Geoscience Reference
In-Depth Information
estimating the passage of time, and this is a theme that we will revisit
later. The use of fragments of rock, or sediment, is discussed here.
DELTAS
The fourth letter of the Greek alphabet, , is applied to triangular
deposits of sediment found occasionally in lakes but more usually at
the mouths of many rivers where they drain into the sea. These
features may be of varied types such as arcuate (the Nile, Rhˆ ne and
Po), bird's foot (Mississippi) and cuspate (Tiber), and may be complex,
comprising more than one river channel and many distributaries each
divided from its neighbour by sand bars and low flat islands. In addi-
tion deltas may consist of many gradually moving complexes: sixteen
lobes have been mapped out in the Mississippi delta, the oldest of
which is 7,000 years old. The best-known deltas in the world are those
of the rivers Ganges and Brahmaputra in northeast India (which form
one system and are generally treated together) which has an area of
105,000 square kilometres, the River Mississippi that flows into the
northern shore of the Gulf of Mexico, the Amazon that feeds sediment
into the westernmid-Atlantic and the River Nile that empties into the
Mediterranean. The River Amazon disgorges 1.2 thousand million
cubic metres of predominantly suspended silt and mud every year,
and as a result the delta front is prograding, or growing out into the
ocean at a rate of 5-10 cm a year. The Ganges is not the longest river in
the world (that honour belongs to the Nile at over 6,600 kilometres)
but it deposits more material than does its African counterpart. This
is because each river system is unique and its physical features and
the volume of material that it carries are controlled by a number of
factors such as the river profile, which broadly is the slope of the river
from source to estuary. The greater the slope, the greater the power
of the water carried within the river to erode the terrain through
which it flows. The Nile flows over largely flat expanses of the north
African continent, in direct contrast to the Ganges which drains the
Himalayas, which are continuing to rise following the collision of
India with the Asian continent some 50 million years ago. This
