Glacial erosion is limited by several factors. One is its foundation; tools (rock and mineral fragments, commonly referred to as rock flour) must remain embedded within moving ice for its full effect to be seen.
Quarrying (or plucking) is used to refer to this process.
Subglacial Erosion
Glacial landscapes exhibit a broad array of erosional conditions. Subglacial erosion refers to any type of sediment removal beneath a glacier; rates range from high removal rates to no erosion at all.
Under stable climate conditions, most glaciers tend to erode relatively slowly due to climatic factors and inertia. Subglacial erosion typically results from quarrying (producing cobble-sized sediment) and abrasion (resulting in silt-sized particles).
These modes of erosion produce a characteristically stoss-and-lee topography with smooth, stream-molded surfaces upstream and steep downstream. Quarrying and abrasion also produce distinctively glacial features such as photogenic surfaces with striae and an obvious glacial polish.
Subaerial Erosion
Subaerial refers to events or features located near or at Earth s surface that are exposed to atmospheric pressure. This term contrasts with subaqueous or submarine events or features that occur beneath bodies of water, or with subglacial events located beneath glacial ice.
Of course, glacially-influenced landscapes such as glaciated valleys have experienced erosion at rates considerably greater than long-term averages (Hallet, Reference Hallet1979); however, it must be remembered that erosion rates for such environments vary considerably over orders of magnitude.
Erosion rates differ considerably depending on several factors. These can include things like the presence or absence of a lubricating layer such as till, the depth to which glacier beds extend and moulin-fed streams’ ability to directly erode bedrock directly through moulin-fed streams, moulin-fed streams’ influence over bedrock erosion rates directly, climate-induced snowline variations due to surface melting, glacier tilts, ice sheet flows, sediment discharge etc all having significant influences on erosion rates.
Basal Erosion
As glacier flow erodes its bed, it also removes subglacial till. Till is a type of subglacial sediment similar to sawdust or fault gouge that not only acts as a lubricant for bedrock but also works to mitigate erosion by reducing stress concentrations, dampening water-pressure fluctuations that would otherwise drive rock-bed sliding, and spreading deformation over an extensive thickness. As glacier flows erode their beds further, till is also removed – an event which plays an integral part in quarrying operations, abrasion and glacier movement alike.
Measuring basal erosion requires making frequent observations over an extended period to understand all of its factors and capture all forces influencing it. Unfortunately, it’s hard to separate abrasion’s effects from those of quarrying; nonetheless, evidence shows that quarrying generally plays a more prominent role than abrasion in producing some classic features of glacier behavior, including photogenic smooth surfaces with striae and polished bedrock surfaces; in many instances this activity is facilitated by plucked clasts from bed surfaces being plucked up from bed (Reference Alley Gurnell and Christianson1987) which often results in steeper surface slopes than would otherwise occur with just abrasion alone.
Overdeepening
Overdeepening is the process by which glaciated landscapes are excavated below their expected levels, most frequently in cirques and the steps of glacial troughs. Overdeepening may occur as a result of rotational flow or rotational abrasion of less resistant rocks, while rotational flow also plays a part.
Glaciated basins typically experience higher erosion rates than non-glaciated basins of equal size and rock type (reference Koppes and Montgomery 2009). This is likely because basal processes erode bedrock faster than they can transport clasts (e.g. quarrying or sediment transport).
Abrasion creates the most distinctive glacial surface features, such as photogenic smoothness with striae and glacial polish (Reference Siman-Tov and others, 2013). Abrasion also plays an integral part in creating the classic “stoss-and-lee topography”, typically featuring minor abrasion on upstream sides of lee features as well as major quarrying operations on downstream sides; its formation may result from coupling between steep surface slopes and bed slopes caused by jointing patterns in bedrock formation facilitated by jointing patterns within bedrock joints.