58
WALLACE H. FULLER
horizons. In places in North American deserts carbonate pans contain significant quantities of siliceous cements (Western Regional Soil Survey
Work Group, 1964). Evaluation of data by Stuart et al. (1961) appears
to support this hypothesis. They found the content of acid-extractable silica
(SiOo
2) in the gray desert soils of northwestern United States (Idaho)
to be highest in the hardest part of the indurated caliche layers.
D. IRON
1. Characteristics
Dark red, brown, and black stains ("desert varnish"), coating gravel,
pebbles, and stones of desert pavements have been attributed to the presence
of iron and manganese oxides. The most generally accepted hypothesis
is that, under certain arid and semiarid climatic conditions, "desert
varnish" results from alternate wetting and drying at relatively high temperatures. Biological conditions, such as the presence of iron oxidizing
microorganisms and/or algae, may have played an important part in the
formation of desert varnishes also. Certainly the common occurrence of
algae living under the harsh desert conditions is well established (Fuller et
al., 1961). Their ability to survive long periods of desiccation and quickly
recover and grow rapidly when it rains, as well as the ability of some to
concentrate iron, makes them prime suspects as a factor in iron coatings
of rocks.
2. Origin
The presence of duripans in deserts containing iron oxides are believed
to be relics or partial relics formed under more humid conditions or under
peculiar circumstances of a high, fluctuating water table. Thin, dehydrated,
iron hydroxide stainings and layers have been observed capping indurated
caliche in the Mohave Desert of the United States. These appear to have
formed during periods of transitory perched water tables.
Desert soils of the U.S.S.R. exhibit little if any redistribution of sesquioxides (or iron oxides) in the profile (Shuvalov, 1949). In fact, Takyr-like
desert soils (extremely arid) show little if any chemical differentiation. On
the other hand, Lobova (1960) provides evidence that ferrugination occurs in desert soils subjected to periodic wetting. Intense heating dehydrates the iron hydroxides to red hematite or yellow-brown limonite. The
yellow and red color of desert sands is due to the presence of these anhydrous forms of iron hydroxides. Iron comes from minerals that, under
desert conditions, weather at a relatively rapid rate to form free hydroxides.
WALLACE H. FULLER
horizons. In places in North American deserts carbonate pans contain significant quantities of siliceous cements (Western Regional Soil Survey
Work Group, 1964). Evaluation of data by Stuart et al. (1961) appears
to support this hypothesis. They found the content of acid-extractable silica
(SiOo
2) in the gray desert soils of northwestern United States (Idaho)
to be highest in the hardest part of the indurated caliche layers.
D. IRON
1. Characteristics
Dark red, brown, and black stains ("desert varnish"), coating gravel,
pebbles, and stones of desert pavements have been attributed to the presence
of iron and manganese oxides. The most generally accepted hypothesis
is that, under certain arid and semiarid climatic conditions, "desert
varnish" results from alternate wetting and drying at relatively high temperatures. Biological conditions, such as the presence of iron oxidizing
microorganisms and/or algae, may have played an important part in the
formation of desert varnishes also. Certainly the common occurrence of
algae living under the harsh desert conditions is well established (Fuller et
al., 1961). Their ability to survive long periods of desiccation and quickly
recover and grow rapidly when it rains, as well as the ability of some to
concentrate iron, makes them prime suspects as a factor in iron coatings
of rocks.
2. Origin
The presence of duripans in deserts containing iron oxides are believed
to be relics or partial relics formed under more humid conditions or under
peculiar circumstances of a high, fluctuating water table. Thin, dehydrated,
iron hydroxide stainings and layers have been observed capping indurated
caliche in the Mohave Desert of the United States. These appear to have
formed during periods of transitory perched water tables.
Desert soils of the U.S.S.R. exhibit little if any redistribution of sesquioxides (or iron oxides) in the profile (Shuvalov, 1949). In fact, Takyr-like
desert soils (extremely arid) show little if any chemical differentiation. On
the other hand, Lobova (1960) provides evidence that ferrugination occurs in desert soils subjected to periodic wetting. Intense heating dehydrates the iron hydroxides to red hematite or yellow-brown limonite. The
yellow and red color of desert sands is due to the presence of these anhydrous forms of iron hydroxides. Iron comes from minerals that, under
desert conditions, weather at a relatively rapid rate to form free hydroxides.