DEFINITION OF GLACIAL TRANSPORT

Glaciers transport enormous amounts of rock debris, including

some large boulders, gravel, sand, and fine silt, called till.

The glacier may carry this at its base, on its surface, or internally.

Glacial deposits are characteristically poorly sorted or

non-sorted, with large boulders next to fine silt. Most of a

glacier’s load is concentrated along its base and sides, because

in these places plucking and abrasion are most effective.

Active ice deposits till as a variety of moraines, which

are ridge-like accumulations of drift deposited on the margin

of a glacier. A terminal moraine represents the farthest point

of travel of the glacier’s terminus. Glacial debris left on the

sides of glaciers forms lateral moraines, whereas where two

glaciers meet, their moraines merge and are known as a medial

moraine.

Rock flour is a general name for the deposits at the

base of glaciers, where they are produced by crushing and

grinding by the glacier to make fine silt and sand. Glacial

drift is a general term for all sediment deposited directly by

glaciers, or by glacial meltwater in streams, lakes, and the

sea. Glacial marine drift is sediment deposited on the

seafloor from floating ice shelves or bergs and may include

many isolated pebbles or boulders that were initially

trapped in glaciers on land, then floated in icebergs that

calved off from tidewater glaciers. The rocks melted out

while over open water and fell into the sediment on the

bottom of the sea. These isolated stones are called dropstones

and are often one of the hallmark signs of ancient

glaciations in rock layers that geologists find in the rock

record. Stratified drift is deposited by meltwater and may

include a range of sizes, deposited in different fluvial or

lacustrine environments.

Glacial erratics are glacially deposited rock fragments

with compositions different from underlying rocks. In many

cases the erratics are composed of rock types that do not

occur in the area they are resting in but are only found hundreds

or even thousands of miles away. Many glacial erratics

in the northern part of the United States can be shown to

have come from parts of Canada. Some clever geologists have

used glacial erratics to help them find mines or rare minerals

that they have located in an isolated erratic—they have used

their knowledge of glacial geology to trace the boulders back

to their sources following the orientation of glacial striations

in underlying rocks. Recently, diamond mines were discovered

in northern Canada (Nunavut) by tracing diamonds

found in glacial till back to their source region.

Sediment deposited by streams washing out of glacial

moraines is known as outwash and is typically deposited by

braided streams. Many of these form on broad plains known

as outwash plains. When glaciers retreat, the load is diminished,

and a series of outwash terraces may form.

See also ICE AGE.

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