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Other common minerals in detrital rocks are feldspars and micas. Because chemical weathering
rapidly transforms these minerals into new substances, their presence in sedimentary rocks
indicates that erosion and deposition were fast enough to preserve some of the primary minerals
from the source rock before they could be decomposed. Particle size is not only a convenient
method of dividing detrital rocks, but the sizes of the component grains also provide useful
information about environments of deposition. Currents of water or air sort the particles by
size—the stronger the current, the larger the particle size carried. Gravels, for example, are
moved by swiftly flowing rivers as well as by landslides and glaciers. Less energy is required to
transport sand; thus, it is common to such features as windblown dunes and some river deposits
and beaches. Very little energy is needed to transport clay, so it settles very slowly.
Accumulations of these tiny particles are generally associated with the quiet waters of a lake,
lagoon, swamp, or certain marine environments. Common detrital sedimentary rocks, in order of
increasing particle size, are shale, sandstone, and conglomerate or breccia.
In contrast to detrital rocks, which form from the solid products of weathering, chemical
sediments derive from ions that are carried in solution to lakes and seas. This material does not
remain dissolved in the water indefinitely, however. Some of it precipitates to form chemical
sediments. These become rocks such as limestone, chert, and rock salt. This precipitation of
material occurs in two ways. Inorganic processes such as evaporation and chemical activity can
produce chemical sediments. Organic (life) processes of water dwelling organisms also form
chemical sediments, said to be of biochemical origin. One example of a deposit resulting from
inorganic chemical processes is the dripstone that decorates many caves. Another is the salt left
behind as a body of seawater evaporates. In contrast, many water-dwelling animals and plants
extract dissolved mineral matter to form shells and other hard parts. After the organisms die,
their skeletons collect by the millions on the floor of a lake or ocean as biochemical sediment.

Task 2. Build up a vocabulary to the text.

Task 3. Provide brief answers to the following:
1. What minerals are most abundant in detrital sedimentary rocks?
2. What is the primary basis for distinguishing among detrital rocks?

Task 4. Look at Figure 2.2, 2.3 A and B. Figure 2.2 shows particle size classification
for detrital sedimentary rocks. Particle size is the primary basis for distinguishing among
various detrital sedimentary rocks. Figure 2.3 A and B identifies sedimentary rocks.
According to the table, sedimentary rocks are divided into three groups—detrital,
chemical, and organic. The main criterion for naming detrital sedimentary rocks is particle
size, whereas the primary basis for distinguishing among chemical sedimentary rocks is
their mineral composition. Using the information from the tables and from the text try to
give as many details as possible about detrital sedimentary rocks.

TEST YOURSELF

Task 1. Tell whether the sentences are true or false. Correct the false ones.
1. Sedimentary rocks contain much of the basic information needed to reconstruct Earth
history.
2. Sedimentary rocks are not associated with many important energy and mineral resources.
3. Sedimentary rock consists of sediment that has been lithified into solid rock.
4. Sediment has three principal sources: (1) as detrital material, which originates and is
transported as solid particles from both mechanical and chemical weathering, which, when
lithified, forms detrital sedimentary rocks; (2) from soluble material produced largely by
chemical weathering, which, when precipitated, forms chemical sedimentary rocks, (3)