2.3 WEATHERING
29
Fig. 2.7. Photograph of carbonate nodular caliche (pale blobs) in overbank floodplain deposits (dark ground
mass) of the Old Red Sandstone (Devonian) Freshwater West, Wales. These nodules are an early stage of soil
formation. They are known to the aboriginals of these parts as "cornstones," because their lime content fertilizes an otherwise barren modern soil. The occurrence of intraformational cornstone conglomerates on fluvial channel floors testifies to their penecontemporaneous origin.
erosion has generally removed the upper part of the soil profile. An interesting exception to this rule has been described by Williams (1969), who discussed the paleoclimate that weathered the Lewisian gneisses prior to the deposition of Torridonian
(Pre-Cambrian) continental sediments in northwest Scotland. Soil horizons within sedimentary sequences are especially characteristic of fluvial and deltaic deposits. Many examples occur beneath coals and lignites. These are pierced by plant rootlets, and often
are white in color due to intensive leaching. Such paleosols are often called seat earths.
Examples are common in the Upper Carboniferous Coal Measures of northern England, in which two types of paleosol have been recognized. Ganisters are silica-rich
horizons, which are quarried for the manufacture of refractory bricks. Tonsteins are
kaolinite-rich seat earths (Bohor and Triplehorn, 1993). It has been argued both that
these are normal desilicified soil horizons, and also that they may be intensely weathered volcanic ash bands (see Section 8.3.2.1). Examples of soils in fluvial environments
include the caliches of the Old Red Sandstone facies of the Devonian in the North Atlantic borderlands. These are sometimes termed "cornstones." They occur as nodules
and concretionary bands within red floodplain siltstones (Fig. 2.7). The occurrence of
reworked cornstone pebbles in interbedded channel sands testifies to their penecontemporaneous origin (Friend and Moody-Stuart, 1970).
2.3.2 Physical Weathering
Four main types of physical weathering are generally recognized: freeze-thaw, insolation, hydration and dehydration, and stress release. Freeze-thaw weathering occurs
where water percolates along fissures and between the grains and crystals of rock. When
29
Fig. 2.7. Photograph of carbonate nodular caliche (pale blobs) in overbank floodplain deposits (dark ground
mass) of the Old Red Sandstone (Devonian) Freshwater West, Wales. These nodules are an early stage of soil
formation. They are known to the aboriginals of these parts as "cornstones," because their lime content fertilizes an otherwise barren modern soil. The occurrence of intraformational cornstone conglomerates on fluvial channel floors testifies to their penecontemporaneous origin.
erosion has generally removed the upper part of the soil profile. An interesting exception to this rule has been described by Williams (1969), who discussed the paleoclimate that weathered the Lewisian gneisses prior to the deposition of Torridonian
(Pre-Cambrian) continental sediments in northwest Scotland. Soil horizons within sedimentary sequences are especially characteristic of fluvial and deltaic deposits. Many examples occur beneath coals and lignites. These are pierced by plant rootlets, and often
are white in color due to intensive leaching. Such paleosols are often called seat earths.
Examples are common in the Upper Carboniferous Coal Measures of northern England, in which two types of paleosol have been recognized. Ganisters are silica-rich
horizons, which are quarried for the manufacture of refractory bricks. Tonsteins are
kaolinite-rich seat earths (Bohor and Triplehorn, 1993). It has been argued both that
these are normal desilicified soil horizons, and also that they may be intensely weathered volcanic ash bands (see Section 8.3.2.1). Examples of soils in fluvial environments
include the caliches of the Old Red Sandstone facies of the Devonian in the North Atlantic borderlands. These are sometimes termed "cornstones." They occur as nodules
and concretionary bands within red floodplain siltstones (Fig. 2.7). The occurrence of
reworked cornstone pebbles in interbedded channel sands testifies to their penecontemporaneous origin (Friend and Moody-Stuart, 1970).
2.3.2 Physical Weathering
Four main types of physical weathering are generally recognized: freeze-thaw, insolation, hydration and dehydration, and stress release. Freeze-thaw weathering occurs
where water percolates along fissures and between the grains and crystals of rock. When