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Basic characteristics of soils
Published in Jonathan Knappett, R. F. Craig, Craig’s Soil Mechanics, 2019
Jonathan Knappett, R. F. Craig
The destructive process in the formation of soil from rock may be either physical or chemical. The physical process may be erosion by the action of wind, water or glaciers, or disintegration caused by cycles of freezing and thawing in cracks in the rock. The resultant soil particles retain the same mineralogical composition as that of the parent rock (a full description of this is beyond the scope of this text). Particles of this type are described as being of ‘bulky’ form, and their shape can be indicated by terms such as angular, rounded, flat and elongated. The particles occur in a wide range of sizes, from boulders, through gravels and sands, to the fine rock flour formed by the grinding action of glaciers. The structural arrangement of bulky particles (Figure 1.6) is described as single grain, each particle being in direct contact with adjoining particles without there being any bond between them. The state of the particles can be described as dense, medium dense or loose, depending on how they are packed together (see Section 1.5).
Sediments and Sedimentary Rocks
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
Although glacier ice movement is much slower than the movement of wind and water is, glaciers can pick up and carry large amounts of clastic material. The finest material is silt-sized rock flour created by grinding that abrades material from bedrock. Much of the material in loess derives from this fine rock flour. However, glaciers pluck and scrape material of all sizes from the ground beneath them, or from valley sides and bottoms as they move forward. Alpine glaciers also gain material from rock falls at their margins. Consequently, clasts of all sizes, including large boulders, can make up a glacier’s load. Figure 8.31 shows much medium-sized to very coarse material in the glacial ice of Patagonia. This clastic material is carried at all levels within glacial ice, including on the surface, although larger and heavier clasts slowly settle and concentrate in the lower levels.
Surface Processes
Published in F.G.H. Blyth, M. H. de Freitas, A Geology for Engineers, 2017
F.G.H. Blyth, M. H. de Freitas
Ice moving over a land surface removes soil and loose rock, exposing the bed-rock below. Englacial material held in the lower part of the moving ice acts as an abrasive and the rock floor over which it is rubbed becomes smoothed and a glaciated surface is thus formed. Sharp corners of blocks held in the sole of a glacier cut grooves or striae in the surfaces over which they are carried, and the striae indicate the direction of movement. Grinding occurs at such points of rock contact and produces fine particles, called rock-flour whose movement polishes the glaciated surface. Protrusions of rock from the valley floor and sides are sheared off by the force of rock blocks pressed against them by the moving glacier. Large rock-obstructions in the path of a glacier are smoothed on their iceward slopes and plucked on the lee side; when exposed to view after the disappearance of the ice they show rounded forms known as roches moutonnees, which are typical of glaciated upland regions.
U–Pb geochronology reveals evidence of a Late Devonian hydrothermal event, and protracted hydrothermal–epithermal system, within the Mount Painter Inlier, northern Flinders Ranges, South Australia
Published in Australian Journal of Earth Sciences, 2020
S. B. Hore, S. M. Hill, A. Reid, B. Wade, N. F. Alley, D. R. Mason
The degree of brecciation varies from incipient and local comminution of basement rocks to incoherent, matrix-supported breccia of granitic rock. Clast abrasion is evident in thin-section (Whitehead, 1976). Some outcrops of granitic breccia consist of extensively fractured and crushed granitic rock in which much of the feldspar has been granulated to a finer grainsize. This rock flour matrix is dominantly quartz and feldspar of granitic origin. Fine-grained silty to sandy sediments can be within the breccia, in some cases with cross-bedded or graded layering. Rounded clasts of hematitic siltstone and breccia are enclosed within the breccia and sediments, suggesting repetition of the process that formed them.