Civil Engineering Reference
In-Depth Information
and moving water deposits are built in steps. Figure 4.3(c) shows a desert environ-
ment. Hot days and cold nights cause thermal weathering of rock mountains which
produces scree slopes. Rare flash floods transport material across the desert floor
(or pediment), depositing coarse material first and fine material later, probably
in fans and layers. Winds cause migration of sand dunes. Large daily tempera-
ture variations with occasional rainfall cause physical alteration of the soils in the
pediment.
These are only three typical depositional environments. They are discussed in more
detail, together with other examples, by Fookes and Vaughan (1986). Much of the
United Kingdom north of a line from the Thames to the Severn estuaries is covered
with a veneer of glacial deposits. Most natural and man-made lakes are currently
collecting layered silt and clay deposits. Large rivers (e.g. Nile, Ganges, Mississippi)
are currently building delta deposits. Modern deserts occur widely throughout Asia,
Australia, Africa, North and South America and theMiddle East. Glacial environments
occur in high latitudes (e.g. Greenland, Antarctica) and at high altitudes.
These typical depositional environments can be recognized in ancient rocks.
For example, the London Clay was deposited in a shallow sea; the Chalk is calcium
carbonate deposited in a warm sea; the New Red Sandstone in the Triassic and the
Old Red Sandstone in the Devonian are ancient desert deposits. The important point
to make here is that you should study present-day depositional environments as an
aid to interpretation of ground investigations; if a geologist can tell you the environ-
ment into which a soil or rock was deposited you have a very good idea of what to
expect.
4.6 Recent geological events
Although the depositional environment has a major influence on the formation of soils
and rocks, they are altered by later geological events such as further deposition or
erosion, folding and faulting and volcanic activity. For soils and soil mechanics the
most significant recent geological events are rising or falling land and sea levels which
lead to continuing deposition or erosion.
Land and sea levels rise and fall relative to one another for a variety of reasons,
including plate movements and mountain building. One of the most important causes
of changes of sea level is temperature change. During an ice age the sea cools and
contracts and ice remains on the land as glaciers; the weight of ice depresses the land
which rebounds as the ice melts. At the end of the last ice age, about 20 000 years ago,
the sea level was about 100 m lower than it is now, so the UK coastline was west of
Ireland and you could have walked to France (if you could cross the large river flowing
through the Straits of Dover).
During a period of rising sea levels (e.g. at the end of an ice age) soils are deposited
around the coasts. If the sea level remains stationary for some time vegetation grows,
which is submerged and decays to peat as the sea level rises again. In the United
Kingdom extensive deposits of this kind are found in the Wash and in the Somerset
Levels. Continuing sea level rise and deposition leads to deposits of normally consol-
idated soils (see Chapter 15) which are soft and weak near the surface but become
stronger with depth. During a period of falling sea level (e.g. at the beginning of an ice
age) the land becomes exposed and subject to weathering, erosion and transportation.
