climate. During the glacial periods there was less
evaporation and therefore less rainfall. North Africa
and the Sahara were then very much drier than the
present day.
Iceland has good examples of desert regions which
are due to a cold climate, and has problems with soil
erosion due to lack of vegetation in many areas.
Shortly after the withdrawal of the ice across
Scandinavia there was a desert with considerable aeolian deposits, which formed before the vegetation
cover developed. Loess deposits are fine-grained
sediments which form largely through aeolian erosion
and transport from deserts and also from glacial
sediments exposed after glacial retreat. Large areas
of China, for example west of Beijing, are covered
by up to several hundred metres of Quaternary loess.
The lack of vegetation naturally means that aeolian
transport and deposition are important in deserts, but
only a relatively small part of the desert areas are
covered by wind-blown sand.
Large areas consist of bare rocks and mountains
with little sediment. In other regions there is only wind
erosion (deflation), which leaves the ground covered
(armoured) by a layer of stones which protect it from
further erosion. Even in desert regions like the Sahara
many areas are dominated by a fluvial drainage pattern. Although several years may pass between rains in
this area, a heavy rainstorm may transport so much
sediment that the fluvial drainage pattern survives for
many years.
Large expanses of wind-blown sand are called ergs.
They may be formed by the coalescence of different
aeolian bedforms. Barchans are crescent-shaped
isolated aeolian dunes with a convex erosion side
and a concave lee side (Fig. 2.27). Barchans are
found mainly on the edges of the erg area, where
there is not enough sand to form a continuous thick
cover. Transverse dunes are common within ergs. Seif
dunes are long, straight sand dunes which may occur
within an erg but are also found in areas with incomplete sand cover. In the central zones very large dunes
form which may be over 100 m high and 1 km long.
Ancient aeolian deposits are recognised by:
1. Their large-scale cross-bedding, up to 20–30 m and
often with wind ripple marks on top of slanting
cross-bedded surfaces (foreset beds).
2. High degree of oxidation which gives a red colour.
3. Good sorting – largely medium- to fine-grained
sand.
4. Lack of fossils or organic material.
The lack of thin silt or clay beds between the crossbedded layers, and the general association with
overlying and underlying sediments, are also important criteria. One of the best-known examples of an
aeolian sandstone is the Navajo Sandstone (Jurassic)
in the Rocky Mountains. In Northern Europe there are
aeolian sandstones in the Lower Permian
(Rotliegendes) and the equivalent Yellow Sand in
northeastern England (Fig. 2.27).
In pre-Devonian times all continental environments
were deserts in the sense that they lacked vegetation.
Also in these rocks, however, we can distinguish
between dry and wet climates, largely from the nature
of the fluvial or aeolian deposits.
Sand dunes are always moving and may end up in
the sea, still partly preserving an aeolian sorting and
grain-size distribution.
2.26 Lacustrine Deposits
How Are Lakes Formed?
In principle we have three types of lake:
1. Lakes of glacial origin:
(a) lakes formed through glacial erosion,
(b) lakes formed through damming by moraines or
by the glaciers themselves.
2. Lakes of tectonic origin:
(a) lakes formed in areas of rapid tectonic subsidence (rifting) or more uniform subsidence,
(b) lakes formed as a result of damming by horsts
which have been elevated through faulting, or
damming by lava etc.
3. Lakes formed by sedimentary processes, e.g.
oxbow lakes in fluvial environments, and delta top
lakes.
Lakes of glacial origin will have a relatively short
lifetime by geological standards. In 10,000–100,000
years most of the lakes in Scandinavia and North
America will have filled up with sediment, if we do
not have another glaciation.
Lakes of tectonic origin, however, will continue to
subside. If the rate of subsidence keeps pace with the
rate of sedimentation, a lake will continue to exist.
Extensive carbonate beds (freshwater carbonates)
and diatom deposits are also common in lacustrine
basins. Some of the largest lakes formed by rifting are
found in East Africa, where a number of lakes occur in
2 Introduction to Sedimentology
63
evaporation and therefore less rainfall. North Africa
and the Sahara were then very much drier than the
present day.
