Natural Science in Archaeology

54 3 Exploitation of Mineral and Rock Raw Materials
If one compares freshly deposited sands (for example, river or beach sands) with
sandstone, the differences are obvious in terms of rock strength, coherence, and
porosity. The term diagenesis is used for the many geologic processes involved
in the post-depositional alteration of sediment into sedimentary rocks. Induration
by cementation of the grains is perhaps the most significant diagenetic process.
Diagenesis excludes those high-temperature and/or high-pressure regimes that form
metamorphic rocks. Sedimentary rocks are inherently bedded. This is frequently
visible because of variations in grain size and to a lesser extent because of variations
in mineralogy.
The major sedimentary rocks that are used in buildings and monuments are listed
in Table 3.2.
Geologic nomenclature has hundreds of names for the varieties of common sedimentary
rocks. Because variations between sedimentary types are often gradational,
all classifications of sedimentary rocks are somewhat arbitrary (Table 3.3). The
pebbles or boulders in conglomerate are often very hard chert or quartz. The author
has marveled at the ability of prehistoric societies to dress blocks of conglomerate
cutting neatly across tough chert boulders, for example, at the Lion Gate at Mycenae
in southwestern Greece. At the other end of the hardness scale, soft shale was
used to carve beads because shaping and drilling it was easy.
Table 3.2 Major sedimentary rocks used in building and monument stone
Sandstones Sandstones are composed of rounded or angular sand size particles cemented
together by silica, iron oxide, or calcium carbonate. Sandstones are often
made up mostly of quartz; they may be well cemented and cohesive or poorly
cemented and friable.
Conglomerates Conglomerates are coarse-grained clastic rocks composed of large rounded
pebbles, cobbles, or boulders set in a fine-grained matrix of sand or silt
cemented by silica, iron oxide, calcium carbonate, or hardened clay.
Limestones Limestones are composed chiefly of calcium carbonate in the form of calcite
and are formed by either detrital, biological, or chemical processes. Many
are highly fossiliferous and clearly represent ancient shell banks or coral
reefs. Chalk and travertine are also limestones.
Evaporites Evaporites are sedimentary rocks composed primarily of the more soluble
minerals such as halite, gypsum, and anhydrite, that are produced from saline
solutions as a result of evaporation.
Table 3.3 Classifi cation of major sedimentary rocks
Clastic Chemical Biogenic
Conglomerate Limestone Chalk
Sandstone Dolomite Some carbonates
Siltstone Chert Chert
Shale Gypsum Coal
Anhydrite Some iron formation
Salt (halite)
Some iron formation