II. DESERT SOILS
47
may be washed from the sand into the soil layers below as restless particles
drop and again are moved across land by winds of varying velocities. The
mineral composition of some North American sand dune material is
reported to be primarily quartz, feldspar, and mica. Clay minerals and siltsized quartz and mica make up clay-silt dunes. Gypsum dunes of sand-particle size are found in deserts (White Sands, New Mexico) as well as
calcareous or lime dunes (Death Valley, California).
3. Vegetation Relationships
Certain desert plants {Sporobolus airoides, A triplex canescens, and
Yucca elatd) appear to grow quite well in gypsum sand dunes where moisture is available. Dune sand and other sand surfaces either under native
or irrigated vegetation, where recent erosion forces have been so active
that morphological characteristics have not developed them into mature
soils yet, are called Regosols or Ergs. These "squatter soils" occur on hummocky, duned, undulating, or rolling outwash plains, till plains, continental
sediment plains, floodplains or aeolian deposits. When stabilized, sands can
develop soil profile differentiations in a surprisingly short time under irrigated agriculture and urbanization.
C. GRAVEL, COBBLES, STONES
Gravel, cobbles, and stones appear on a variety of topographical positions such as stream outwash deposits, sediment plains, alluvial flood
plains, volcanic plateaus, and alluvial fans and on consolidated upland such
as foothills, denuded hills, lower mountains, steep upland, and residual
upland mountains, most of which are highly dissected and subjected to
erosion (see Fig. 8). Both arid and semiarid climates are represented in
these topographical positions. Soil development is usually poor if at all,
and the soils are immature and vegetation is sparse. These surfaces are
characterized by pockets of unconnected soils and thin or shallow soils
(see Fig. 9 ) . They are predominantly Entisols occurring upon soil material
of such recent age, or in areas where erosion and sedimentation are so
active, that there has been little or no effect of climate and vegetation to
produce morphologic character. None have a B horizon and the A horizon
is usually thin.
Alluvial desert soils, as the name implies, appear on fans, low terraces
and floodplains. The C horizon, parent material, is stratified alluvium,
which often is stony or gravelly.
Lithosols occur on eroded and abraded ridges, hills, and plateaus as
very thin soils with no B horizon. A weak horizon of lime enrichment may
appear at or near the rock surface. These soils may be stony and rocky
47
may be washed from the sand into the soil layers below as restless particles
drop and again are moved across land by winds of varying velocities. The
mineral composition of some North American sand dune material is
reported to be primarily quartz, feldspar, and mica. Clay minerals and siltsized quartz and mica make up clay-silt dunes. Gypsum dunes of sand-particle size are found in deserts (White Sands, New Mexico) as well as
calcareous or lime dunes (Death Valley, California).
3. Vegetation Relationships
Certain desert plants {Sporobolus airoides, A triplex canescens, and
Yucca elatd) appear to grow quite well in gypsum sand dunes where moisture is available. Dune sand and other sand surfaces either under native
or irrigated vegetation, where recent erosion forces have been so active
that morphological characteristics have not developed them into mature
soils yet, are called Regosols or Ergs. These "squatter soils" occur on hummocky, duned, undulating, or rolling outwash plains, till plains, continental
sediment plains, floodplains or aeolian deposits. When stabilized, sands can
develop soil profile differentiations in a surprisingly short time under irrigated agriculture and urbanization.
C. GRAVEL, COBBLES, STONES
Gravel, cobbles, and stones appear on a variety of topographical positions such as stream outwash deposits, sediment plains, alluvial flood
plains, volcanic plateaus, and alluvial fans and on consolidated upland such
as foothills, denuded hills, lower mountains, steep upland, and residual
upland mountains, most of which are highly dissected and subjected to
erosion (see Fig. 8). Both arid and semiarid climates are represented in
these topographical positions. Soil development is usually poor if at all,
and the soils are immature and vegetation is sparse. These surfaces are
characterized by pockets of unconnected soils and thin or shallow soils
(see Fig. 9 ) . They are predominantly Entisols occurring upon soil material
of such recent age, or in areas where erosion and sedimentation are so
active, that there has been little or no effect of climate and vegetation to
produce morphologic character. None have a B horizon and the A horizon
is usually thin.
Alluvial desert soils, as the name implies, appear on fans, low terraces
and floodplains. The C horizon, parent material, is stratified alluvium,
which often is stony or gravelly.
Lithosols occur on eroded and abraded ridges, hills, and plateaus as
very thin soils with no B horizon. A weak horizon of lime enrichment may
appear at or near the rock surface. These soils may be stony and rocky