the ancient rock. Geology The Acropolis is a klippe consisting of two lithostratigraphic units, the Athens schist and the overlying Acropolis limestone. The Athens schist is a soft reddish rock dating from the upper Cretaceous period. The original sediments were deposited in a river delta approximately 72 million years ago. The Acropolis limestone dates from the upper Jurassic period, predating the underlying Athens schist by about 30 million years. The Acropolis limestone was thrust over the Athens schist by compressional tectonic forces (Plate tectonics), forming a nappe or overthrust sheet. Erosion of the limestone nappe led to the eventual detachment of the Acropolis, forming the present-day feature. Where the Athens schist and the limestone meet there are springs and karstic caves. Many of the hills in the Athens region were formed by the erosion of the same nappe as the Acropolis. These include the hills of Lykabettos, Areopagus, and Mouseion. The marble utilized to construct the buildings of the Acropolis was sourced from the quarries of Mount Pentelicus, a mountain to the northeast of the city. Geological issues The limestone that the Acropolis is built upon is unstable because of the erosion and tectonic shifts that the region is prone to. This instability can cause rock slides that cause damage to the historic site. Various measures have been implemented to protect the site including retaining walls, drainage systems, and rock bolts. These measures work to counter the natural processes that threaten the historic site. Gallery See also Landscaping of the Acropolis of Athens References Notes Bibliography Cohen, Beth. (2010). "Deconstructing the Acropolis: The Acropolis Museum, Athens, opened 20 June 2009 by Bernard Tschumi Architects." American Journal of Archaeology 114:745–753. Goette, Hans Rupprecht. (2001). Athens, Attica and the Megarid: An Archaeological Guide. London and New York: Routledge. Harris, Diane. (1995). The Treasures of the Parthenon and Erechtheion. New York: Oxford Univ. Press. Neils, Jenifer, ed. (1996). Worshipping Athena: Panathenaia and Parthenon. Madison: Univ. of Wisconsin Press. Pollitt, Jerome J. (1990). The Art of Ancient Greece: Sources and Documents. New York: Cambridge Univ. Press. External links The Acropolis of Athens (Greek Government website) The Acropolis Restoration Project (Greek Government website) The Acropolis of Athens Virtual Tour The Acropolis Museum and the Goddess Athena The Glafka Project UNESCO World Heritage Centre — Acropolis, Athens Ancient Athens 3D Excerpt on the geology of Athens from: A Geological Companion to Greece and the Aegean by