Basics of Environmental Science

26 / Basics of Environmental Science

Sandstones are perhaps the most familiar sedimentary rocks, consisting mainly of sand grains, made

from quartz (silica, SiO

2

) which crystallized originally into igneous rock. Clay particles, much smaller

than sand grains, can pack together to make mudstones. Sediments rich in calcium carbonate, often

consisting mainly of the remains of shells and containing many fossils, form limestone and dolomite

(sometimes known as ‘dolostone’ to distinguish it from the mineral called dolomite) (HOLMES,

1965, ch. VI, pp. 118–141). Particles deposited as sediments are changed into rock by the pressure of

later deposits lying above them and the action of cementing compounds subsequently introduced

into them. The process, occurring at low temperature, is called ‘diagenesis’. Some sedimentary

rocks are very hard and many, especially sandstones and limestones, make excellent and durable

building stone. Once formed, a sedimentary rock is subject to renewed weathering, especially if it is

exposed at the surface, so sedimentary rocks continually form and re-form.

Sediments are deposited in horizontal layers, called ‘beds’, but subsequent movements of the crust

often fold or fracture them. It is not unusual for beds to be folded until they are upside down, and the

reconstruction of the environmental conditions under which sediments were deposited from the

study of rock strata often begins by seeking to determine which way up they were when they formed.

All in all, the interpretation of sedimentary structures can be difficult.^6 Figure 2.4 shows the sequence

of events by which sedimentary structures may be folded, sculptured, and then subside to be buried

beneath later beds producing an unconformity.

Figure 2.4 Stages in the development of an unconformity