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Glacial Till Properties

Glacial Till Properties

The Earth, as we know it, is constantly changing and evolving, and with that evolution comes geological formations and structures that never cease to amaze us. We bring you information on glacial till, along with their properties, types, and the subsequent land formations made by them.
Rashmi Sunder
Workings of Mother Nature
An Esker is a long, snake-like ridge of glacial till deposited by a stream that ran under or within a glacier.
Our planet is one of its kind in the solar system. Geologists and geographers have charted the course of its physical evolution dating back to the original formation through collision with suspended matter in the system. We know of mountains and plains, plateaus and rivers, and everything in between. That obviously includes how these land forms were formed. But as we know now, the physical properties of the Earth are constantly evolving and reshaping themselves, and this process can span centuries or even take days.
One of the interesting formations that is found is that of a glacial till. When we think of tilling, we think of agricultural methods like digging, overturning, and stirring. But glacial till is a natural formation. It is essentially the unsorted sediment deposited directly by glacial ice. It is a soft rock identified by large angular rock fragments on the surface and within the soil. Glacial till forms a part of glacial drift, which is the coarsely graded and heterogeneous sediments of a glacier.

So how is glacial till formed, you ask?
FORMATION OF GLACIAL TILL
Glacial till, as we described earlier, consists of sedimentary deposits made by the movement of glacial ice. It is a composition made of materials like clay, sand, boulders, minerals, and gravel. This material comes from subglacial erosion and entrainment (which is the motion of either solid or fluid material) of the moving glacial ice of previously unconsolidated sediments. When glacial plucking or abrasion (friction cause by rubbing of rocks which causes them to disintegrate or form roughness on the surface) occurs, then the bedrock can also erode. This can result in the formation of clasts (which is a chunk or a small bit of rock broken off an outcrop by physical weathering) of various sizes, which get incorporated into the glacier's bed. Eventually, the sedimentary assemblage forming this layer will be abandoned some distance downward from various sources. This is the process of glacial till deposition.
Now that we know how till is formed, let us look at the different types.
TYPES OF GLACIAL TILL
Tillite
When till has been indurated or lithified by subsequent burial into solid rock, it is known as tillite.

Further, till is divided based on the following basis:

Primary Deposits
This is the type of till that is directly laid down by glacial movement.

Secondary Deposits
This is the till that has undergone reshaping and eventual weathering by glacial movement and other forces.
Glacial Till
Depositional Settings of Glacial Till
Primary deposits can be further divided into the following:
Subglacial Tills
This is the kind of till that is deposited within the glacier, and has been further divided into the following:
  • Lodgement tills- This is sediment that has been deposited by plastering of glacial debris from a sliding glacier bed, or in simple terms, it's the till lodged by ice onto the substrate.
  • Deformation till- This is sediment that has been disaggregated and (usually) homogenized by shearing in the subglacial deformed layer. This is essentially the deformation of non-glacial sediments.
  • Melt-out tills- They are released by melting of stagnant or slowly moving, debris-rich glacier ice and deposited without any subsequent transportation or weathering. This is further divided into subglacial melt-out till (melting of debris-rich ice at the bottom of the glacier) and supraglacial melt-out till (which we will read about in the following section).
Supraglacial Tills
Supraglacial tills form on the surface of the glacier or glacial ice. This is divided further into the following:
  • Sublimation till- This is similar to melt-out till, except the ice is lost through sublimation rather than melting. It often occurs only in extremely cold and arid conditions like Antarctica.
  • Supraglacial melt-out till- Mentioned earlier, this is the melting of ice on the glacier surface; for example, on lateral and medial moraines (which we will discuss later).
Ablation Tills
Ablation till accumulates initially in a supraglacial position and is later brought down to the ground surface by melting of ice within the glacier, known as undermelting. This is further divided into the following:
  • Flow till - It consists of debris that has built up on glacial ice, and after saturation with melted water, it becomes so unstable that it flows or slumps into nearby hollows or crevices.
Melt-out tills are also considered to be a part of the ablation till, but that is based on the way you classify glacial tills.
We know now about the different types of tills. But what are its properties and characteristics? Before we get into it, you might notice that we have mentioned "properties" and "characteristics" as separate terms. In general terms, they both mean one and the same thing. But when we are talking science, they take on slightly deviated meanings. "Properties" refer to the general attributes that are possessed by all members of the specific class, whereas "characteristics" refer to the specific attributes that are intrinsic to a specific matter. If you think about it, property is a noun and characteristics are the adjectives. Now that this is out of the way, let us look at how tills are characterized and what are its properties.
PROPERTIES AND CHARACTERISTICS OF GLACIAL TILL
The following are the general properties of glacial till:
  • They are dense and stiff, and this density and stiffness is a function of the mode of transportation rather than the process of consolidation.
  • Ablation deposits within lodgment/deformation till contain less dense and softer material.
  • The degree of consolidation depends on drainage profile and stiffness of underlying soils.
  • They behave as a 'drained' material because of the stiffness even though they have low permeability.
The following are the characteristics of subglacial tills:
TYPE PARTICLE SHAPE PARTICLE SIZE DISTRIBUTION FABRIC PACKING LITHOLOGY STRUCTURE
Lodgement Rounded edges, spherical, striated and faceted Bi-modal (silt-clay and gravel
plus) or multi-modal
Elongated particles are
aligned with flow direction
Dense and well consolidated Local rock types dominate Structureless, but with shear planes and foliations
Subglacial melt-out Rounded edges, spherical, striated and faceted Bi-modal (silt-clay and
gravel plus) or multi-modal (sorting is possible)
Elongated particles are aligned with flow direction (but less than lodgment) Dense and well consolidated (but less than lodgment) Local rock types dominate Normally structureless but sometimes stratified (not sheared)
Deformation Depends on parent
material
Diverse, may include rafts of original material Strong fabric in the direction of shear Dense and well consolidated Depends on parent m

