Chapter 18 Glaciation
Sediments transported and deposited during the Pleistocene glaciations are abundant throughout Canada and much of the northern USA. They are important sources of construction materials and are valuable as reservoirs for groundwater. Because they are almost all unconsolidated, they have significant implications for mass wasting.
Figure 16.4.1 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 icefield 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 in Figure 16.4.2. There are many types of glacial sediment generally classified by whether they are transported on, within, or beneath the glacial ice. The main types of sediment in a glacial environment are described below.
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 terminal moraine. The end moraine that represents the farthest advance of the glacier is a end moraine. Supraglacial and englacial sediments can also be deposited when the ice melts. Sediments transported and deposited by glacial ice are known as till.
Subglacial sediment (e.g., lodgement till) is material that has been eroded from the underlying rock by the ice, and is moved by the ice. It has a wide range of grain sizes (in other words it is poorly sorted), including a relatively high proportion of silt and clay. The larger clasts (pebbles to boulders in size) tend to become partly rounded by abrasion. Lodgement till forms as a sheet of well-compacted sediment beneath a glacier, and ranges from several centimetres to many metres in thickness. Lodgement till is normally unbedded. An example is shown in Figure 16.4.3a.
Supraglacial 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 16.0.1) and, where two glaciers meet, medial moraines. (Medial moraines are visible on the Aletsch Glacier in Figure 16.3.4.) 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 in Figure 16.4.3b. When supraglacial sediments become incorporated into the body of the glacier, the