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TABLE 17.4

A simplified classification scheme for clastic sediments
and rocks
Sediment Name Size Range Rock
Boulder Over 256 mm
(10 in)
Gravel 2–256 mm Conglomerate
(0.08–10 in) or breccia*
Sand 1/16–2 mm Sandstone
(0.025–0.08 in)
Silt (or dust) 1/256–1/16 mm Siltstone**
(0.00015–0.025 in)
Clay (or dust) Less than 1/256 mm Claystone**
(less than 0.00015 in)
FIGURE 17.15 This is a piece of breccia, a sedimentary rock
*Conglomerate has a rounded fragment; breccia has an angular fragment. that formed from the consolidation of large angular fragments into
**Both also known as mudstone; called shale if it splits along parallel planes.
a solid rock. This sample with gravel-sized and smaller fragments is
from Colorado.
down, so there are a wide range of sizes of clastic sediments
(Table 17.4). The largest of the clastic sediments, boulders and
gravel, are the raw materials for the sedimentary rock that is
called conglomerate or breccia, depending on whether the frag-
ments are well-rounded or angular (Figure 17.15). Sandstone, as
the name implies, is a sedimentary rock formed from sand that
has been consolidated into solid rock (Figure 17.16). The small-
est clastic sediments, silt and clay, are consolidated into solid
siltstone and claystone. If either of these sedimentary rocks tends
to break along planes into flat pieces, it is called shale. Note that
when a clastic sediment is referred to as clay, it means a sedi-
ment size (less than 1/256 mm) and not the name of the clay
mineral. When deposited from the air, clay- and silt-sized par-
ticles are commonly called dust.
Dissolved rock materials form chemical sediments that
are removed from solution to form sedimentary rocks. The
dissolved materials are ions from minerals and rocks that have FIGURE 17.16 This is a piece of sandstone, a sedimentary rock
been completely broken down. Once they are transported to that formed from the consolidation of sand grains into a solid rock.
lakes or oceans, the dissolved ions are available to make sedi- This sample with iron oxide banding is from Tasmania, Australia.
ments through one of three paths. These are (1) chemical pre-
cipitation from solution, (2) crystallization from evaporating
water, or (3) biological sediments. The most abundant chemi-
cal sedimentary rocks are the carbonates and evaporates. The
carbonates are limestone and dolomite (Figure 17.17). Lime-
stone is composed of calcium carbonate, which is also the
composition of the mineral called calcite. Dolomite probably
formed from limestone by the replacement of calcium ions
with magnesium ions (both belong to the same chemical fam-
ily). Limestone is precipitated directly from freshwater or salt
water or indirectly by the actions of plants and animals that
form shells of calcium carbonate.
Most sediments are deposited as many separate particles
that accumulate in certain environments as loose sediments.
Such accumulations of rock fragments, chemical deposits, or
animal shells must become consolidated into a solid, coherent
FIGURE 17.17 This is a piece of limestone, a sedimentary
mass to become sedimentary rock. There are two main parts to rock that formed underwater—sometimes with the remains of
this lithification, or rock-forming process: (1) compaction and marine organisms. Can you find the mold of a brachiopod fossil
(2) cementation (Figure 17.18). on this sample?

17-13 CHAPTER 17 Rocks and Minerals 445