Geoscience Reference
In-Depth Information
and of new minerals, produced during weathering. These
include sandstones, mudstones, and conglomerate.
Chemical-organic sediments are formed from materials
transported in solution and from materials manufactured
by plants and animals, such as limestone, dolomite,
ironstone, and evaporites. The two main characteristics
used to classify sedimentary rocks are the grain size and
composition. Sedimentary rocks are widely used for
building, with sandstones and limestones being the
Sedimentary rocks are formed from fragments of rock
eroded from the weathered land surface, transported by
water, wind, and ice, and deposited in coastal plains,
deltas, and seas. The loose sediments are deposited in
layers (beds) which become compacted and cemented to
form bedded rock. Detrital sediments are formed by the
accumulation of transported particles of old minerals,
Table 7
Geological classification of sedimentary rocks for engineering purposes (a modified form of this
table appears in BS 5930:1999)
Grain size (mm)
Bedded rocks
>20
Grain size
At least 50% of
Chemical and
description
grains are of
carbonaceous
carbonate
rocks
2-20
Rudaceous
Conglomerate
Limestone
Halite
Rounded boulders, cobbles,
and gravel cemented in a
Magnesian
Anhydrite
finer matrix
limestone
Breccia
Irregular rock fragments in
Dolomite
Gypsum
a finer matrix
0.6-2
Coarse
Sandstone
Angular or rounded grains,
commonly cemented by
clay, calcitic, or iron
minerals
0.2-0.6
Medium
Orthoquartzite
Quartz grains and
siliceous cement
Arkose
Many feldspar grains
0.06-0.2
Fine
Greywacke
Many rock chips
0.002-0.06
Argillaceous
Mudstone
Siltstone
Chalk
Mostly silt
<0.002
Shale
Claystone
Fissile
Mostly clay
Amorphous or cryptocrystalline
Chert
Coal
Occurs as nodules and beds in limestone
and calcareous sandstone
Flint
Lignite
Occurs as bands of nodules in the Cretaceous
Chalk of western Europe
