Glacial erosion produces very distinctive landforms, including glacial striations lines, cirques and drumlins.
Glacial erosion takes place through abrasion: moving ice scrapes the bedrock like sandpaper, leaving marks called glacial striations marks behind.
Erosion is most prevalent in active glaciated valleys; erosion tends to be much lower in nonglaciated regions.
Abrasion occurs when rocks embedded in glacier ice scrape against bedrock, grinding and scratching its surface, leaving behind long, straight scratches (called striations marks ) on its surface that serve as evidence that glaciers once covered an area. These marks serve as one of the strongest indicators that a glacier once covered an area.
Glacial ice can be extremely abrasive due to the immense amounts of rock, sediment, and debris pounded into its basal ice by glacier weight; or when exposed directly to hydraulic pressure from subglacial melt water.
Rainwater falling onto glaciers can also intensify erosion by lubricating the interface between the ice and rock beneath, enabling faster sliding across it, increasing abrasion and erosion rates.
Plucking refers to the process by which glacial erosion ploughs or ejects rock particles from beneath the surface of a glacier, creating erosional action beneath its surface. This slow-speed process depends on bedrock lithology and subglacial water pressure fluctuations for its speed; typically occurring along preexisting cracks in bedrock beds.
When combined, plucking and abrasion create long scratches or “glacial striations,” providing evidence that glaciers once covered this region and helped shape its landscapes as we know them today.
Nunataks are rocky outcrops left behind by glaciers and can be highly identifiable. Famous examples of nunataks include the Matterhorn in Switzerland and Queen Louise Land in Greenland; but more subtly formed drumlins (see Figure 4.3.1) can also be nunataks; these feature smooth surfaces near their down-ice end with sharp angles or even corners similar to what you see here.
Glacial landforms can be formed through erosion and plucking processes, producing features like cirques, aretes, rock basins and fjords on an extensive scale that most people can recognize after only taking a few courses in geology.
Some features created by glacial erosion have an enormous influence on local human communities. One such landscape feature created by glacial erosion is the corrie (an armchair-shaped hollow filled with water that often draws mountain climbers).
Tarns are high altitude lakes formed when water accumulates within a corrie. Yosemite National Park’s rock formations were shaped by glacial erosion and now provide some of the world’s most challenging climbing terrain.
Freeze-thaw weathering (also referred to as frost wedging) is a physical process which causes rocks to disintegrate over time. It works when water seeping into cracks at night freezes and expands by an average of 9% when freezing at night, gradually breaking apart the rock over time. Thermal stress can further deteriorate it further with outer layers flaking off into thin sheets, also known as exfoliation.
Cryogenic processes lead to the formation of cracks and fissures in rocks, made worse by repeated expansion and contraction caused by freezing and thawing water.
Freeze-thaw weathering is one of the primary drivers of erosion in glaciated environments, eroding bedrock more aggressively along the central flowline than its lateral margins and producing long, narrow valleys with steep walls, as well as leading to the creation of cirques, gorges and surface alluvial fans.