What is Glacial Till?
Glacial till is the deposition of sediment by a glacier. It overlaps glacier forefields, can be assembled to create moraines and other glacier landforms, and is omnipresent in glacial environments. It contains a very distinctive composition which typically appears from the fact that glaciers not only grind rocks, fragmenting them into fine, small pieces but they also take off large chunks of rock. This reflects that glaciers transport everything from big boulders to small grains smaller than sand.
What is a Till?
till is any substance laid down directly or revamped by a glacier. Essentially, it is a mixture of rock fragments and gibber stones in a fine-grained muddy or sandy matrix. The absolute composition of any specific till, however, depends on clay content, boulder size and their blend. However, in glacial landform, the type of deposition is called lodgment till.
Types of Glacial Till
There are different types of glacial sediment usually categorized by whether they are transported on, within, or under the glacial ice. The primary types of sediment in a glacial environment are defined as below.
Supraglacial (on top of the ice) and englacial (within the ice) sediments which skid off the melting front of a stationary glacier are able to create a ridge of unsorted sediments known as end moraine. The end moraine which reflects the farthest advance of the glacier is called a terminal moraine. Sediments carried and amassed by glacial ice are called till.
Subglacial sediment (example is lodgement till) is a substance which has been eroded from the underlying rock by the ice, and is shifted by the ice. Supraglacial sediments are principally extracted from freeze-thaw eroded substance that has fallen onto the ice from rocky slopes above. These sediments create lateral moraines and, where two glaciers meet, medial moraines.
Glaciated Till
The sediments generated by glacial grinding are quite distinctive. Glacial till consist of sediments of every size, from minute particles smaller than a grain of sand to huge stones, all jumbled up together.
These sediments and rocks are blended altogether in a jumble after they are deposited. On the contrary, rocks and sediments collected by rivers settle out as the water slows down the speed, so huge boulders are frequently dropped before small grains of sand. Instead of jumbling sediments of every size, rivers organize them in a manner that viscous glaciers cannot.
Glacial Flour
Glacier flour defines the component of glacier sediment which is way finer than sand. This substance contains similar consistency to flour, which is why for its name. Since this sediment is so fine, it is effortlessly transported by and suspended in water. It is that flour tiniest size of sediment (much smaller than sand) and is accountable for the milky, colored water in the rivers, ponds, and lakes that are nourished by glaciers. Glacier lakes consist of a large-scale of beautiful colors which appears as sunlight scatters when it hits sediment particles in the water.
Quick Facts
When a glacier melts, all of the sand, mud and rock that it was bearing get left behind. This mixture of sediment is what we call till.
Blocks of ice left behind in the till steadily melt developing depressions termed as kettle holes. These can fill with water creating kettle lakes.
An erratic is a boulder which has been carried a long way by a glacier. Erratics are different types of rock than the local bedrock.
Till might just be a meter or two thick or tens of metres thick based on how much residue was in the ice.
Several tills can get constructed if glaciers advance and melt a number of times.
The composition of till demonstrates the geology of the region the ice has flowed over. This typically implies that till from various parts of a massive glacier will contain different types of minerals and rocks.
At times precious minerals can be detected to their bedrock source.
Geologists compare maps that display where there are high concentrations of minerals like gold or platinum with maps that depict which direction the glacier shifted. This is what they call drift prospecting.
FAQs on Glacial Till
1. What exactly is glacial till?
Glacial till is a type of sediment that is directly deposited by a glacier as it melts and retreats. It is characteristically unsorted, meaning it contains a wide mixture of materials ranging from fine clay and silt to sand, gravel, and large boulders, all jumbled together without any layering.
2. What are the main characteristics of glacial till?
The key characteristics that help identify glacial till include:
- Poor Sorting: It is a diamicton, a mixture of rock fragments of all sizes, from fine clay to massive boulders.
- Angular Clasts: The rocks and pebbles within the till are often sharp and angular, as they haven't been rounded by flowing water.
- No Stratification: Unlike sediments deposited by water or wind, till lacks distinct layers and appears as a massive, jumbled deposit.
- Variable Composition: Its mineral and rock content reflects the geology of the terrain the glacier has travelled over.
3. Why is glacial till sometimes called 'boulder clay'?
Glacial till is often called boulder clay because this name aptly describes its most common composition. It typically consists of a fine-grained matrix of clay, which binds together larger, unsorted rock fragments, including pebbles, cobbles, and boulders. This term highlights the two dominant components of the deposit.
4. How are the different types of glacial till classified?
Glacial till is mainly classified based on its position of deposition relative to the glacier. The two primary categories are:
- Subglacial Till: This is material deposited at the base of the glacier. It includes lodgement till (plastered onto the ground by the moving ice) and melt-out till (released as the basal ice melts in place). It is typically dense and compacted.
- Supraglacial Till: This is material transported on or near the glacier's surface and deposited as the ice melts away. It includes melt-out till from the surface and flow till, which slides or flows off the glacier's snout. It is generally looser than subglacial till.
5. What is the difference between glacial till and a moraine?
The key difference is that glacial till is the material, while a moraine is the landform. A moraine is a ridge, mound, or sheet-like accumulation of glacial till that is formed at the edges, front, or beneath a glacier. In simple terms, moraines are made of glacial till.
6. How does the formation of glacial till influence the landscape?
Glacial till significantly shapes landscapes by forming extensive, gently rolling plains known as till plains. These deposits blanket the underlying bedrock, smoothing out the pre-existing topography. Furthermore, accumulations of till create distinct glacial landforms such as moraines, drumlins, and flutes, which provide geologists with clear evidence of past glacial movement and extent.
7. Can glacial till create fertile soil, and if so, how?
Yes, glacial till can form very fertile soils. This is because the grinding action of the glacier pulverises a wide variety of rock types, releasing a rich mix of essential minerals and nutrients. Unlike soils developed from a single parent rock, till-derived soils are geochemically diverse, providing a broad spectrum of nutrients necessary for agriculture. Many of the world's most productive farming regions, such as the American Midwest, are located on top of ancient till plains.
