Page 68 - Fundamentals of Geomorphology

GEOMORPHIC MATERIALS AND PROCESSES 51

Table 3.1 Size grades of sedimentary particles
Particle names Particle diameter Deposits

φ (phi) units a mm Unconsolidated Consolidated examples
examples
Gravel b Boulders <−8 > 256 Rudaceous deposits
Cobbles −6to −8 64–256 Till Conglomerate, breccia,
gritstone
Pebbles −2to −6 4–64
Granules −1to −2 2–4
Sand Very coarse sand 0 to −1 1–2 Arenaceous deposits
Coarse sand 1 to 0 0.5–1 Sand Sandstone, arkose,
greywacke, flags
Medium sand 2 to 1 0.25–0.5
Fine sand 3 to 2 0.125–0.25
Very fine sand 4 to 3 0.0625–0.125
Silt 8 to 4 0.002–0.0625 Argillaceous deposits
Clay > 8 < 0.002 Clay, mud, silt Siltstone, claystone,
mudstone, shale, marl
Notes:
a The phi scale expresses the particle diameter, d, as the negative logarithm to the base 2: φ =−log d
2
b The subdivisions of coarse particles vary according to authorities

rocks, such as granite, it may produce convex hills known vapour circulating through the still open pores and fis-
as exfoliation domes. sures that comes into contact with the ice condenses
and freezes. The result is that ice lenses grow that push
Frost action up the overlying layers of soil. This process is called
frost heaving and is common in glacial and periglacial
Water occupying the pores and interstices within a soil environments (cf. p. 66).
or rock body expands upon freezing by 9 per cent.
This expansion builds up stress in the pores and fis- Heating and cooling
sures, causing the physical disintegration of rocks. Frost
weathering or frost shattering breaks off small grains Rocks have low thermal conductivities, which means
and large boulders, the boulders then being fragmented that they are not good at conducting heat away from
into smaller pieces. It is an important process in cold their surfaces. When they are heated, the outer few
environments, where freeze–thaw cycles are common. millimetres become much hotter than the inner por-
Furthermore, if water-filled fissures and pores freeze tion and the outsides expand more than the insides.
rapidly at the surface, the expanding ice induces a hydro- In addition, in rocks composed of crystals of different
static or cryostatic pressure that is transmitted with equal colours, the darker crystals warm up faster and cool down
intensity through all the interconnected hollow spaces to more slowly than the lighter crystals. All these thermal
the still unfrozen water below. The force produced is stresses may cause rock disintegration and the formation
large enough to shatter rocks, and the process is called of rock flakes, shells, and huge sheets. Repeated heating
hydrofracturing (Selby 1982, 16). It means that frost and cooling produces a fatigue effect, which enhances
shattering can occur below the depth of frozen ground. the thermal weathering. The production of sheets by
In unsaturated soils, once the water is frozen, the water thermal stress was once called exfoliation, but today