4.4 GLACIAL PROCESSES
113
dicular to the prevailing wind direction. Penecontemporaneous deformation of bedding
is quite common in dunes. It takes the form of both fracturing and slipping, as well as
various fold structures (McKee, 1971, 1979).
In conclusion, one can see that eolian traction deposits are broadly similar to aqueous eolian ones. They have similar modes of grain transport and thus similar bed forms.
Much remains to be learned of the factors that control the morphology of eolian dunes,
and of their elusive internal structure.
4.3.2 Eolian Sedimentation from Suspension
Most eolian sediment is deposited by wind blowing over desiccated alluvium. The gravel
remains behind. The sand saltates into dunes. The silt and clay are blown away in suspension. This much is known, and there is no question of the competence of wind to
transport large amounts of silt and clay in suspension. It has been calculated that between 25 and 37 million tons of dust are transported from the Sahara throughout the
longitude of Barbados each year. This quantity of dust is sufficient to maintain the present rate of pelagic sedimentation in the entire North Atlantic (Prospero and Carlson,
1972). Dust is transported in the winds of deserts, but little is actually deposited out of
suspension in the way that mud settles on the sea floor. Most silt and clay is finally deposited in playas, following rains and flash floods. Desiccation and cohesion tend to prevent this material from being recycled. The dust suspensions of periglacial deserts are
different from those of tropic deserts. Clay is largely absent. They are high in silica particles of medium silt grade produced by glacial action. This material is termed loess
(Berg, 1964). Extensive deposits of Pleistocene loess occur in a belt right across the
Northern Hemisphere to the south of the maximum extent of the ice sheets. Loess occurs in laterally extensive layers, often of great thickness. It is slightly calcareous, massive, and weathers into characteristic polygonal shrinkage cracks. While most authorities agree that loess was transported by eolian suspensions (i.e., as dust clouds), there is
some debate as to whether it settled out of free air, or whether it was actually deposited
as a result of fluvial action (e.g., Smalley, 1972; Tsoar and Pye, 1987). Some authorities
argue for a polygenetic origin of loess, suggesting that it may form by wind action or by
in-place cryogenesis of diverse rock types (Popov, 1972).
4.4 GLACIAL PROCESSES
Several types of sedimentary deposit are associated with glaciation (Denoux, 1994;
Miller, 1996; Bennett and Glaisner, 1996; Hooke, 1998). These include loess (just described), varved clays, and the sands and gravels of fluvioglacial outwash plains. These
deposits, though associated with glaciation, are actually eolian, aqueous suspension
and traction current deposits, respectively.
Ice itself transports and deposits one rock type only, termed diamictite (Flint et al.,
1960). This is a poorly sorted sediment from boulders down to clay grade. Much of the
clay material is composed of diverse minerals, but largely silica, formed by glacial pulverization. Clay minerals are a minor constituent. The boulders show a wide size range,
are often angular, and sometimes grooved where ice has caused the sharp corner of one
boulder to scratch across a neighbor. Statistical analysis of the orientation of the larger
113
dicular to the prevailing wind direction. Penecontemporaneous deformation of bedding
is quite common in dunes. It takes the form of both fracturing and slipping, as well as
various fold structures (McKee, 1971, 1979).
In conclusion, one can see that eolian traction deposits are broadly similar to aqueous eolian ones. They have similar modes of grain transport and thus similar bed forms.
Much remains to be learned of the factors that control the morphology of eolian dunes,
and of their elusive internal structure.
4.3.2 Eolian Sedimentation from Suspension
Most eolian sediment is deposited by wind blowing over desiccated alluvium. The gravel
remains behind. The sand saltates into dunes. The silt and clay are blown away in suspension. This much is known, and there is no question of the competence of wind to
transport large amounts of silt and clay in suspension. It has been calculated that between 25 and 37 million tons of dust are transported from the Sahara throughout the
longitude of Barbados each year. This quantity of dust is sufficient to maintain the present rate of pelagic sedimentation in the entire North Atlantic (Prospero and Carlson,
1972). Dust is transported in the winds of deserts, but little is actually deposited out of
suspension in the way that mud settles on the sea floor. Most silt and clay is finally deposited in playas, following rains and flash floods. Desiccation and cohesion tend to prevent this material from being recycled. The dust suspensions of periglacial deserts are
different from those of tropic deserts. Clay is largely absent. They are high in silica particles of medium silt grade produced by glacial action. This material is termed loess
(Berg, 1964). Extensive deposits of Pleistocene loess occur in a belt right across the
Northern Hemisphere to the south of the maximum extent of the ice sheets. Loess occurs in laterally extensive layers, often of great thickness. It is slightly calcareous, massive, and weathers into characteristic polygonal shrinkage cracks. While most authorities agree that loess was transported by eolian suspensions (i.e., as dust clouds), there is
some debate as to whether it settled out of free air, or whether it was actually deposited
as a result of fluvial action (e.g., Smalley, 1972; Tsoar and Pye, 1987). Some authorities
argue for a polygenetic origin of loess, suggesting that it may form by wind action or by
in-place cryogenesis of diverse rock types (Popov, 1972).
4.4 GLACIAL PROCESSES
Several types of sedimentary deposit are associated with glaciation (Denoux, 1994;
Miller, 1996; Bennett and Glaisner, 1996; Hooke, 1998). These include loess (just described), varved clays, and the sands and gravels of fluvioglacial outwash plains. These
deposits, though associated with glaciation, are actually eolian, aqueous suspension
and traction current deposits, respectively.
Ice itself transports and deposits one rock type only, termed diamictite (Flint et al.,
1960). This is a poorly sorted sediment from boulders down to clay grade. Much of the
clay material is composed of diverse minerals, but largely silica, formed by glacial pulverization. Clay minerals are a minor constituent. The boulders show a wide size range,
are often angular, and sometimes grooved where ice has caused the sharp corner of one
boulder to scratch across a neighbor. Statistical analysis of the orientation of the larger