Geology Reference
In-Depth Information
Table 2.1 Porosities and permeabilities of rocks and sediments
Material
Representative porosity
Permeability range
(litres/day/m
2
)
(per cent void space)
Unconsolidated
Clay
50-60
0
.
0004-0
.
04
Silt and glacial till
20-40
0
.
04-400
Alluvial sands
30-40
400-400,000
Alluvial gravels
25-35
400,000-40,000,000
Indurated: sedimentary
Shale
5-15
0
.
000004-0
.
004
Siltstone
5-20
0
.
0004-40
Sandstone and conglomerate
5-25
0
.
04-4,000
Limestone
0
.
1-10
0
.
004-400
Indurated: igneous and metamorphic
Volcanic (basalt)
0
.
001-50
0
.
004-40
Granite (weathered)
0
.
001-10
0
.
0004-0
.
4
Granite (fresh)
0
.
0001-1
0
.
000004-0
.
0004
Slate
0
.
001-1
0
.
000004-0
.
004
Schist
0
.
001-1
0
.
00004-0
.
04
Gneiss
0
.
0001-1
0
.
000004-0
.
004
Tuff
10-80
0
.
0004-40
Source:
Adapted from Waltz (1969)
and erosion. A case in point is the Folkestone Sands
of south-east England, which form a low relief feature
in the northern and western margins of the Weald,
though it is overshadowed by the impressive Hythe
Beds cuesta. Interestingly, the Hythe Beds comprise
incoherent sands over much of the Weald, but in the
west and north-west they contain sandstones and chert
beds, and in the north and north-east the sands are
partly replaced by interbedded sandy limestones and
loosely cemented sandstones. These resistant bands
produce a discontinuous cuesta that is absent in
the south-eastern Weald, but elsewhere rises to form
splendid ramparts at Hindhead (273 m), Blackdown
(280 m), and Leith Hill (294 m) that tower above the
Low Weald (Jones 1981, 18). However, in general,
hillslopes on the aforementioned sandstones are rarely
steep and usually covered with soil. Massive and more
strongly cemented sandstones and gritstones normally
form steep slopes and commonly bear steep cliffs and
isolated pillars. They do so throughout the world.
Details of the influence of rocks upon relief will be
discussed in Chapters 4 and 5.
Science
(Box 2.3). Here is how he described it in
old-fashioned but most elegant language:
in vapour by the atmosphere, imparts in its descent, fertility to
the earth, and becomes the great cause of vegetation and of life;
but now we find, that this vapour not only fertilizes, but creates
the soil; prepares it from the soil rock, and, after employing it
in the great operations of the surface, carries it back into the
We have long been accustomed to admire that beautiful con-
trivance in Nature, by which the water of the ocean, drawn up
