4.4 Glacial Deposits
Sediments transported and deposited during the Pleistocene glaciations are abundant throughout Canada and the northern US. They are important sources of construction materials and as reservoirs for groundwater, and, because they are almost all unconsolidated, they have significant implications for mass wasting.
Figure 4.4.1 illustrates some of the ways that sediments are transported and deposited. The Bering Glacier is the largest in North America, and although most of it is in Alaska, it flows from an ice field that extends into southwestern Yukon. The surface of the ice is partially, or in some cases completely covered with rocky debris that has fallen from surrounding steep rock faces. There are muddy rivers issuing from the glacier in several locations, depositing sediment on land, into Vitus Lake and directly into the ocean. There are dirty icebergs shedding their sediment into the lake. And, not visible in this view, there are sediments being moved along beneath the ice.
The formation and movement of sediments in glacial environments is shown diagrammatically on Figure 4.4.2. The main types of sediment in a glacial environment are as follows: Supraglacial (on top of the ice) and englacial (within the ice) sediments that slide off the melting front of a stationary glacier can form a ridge of unsorted sediments called an end moraine. The end moraine that represents the furthest advance of glacier is a terminal moraine.
Sub-glacial till (the most abundant of which is lodgement till) is material that has been eroded from the underlying rock by the ice, and is moved by the ice, and emplaced on the bed by friction generated by the weight of overlying ice. It has a wide range of grain sizes, including a relatively high proportion of silt and clay. The larger clasts (pebbles to boulders in size) tend to get partly rounded by abrasion. When a glacier eventually melts the lodgement till is exposed as a sheet of well-compacted sediment ranging from several centimetres to many metres in thickness. Lodgement till is normally unbedded. An example is shown on Figure 4.4.3a.
Supra-glacial sediments are primarily derived from freeze-thaw eroded material that has fallen onto the ice from rocky slopes above. These sediments form lateral moraines (Figure 4.4.4) and, where two glaciers meet: medial moraines. (Medial moraines are visible on the Aletsch Glacier on Figure 4.3.3.) Most of this material is deposited on the ground when the ice melts, and is therefore called ablation till, a mixture of fine and coarse angular rock fragments, with much less sand, silt and clay than lodgement till. An example is shown on Figure 16.30b. When supraglacial sediments get incorporated into the body of the glacier they are known as englacial sediments (Figure 4.4.2).
Massive amounts of water flow on the surface, within and at the base of a glacier, even in cold areas and even when the glacier is advancing. Depending on its velocity, this water is able to move sediments of various sizes and most of that material is washed out of the lower end of the glacier and deposited as outwash sediments. These sediments accumulate in a wide range of environments in the proglacial region (the area in front of a glacier), most in fluvial (river) environments, but some in lakes and some in the ocean.
A large proglacial plain of sediment deposition is called a sandur, and within that area glacio-fluvial deposits can be tens of metres thick. The sandur shown on Figure 4.4.5 covers an area of over 1000 km2 along the southern coast of Iceland near to Vatnajökull (the largest glacier in Iceland). Glacio-fluvial sediments are generally similar to sediments deposited in normal fluvial environments, and are dominated by silt, sand and gravel. The grains tend to be moderately well rounded, and the sediments have sedimentary structures (e.g., bedding, cross bedding, clast imbrication) that are similar to those formed by non-glacial streams (Figure 4.4.6).
In situations where a glacier is receding, a block of ice might become separated from the main ice sheet and then could get buried in glacio-fluvial sediments. When the ice block eventually melts a depression will form, and if this fills with water it is known as a kettle lake (Figure 4.4.7).
A subglacial stream will create its own channel within the ice, and sediments that are being transported and deposited by the stream will build up within that channel. When the ice recedes, that sediment will remain to form a long sinuous ridge known as an esker. Eskers are most common in areas of continental glaciation. They can be several metres high, tens of metres wide and tens of kilometres long (Figure 4.4.8).
Outwash streams can flow into proglacial lakes where glacio-lacustrine sediments are deposited. These are dominated by silt- and clay-sized particles and are typically laminated on the millimetre scale. In some cases varves develop: a series of beds that each has distinctive summer and winter layers. Relatively coarse sediments are deposited in the summer when melt discharge is high, and finer sediments are deposited in the winter, when discharge is very low. Ice bergs are common on pro-glacial lakes (see Figure 4.2.12), and most of them contain englacial sediments of various sizes. As these bergs melt, the released clasts sink to the bottom and get incorporated into the glacio-lacustrine layers as drop stones (Figure 4.4.9a).
The processes that occur in proglacial lakes can also take place where a glacier terminates in the ocean. These are called glacio-marine sediments (Figure 4.4.9b).
Exercise 4.4 Identify Glacial Depositional Environments
Figure 4.4.1 shows the Bering Glacier in Alaska. Glacial sediments of many different types are being deposited throughout this area. Identify where you would expect to fine the following: a) glacio-fluvial sand, b) lodgement till, c) glacio-lacustrine clay with drop stones, d) ablation till, and e) glacio-marine silt and clay.
Exercise answers are provided Appendix 2
- Figure 4.4.1 NASA Earth Observatory‘s Image of the Day for Aug 3, 2004, public domain
- Figure 4.4.2 Steven Earle, CC BY 4.0
- Figure 4.4.3 Photos by Steven Earle, CC BY 4.0
- Figure 4.4.4 Photo by Steven Earle, CC BY 4.0
- Figure 4.4.5 Photo by Steven Earle, CC BY 4.0
- Figure 4.4.6 Photos by Steven Earle, CC BY 4.0
- Figure 4.4.7 Photo by Steven Earle, CC BY 4.0
- Figure 4.4.8 Photo from Agriculture and Agri-food Canada, https://sis.agr.gc.ca/cansis/images/nt/locsf/index.html, Open Government Licence – Canada,
- Figure 4.4.9 Photos by Steven Earle, CC BY 4.0