According to Virginia International University (After Bennett and Glasser, 2009)
These are the characteristics of supraglacial tills:

TYPE PARTICLE SHAPE PARTICLE SIZE DISTRIBUTION FABRIC PACKING LITHOLOGY STRUCTURE
Meltout Mostly angular, not
striated or faceted
Coarse and unsorted
(similar to talus) Sorted if
water-worked
Poorly developed Poorly consolidated Can be quite variable
(depends on size of glacier)
Structureless, but with possible crude bedding
Flow Mostly angular, not striated or faceted Coarse and unsorted
Individual flow packages may be sorted.
Variable. Individual flow packages may show a flow fabric. Poorly consolidated Can be quite variable May see evidence of
individual flow packages
Sublimation Rounded edges, equant, striated and faceted Bi-modal (silt-clay and gravel plus) or multi-modal Elongated particles are aligned with flow direction Poorly consolidated Variable Stratified with possible fold
structures

According to Virginia International University (After Bennett and Glasser, 2009)
Finally, let us look at the types of landforms made by glacial tills.
LANDFORMS PRODUCED BY GLACIAL TILL
Glacial Landforms
Glacial Landforms
Glacial tills form varied landforms based on the method of transportation, consolidation, the sediments present in the till, and the location where it is deposited.
These are some of the types of landforms that tills can be turned into:
Glacial Till Soil
Glaciers are extremely effective at eroding and transporting these materials and everything from clay- to boulder-sized particles are moved as one large mass. As a result, ground-up bedrock, plant fragments, and even animal remains can be found in glacial till. Over time, with the process of erosion and weathering, the components break down to form soil that is very rich in minerals, which can be turned into a high-grade farmland.
Moraine
Moraine
Moraine
Moraines are deposits of glacial till that are left behind when a glacier recedes or that are carried on the top layer of alpine glaciers. These can be divided further:
  • Lateral Moraines- These consist of rock debris and sediments that have been loosened from the walls beside a valley glacier and have been built up in ridges along the sides of it.
  • Medial Moraines- These are long ridges of till that result when lateral moraines come together as two tributary glaciers and merge to form a single glacier. As more tributary glaciers join the main body of ice, a series of roughly parallel medial moraines develop on the surface of main glacier.
  • End Moraines- This is a large, crescent-shaped pile of till that builds up at the end of a glacier. This is further divided into:
  1. Terminal Moraine: It is the ridge of till that marks the farthest advance of the glacier before it starts to recede.
  2. Recessional Moraine: It develops at the front of the receding glacier, and a series of such moraines mark the path of a retreating glacier.
  3. Ground Moraines- This is the thin, widespread layer of till that is deposited across the surface as an ice sheet melts.
Drumlins
When ground moraine material gets reshaped by subsequent glaciers into streamlined hills which are long, narrow, rounded ridges of till whose long axes parallel the direction the glacier traveled, it is known as a drumlin.
Outwash and Outwash Plain
When till is released from the ice into the flowing water, the sediments get deposited by glacial meltwater, and this is called an outwash. Since they have been transported by running water, the outwash deposits are braided, sorted, and layered. The broad front of outwash associated with an ice sheet is called an outwash plain.
Kames
These are the steep‐sided mounds of stratified till that were deposited by meltwater in depressions or openings in the ice or as short‐lived deltas or fans at the mouths of meltwater streams.
Kettle and Kettle Lakes
Sometimes, the rapid buildup of sediments can bury isolated blocks of ice. When this ice melts, the resulting depression is called a kettle. Kettle lakes are bodies of water that occupy such kettles.
Eskers
These are the long, winding ridges of outwash that are deposited in streams flowing through ice caves and tunnels at the base of the glacier. They are generally well sorted and cross‐bedded, and as such, esker sands and gravels eventually choke off the waterway.
Varve
It consists of one light‐colored bed and one dark‐colored bed that represent a single year's deposition. The light‐colored layer is mainly silt that was deposited rapidly during summer, and the dark layer consists of clay and organic material that was formed during the winter. The age of a glacial lake can be determined from the number of varves that have formed at the bottom of the lake.
We have covered most of the aspects of glacial tills, and hopefully this article has furthered your understanding on the subject.