Engineering Geology

Chapter 3
Other small ridge-like kames accumulate in crevasses in stagnant or near-stagnant ice. Many
kames do not survive deglaciation for any appreciable period of time.
Eskers are long, narrow, sinuous, ridge-like masses of stratified drift that are unrelated to surface
topography (Fig. 3.29). For example, eskers may climb up valley sides and cross low
watersheds. They represent sediments deposited by streams that flowed within channels in
a glacier. Although eskers may be interrupted, their general continuity is easily discernible
and, indeed, some may extend lengthwise for several hundred kilometres. Eskers may reach
up to 50 m in height, and they range up to 200 m in width. Their sides are often steep. Eskers
are composed principally of sands and gravels, although silts and boulders are found within
them. These deposits are generally cross-bedded.
Other Glacial Effects
Ice sheets have caused diversions of drainage in areas of low relief. In some areas that were
completely covered with glacial deposits, the post-glacial drainage pattern may bear no relationship
to the surface beneath the drift, indeed moraines and eskers may form minor water
divides. As would be expected, notable changes occurred at or near the margin of the ice.
Lakes were formed there that were drained by streams whose paths disregarded pre-glacial
relief. Evidence of the existence of pro-glacial lakes is to be found in the lacustrine deposits,
terraces and overflow channels that they leave behind.
Where valley glaciers extend below the snowline, they frequently pond back streams that flow
down the valley sides, giving rise to lakes. If any col between two valleys is lower than the
surface of the glacier occupying one of them, then the water from any adjacent lake dammed
by this glacier eventually spills into the adjoining valley, and in so doing, erodes an overflow
channel. Marginal spillways may develop along the side of a valley at the contact with the ice.
The enormous weight of an overlying ice sheet causes the Earth’s crust beneath it to sag.
Once the ice sheet disappears, the land slowly rises to recover its former position and,
thereby, restores isostatic equilibrium. Consequently, the areas of northern Europe and North
America presently affected by isostatic uplift more or less correspond with those areas that
were formerly covered with ice. At present, the rate of isostatic recovery, for example, in the
centre of Scandinavia, is approximately one metre per century. Isostatic uplift is neither regular
nor continuous. Consequently, the rise in the land surface so affected has been overtaken at
times by a rise in sea level. The latter was caused by melt water from the retreating ice sheets.
With the advance and retreat of ice sheets in Pleistocene times, the level of the sea fluctuated.
Marine terraces (strandlines) were produced during interglacial periods when the sea was
at a much higher level. The post-glacial rise in sea level has given rise to drowned coastlines
123