Geology Reference
In-Depth Information
thermal stresses set up in the rock itself. Finally, the growth of segregated ice lenses at
depth, from water migrating towards the freezing plane, may force rock layers apart.
Ice-rich brecciated bedrock can be observed in many permafrost regions today (Büdel,
1977; see English translation, 1982, p. 82; French et al., 1986) (see Section 7.3.3). It is the
result of the growth of segregated ice, often reticulate in structure (Mackay, 1974b; Murton
and French, 1994). In sedimatary bedrock, expanded joints (see Figures 7.6C, D) may
cause buckling of adjacent rock (see Section 4.5.1).
Brecciated bedrock, often to depths of 3-6 m, can be observed in a number of litholo-
gies in mid-latitudes today (Figure 13.2). For example, in central and southern England,
brecciation has been described from Portland Limestone, Mercer Mudstone, Upper Lias
clay, and Chalk (Bradshaw and Smith, 1963; Horswill and Horton, 1976; Murton, 1996a).
The Chalk is especially suited to brecciation by ice segregation because it is relatively soft,
and highly porous and permeable (Williams, 1987). Brecciation takes the form of a near-
surface mantle of loose bedrock in which vertical and horizontal joints are spaced a few
centimeters apart (see Figure 7.6B). Typically, the lower contact with unbrecciated bedrock
is gradual.
Brecciated bedrock has attracted only limited attention from periglacial geomorpholo-
gists but is probably far more widespread in occurrence than is currently reported. One
Figure 13.2.
Brecciated Chalk bedrock. The section shows, in ascending sequence, unweathered
chalk, brecciated chalk, an involuted layer, and wind-blown sediment (“brickearth”). Isle of Thanet,
Kent, southern England.
