Denudation, weathering and mass wasting - Fundamentals of the Physical Environment - page 289

Figure 13.5
Discontinuities at all
geological scales.

Sandstone

Sediments

Crust

Limestone

Igneous
Pluton

Mantle

Shale

Tectonic Structural Lithological

Quartz

Quartz
crystal
flaw

Feldspar

Mica Flakes

Mineralogical Crystalline

Offshore
Mesozoic,
Cenozoic

Upper
Palaeozoic

Precambrian and Lower Palaeozoic Mesozoic Cenozoic Mesozoic

NW Snowdonia Berwyn Hills Longmynd Cotswolds Chilterns North

Downs

South
Downs

SE

km Wenlock

Edge

km

1 1

Midland
Plain

London
Basin

English
Channel

Irish
Sea

Weald

0.5 0.5

0

0

Palaeozoic
and Precambrian
basement

-0.5 -0.5

YOUNGEST

Cenozoic
sediments

Cretaceous chalk
and sandstone

Jurassic limestone
& sandstone

Mesozoic
clays, sands

Permo- Triassic
sediments

Carboniferous
sediments

OLDEST

Precambrian
basement

Cambrian
marine sediments

Ordovician volcanics
& marine sediments

Ordovician
plutons

Silurian marine
sediments

Devonian
sediments

Figure 13.6 Geological cross-section from Anglesey to Brighton (450 km), linking structure and relief. Older, resistant igneous
and metamorphic rocks form higher ground in Wales than younger, less resistant sedimentary rocks in central and south-east
England. Mesozoic sediments in the Irish Sea basin suggest that Wales may have been exhumed from beneath a Mesozoic cover.

during the various processes of rock formation. Pore
water in moderate quantities exerts a 'suction' force,
through surface tension, and its presence depends on rock
porosity. Normal stress is the anchoring weight of rock
overlying a point in the mass and depends on rock density
and gravity. Internal friction strength is generated at the
contact points of constituent grains and minerals,
determined by their size, shape and packing arrangement

( texture ). It is additional to cohesion, and is given as an
angle along which shearing would take place; the higher
the angle the greater the strength. Its significance is
seen clearly in a dry scree or pile of loose dry sand, where
the 'angle of rest' equals the friction angle. Mohr-
Coulomb criteria will help us to understand some
principal failure conditions and forms later; meanwhile,
typical component strengths are shown in Table 13.1 .