Natural Science in Archaeology

58 3 Exploitation of Mineral and Rock Raw Materials
content renders these rocks amenable for use as chipped or ground stone implements.
Of the many shales found in Britain only one, a bituminous oil-shale, was
worked in any quantity. When freshly quarried its texture and hardness approximated
those of slate, and, because of its color, if smoothed and polished with beeswax
it resembled jet. This shale had intermittent use in Neolithic and Bronze ages
and became the basis of an industry from the Iron Age through the Roman Period.
The products included armlets, whorls, beads, finger rings, and even furniture and
statues (Lawson 1976).
Claystone is an indurated clay. If it has some fissility approximately parallel to
bedding, then it is also referred to as shale. An argillite is a rock derived from shale
or siltstone that has undergone a higher degree of induration than present in shales
and siltstones. The name siltstone refers to a rock whose chief constituent is siltsized
grains.
3.4.3 Pyroclastic Sedimentary Rocks
Another group of clastic rocks is derived from explosive volcanism. Sometimes
these are classified as igneous rocks, but, because they are deposited as sediments
on land or in the water, they should be classified with the sedimentary rocks. Fisher
and Schmincke (1984) give a complete description of these pyroclastic rocks.
3.5 Metamorphic Rocks
Metamorphic rocks are formed by major alteration of preexisting rocks as the result
of high temperature, high pressure, and/or chemically active fluids. Sedimentary
(and some igneous) rocks undergo mineralogical and textural changes in response
to the imposed conditions. The degree of metamorphism varies with conditions, and
specific mineral assemblages, referred to as “metamorphic facies” are associated
with specific temperature/pressure conditions.
The bulk composition of the preexisting rocks and the temperature and pressure
of the metamorphism control the mineral composition of metamorphic rocks
(Table 3.4). There are a variety of names in the geologic literature to account for
mineralogical or textural variation within the major metamorphic rock types. It
should be noted that not all quartzites are of metamorphic origin. A quartz sandstone
Table 3.4 Types of metamorphic rocks
Through high-temperature (>>) and high-pressure (>>) transformation of sedimentary rocks.
Limestone >> Marble
Sandstone >> Quartzite
Shale >> Slate >> Phyllite >> Schist >> Gneiss