texture: Till is typically composed of a mixture of different-sized particles, including rocks, boulders, pebbles, sand, silt, and clay. It is usually unsorted and unstratified, with no clear pattern of layering or bedding.

2. Location: Till deposits are typically found in areas that were covered by glaciers during past glacial periods, including much of North America, Europe, and parts of Asia. They may be found in a variety of landforms, including moraines, drumlins, eskers, and kames, which are all formed by the movement of glaciers and the deposition of sediment.

3. Geological context: Till deposits are often associated with other types of sedimentary rocks, such as sandstone, shale, or limestone, which can help to confirm their glacial origin. They may also contain fossils or other evidence of past environmental conditions, such as ancient soils or buried plant material.

4. Physical properties: Till can be identified based on its physical properties, such as its hardness, color, and shape. For example, some tills may be very hard and difficult to excavate, while others may be loose and crumbly. The color and shape of the rocks and other particles within the till can also provide clues to its origin and history.

To confirm the identification of till, additional tests may be conducted in a laboratory setting, such as grain size analysis, mineralogical analysis, or magnetic susceptibility measurements. However, in many cases, the physical characteristics and geological context of till are sufficient to identify it in the field.