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.