Natural Science in Archaeology

11.6 Weathering and Decomposition 279
The historical buildings in Upper Egypt were built largely of local sandstone.
The Horus temple at Edfu was completely constructed of local sandstone. The Abu
Simbel temples were cut in the Nubian sandstone quarry in South Aswan. These
sandstones are composed mainly of quartz grains cemented by ferruginous, siliceous,
carbonaceous, and clay cements. Diurnal and seasonal changes in temperature
and relative humidity are the principal threats to monuments constructed of
these rocks. The temperature and humidity variations cause the growth of halite and
gypsum crystals that result in cracking and structural failure.
A dramatic case of change in environment is seen in the removal of the large
obelisks from Egypt. Sculpted in the middle of the second millennium BCE, these
monuments stood in Egypt for 3000 years with little surficial damage. Over a century
ago, three were exported, one to Paris about 1840, one to London about 1870,
and one to New York about 1890. In the more humid and polluted atmospheres of
these cities, the three obelisks have suffered major deterioration. Sulfur and nitrogen
acids in the atmosphere have done most of the damage. The recent increase of
vehicle emissions in Cairo does not augur well for the preservation of those surviving
in Egypt.
An important part of our culture is chiseled in stone, and these stones are slowly
weathering away. Even ancient Greek and Roman writers were aware of this phenomenon.
Based on the rate the limestone has weathered in the great pyramids of
Egypt, now nearly 5000 years old, they could last another 100,000 years. This is
due largely to the low rainfall. However, the recent dramatic increase in the acidity
of the rain could reduce this time to substantially less than 10,000 years. Their companion
structure, the Sphinx, is not so “lucky”. In recent years the Sphinx has deteriorated
rapidly. The Sphinx is carved from the natural limestone of the Giza Plateau.
The lowest stratum of the Sphinx is composed of member I, which is hard rock of
a reef limestone (Figs. 11.11 and 11.12). This rock has not weathered appreciably.
Fig. 11.11 The Sphinx was carved in situ from bedrock