Natural Science in Archaeology

3.4 Sedimentary Rocks 53
quartz diorites from the Eastern Desert were exploited in ancient times. Granites
and related quartz diorites also have been used as grindstones in the Bronze Age of
Korea, where syenites were used for axe blades (Lee et al. 2006).
Gabbros are mafic intrusive rocks composed mainly of plagioclase and augite,
sometimes with olivine. Gabbros are rarely porphyritic. In Croatia, six magmatic
rock provinces provided the majority of the raw materials for Neolithic tool industries.
The most common rock types found at Croatian sites are locally-derived
gabbros and diabases (Paunovic 2001).
The Romans maintained a major gabbro quarry in Wadi Umm Wikala early in
the first millennium CE. This site, named Ophites, was the next largest quarry in
the Eastern Desert of Egypt after those of Mons Claudianus and Mons Porpyrites
farther north. The Romans exported gabbro from this quarry for use as columns,
basins, and wall and pavement tiles in Rome, Pompeii, Herculaneum, and elsewhere.
Pliny described this rock which he said was named ophites (Sidebotham
et al. 2001).
Rocks of mafic composition but intermediate in grain size between gabbros and
basalts are called dolerites, especially in Great Britain. Local dolerite was used in
Mesolithic and Neolithic axes in east central Sweden (Kars et al. 1992). There was
an early Neolithic axe factory using dolerite rocks at Le Pinacle, Jersey, Channel
Islands (Patton 1991). Dolerite axes are the most abundant lithologic type in Brittany.
Most were produced at the great axe-manufacturing center at Plus-sulien
(Le Roux 1971). In North America the term diabase means essentially the same
thing as dolerite.
Tektites are small bodies of silicate glass formed by impact melting. They are
normally dumbbell, spheroid, or tear-shaped. The term tektite comes from the Greek
tektos meaning melted. Tektites have been found on every continent but always in
restricted areas.
In addition to the utilization of igneous rocks themselves, many deposits of
useful minerals such as metals result during the formation of igneous rocks. Following
the solidification of igneous rocks from crystallization of the magma, associated
metal-rich, hot water (hydrothermal) solutions have formed important metal
deposits.
3.4 Sedimentary Rocks
Sedimentary rocks form at low temperatures and pressures near the earth’s surface
when sediment that was deposited by water, wind, or ice becomes lithified. Chemical
sedimentary rocks form by precipitation from solution. Sedimentary rocks cover
roughly three-quarters of the surface of the continents. Based on stratigraphic measurements,
the percentages of the common sedimentary rocks are approximately:
shale 45%, sandstone 25%, limestone 20%, and other 10%. However, geochemical
and other calculations based on the products of weathering of igneous and metamorphic
rocks indicate that shales could comprise up to 80%.