Iceland has good examples of desert regions which
are due to a cold climate, and has problems with soil
erosion due to lack of vegetation in many areas.
Shortly after the withdrawal of the ice across
Scandinavia there was a desert with considerable aeolian deposits, which formed before the vegetation
cover developed. Loess deposits are fine-grained
sediments which form largely through aeolian erosion
and transport from deserts and also from glacial
sediments exposed after glacial retreat. Large areas
of China, for example west of Beijing, are covered
by up to several hundred metres of Quaternary loess.
The lack of vegetation naturally means that aeolian
transport and deposition are important in deserts, but
only a relatively small part of the desert areas are
covered by wind-blown sand.
Large areas consist of bare rocks and mountains
with little sediment. In other regions there is only wind
erosion (deflation), which leaves the ground covered
(armoured) by a layer of stones which protect it from
further erosion. Even in desert regions like the Sahara
many areas are dominated by a fluvial drainage pattern. Although several years may pass between rains in
this area, a heavy rainstorm may transport so much
sediment that the fluvial drainage pattern survives for
many years.
Large expanses of wind-blown sand are called ergs.
They may be formed by the coalescence of different
aeolian bedforms. Barchans are crescent-shaped
isolated aeolian dunes with a convex erosion side
and a concave lee side (Fig. 2.27). Barchans are
found mainly on the edges of the erg area, where
there is not enough sand to form a continuous thick
cover. Transverse dunes are common within ergs. Seif
dunes are long, straight sand dunes which may occur
within an erg but are also found in areas with incomplete sand cover. In the central zones very large dunes
form which may be over 100 m high and 1 km long.
Ancient aeolian deposits are recognised by:
1. Their large-scale cross-bedding, up to 20–30 m and
often with wind ripple marks on top of slanting
cross-bedded surfaces (foreset beds).
2. High degree of oxidation which gives a red colour.
3. Good sorting – largely medium- to fine-grained
sand.
4. Lack of fossils or organic material.
The lack of thin silt or clay beds between the crossbedded layers, and the general association with
overlying and underlying sediments, are also important criteria. One of the best-known examples of an
aeolian sandstone is the Navajo Sandstone (Jurassic)
in the Rocky Mountains. In Northern Europe there are
aeolian sandstones in the Lower Permian
(Rotliegendes) and the equivalent Yellow Sand in
northeastern England (Fig. 2.27).
In pre-Devonian times all continental environments
were deserts in the sense that they lacked vegetation.
Also in these rocks, however, we can distinguish
between dry and wet climates, largely from the nature
of the fluvial or aeolian deposits.
Sand dunes are always moving and may end up in
the sea, still partly preserving an aeolian sorting and
grain-size distribution.
2.26 Lacustrine Deposits
How Are Lakes Formed?
In principle we have three types of lake:
1. Lakes of glacial origin:
(a) lakes formed through glacial erosion,
(b) lakes formed through damming by moraines or
by the glaciers themselves.
2. Lakes of tectonic origin:
(a) lakes formed in areas of rapid tectonic subsidence (rifting) or more uniform subsidence,
(b) lakes formed as a result of damming by horsts
which have been elevated through faulting, or
damming by lava etc.
3. Lakes formed by sedimentary processes, e.g.
oxbow lakes in fluvial environments, and delta top
lakes.
Lakes of glacial origin will have a relatively short
lifetime by geological standards. In 10,000–100,000
years most of the lakes in Scandinavia and North
America will have filled up with sediment, if we do
not have another glaciation.
Lakes of tectonic origin, however, will continue to
subside. If the rate of subsidence keeps pace with the
rate of sedimentation, a lake will continue to exist.
Extensive carbonate beds (freshwater carbonates)
and diatom deposits are also common in lacustrine
basins. Some of the largest lakes formed by rifting are
found in East Africa, where a number of lakes occur in
2 Introduction to Sedimentology
63