Environmental Engineering Reference
In-Depth Information
Erosion
During the long, cold, moist periods of the Pleistocene, snow and ice accumulated to
tremendous thicknesses, estimated to be over 5000 ft in the New England states. As the
huge mass grew, it flowed as a rheological solid, acquiring a load of soil and rock debris.
Loose material beneath the advancing ice was engulfed and picked up, and blocks of pro-
truding jointed rock were plucked from the downstream side of hills to be carried as boul-
ders, often deposited hundreds of miles from their source over entirely different rock
types (hence the term
glacial erratics
).
As the debris accumulated in the lower portions of the glacier it became a giant rasp,
eroding and smoothing the surface. The abrasive action denuded the rock surface of soil
and weathered rock, and often etched the remaining rock surface with striations. As the
weather warmed, the glacier began to melt and recede, and streams and rivers formed to
flow from the frontal lobes of the ice mass.
Deposition
Glacial drift
is a general term for all deposits having their origin in glacial activity, and is
divided into two broad groups: till and stratified drift.
•
Till
is unstratified drift deposited directly from the ice mass, and
•
Stratified drift
is deposited by flowing water associated with melting ice.
Deposits are often classified by landform. The modes of occurrence and depositional fea-
tures of the more common glacial deposits are given in
Figure 7.77.
Moraines
The term moraine is often used synonymously with till or drift, but when used with mod-
ifiers it more correctly denotes a particular landform which can consist of only till, of mix-
tures of till and stratified drift, or of only stratified drift.
Ground moraine
denotes drift (till) deposited beneath the advancing ice, forming sheets
over the landscape. The surface is characteristically gently rolling and lacks ridge-like
forms.
Terminal moraine
is a ridge-like feature built along the forward margin of a glacier dur-
ing a halt in its advance and prior to its recession. Composed of various mixtures of drift,
a terminal moraine marks the farthest advance of the glacier. The topographic expression
as evident on the south shore of Long Island, New York, is given in
Figure 7.86.
At this
location it rises 100 ft above the surrounding outwash plain.
Moraine plain
(outwash plain) is deposited by the meltwaters of the glacier as its destruc-
tion begins during the occurrence of warmer temperatures.
Kettle moraine
refers to a terminal moraine with a surface marked with numerous
depressions (kettle holes), which result from the melting of large blocks of ice remaining
on the surface or buried at shallow depths.
Recessional moraines
are ridge-like features built of drift along the margins of the glacier
as they recede from their location of farthest advance. They represent a temporary stand
from retreat allowing an increase in deposition.
Interlobate or intermediate moraine
is a ridge-like feature formed between two glacier lobes
pushing their margins together to form a common moraine between them. They usually
trend parallel to the ice movement.
Frontal moraine
is an accumulation of drift at the terminus of a valley or alpine glacier,
but the term is also used to denote a terminal continental moraine.
