Natural Science in Archaeology

3.4 Sedimentary Rocks 55
Clastic particles are derived from preexisting rocks and transported some distance
from their origin. There are three major types of clastic sediments: carbonate,
terrigenous, and pyroclastic. Carbonate clastics are usually deposited in marine
waters and consist primarily of skeletal grains, oolites, and some carbonate sand
from the continents. Terrigenous clastics are the most common and are ultimately
derived from outside the basin of deposition. Silicate minerals predominate in these
sediments. Pyroclastic particles, such as volcanic ash, are derived directly from
volcanic explosions.
3.4.1 Carbonate Sedimentary Rocks
Limestones are sedimentary rocks in which the carbonate minerals calcite and dolomite
make up 50% or more of the rock. The balance is usually quartz, chert, and
clay. Chert is sometimes a major constituent, occurring as nodules along certain
bedding planes. These nodules may coalesce to form essentially continuous beds.
The average limestone contains only about 5% quartz.
The name “limestone” covers a very broad range of sedimentary rocks. All are
composed chiefly of calcium carbonate, but the method of formation varies and
includes chemically precipitated, biogenic, and clastic rocks. For utility as a building
or monumental stone, limestones range from friable and structurally weak to
dense, compact rocks of great structural strength.
Limestone tools were used by the Paleolithic inhabitants at Choukoutien, China
(Movius 1948), but limestone is too soft to have been widely used as a raw material
for stone tools. The Great Pyramid of Cheops at Giza was constructed of nummulitic
limestone. All the stone for Mayan structures appears to have come from local
quarries. Most of the region of Mayan civilization is underlain by limestone, thus
this rock was used for their extensive rock structures. Even much of their mortar
consisted of sascab, which was made from crushed and burnt limestone that mimicked
the properties of cement.
Dolomites are those varieties of limestone where more than half of the carbonate
is the mineral dolomite. The name comes from the French geologist de Dolomieu.
In terms of archaeomineralogy, dolomites were not good candidates for sources of
lime plaster. Limestones are either mostly calcite or mostly dolomite, so ancient
craftspeople seeking to make lime mortar would have soon identified the appropriate
source rock.
Chalk is a porous, fine-grained, white, coccolith-rich limestone, usually of Cretaceous
age. The name “Cretaceous” comes from the Latin word “creta”, meaning
chalk; the name was applied to this geologic time period because of the extensive
English chalk beds of this age. Europe‘s tallest prehistoric mound, Silbury Hill in the
British isles, is built of chalk. Chalk occurs widely in northwest Europe and extends
eastward to Russia. It also covers areas in central North America. It is more than
98% calcite. The silica fraction of the original deposits has frequently been diagenetically
converted to flint. Ancient authors often referred to chalk. Livy reported