Opposite the East Bank city of Thebes and its many temples, the West Bank of the Nile served for thousands of years as a vast cemetery for its citizenry and officialdom. For five hundred years, during Dynasties 17 to 20, it was also the burial place of Egypt's rulers. Several locations are known to have been set aside here as royal necropoleis, the most famous of which is the Valley of the Kings (KV).
KV actually consists of two valleys that run northeast from the slopes of a prominent ridge along a high plateau into the Western Desert. This plateau is composed mainly of limestone strata, the Theban Formation, laid down about thirty-five to fifty-six million years ago, with strata of "Isna shale" intermittently lying among them. These strata were forced upwards later in the Tertiary, and during the Pleistocene, heavy rains cut numerous wadis deep into the limestone as water drained off the plateau toward what eventually become the Nile Valley.
The quality of limestone in the Valley of the Kings varies from extremely fine and structurally sound (as in KV 57, belonging to Horemheb), to fractured and weak, as in the lower reaches of KV 11, only a few meters away. Often, as a tomb was cut, it would pass through several limestone strata, each of different quality, and the plan of a tomb was sometimes altered to acknowledge that fact (as in KV 20).
The Isna Shale (also called Montmorillonite) that intermittently appears in the Valley of the Kings is an especially weak and unstable stone that posed problems for ancient quarrymen and modern conservators. When the shale comes in contact with moisture it expands, causing parts of a hillside literally to break apart. This has happened in the past, and has done damage to several tombs in the Valley, including KV 7 (Rameses II), KV 13 (Bay), KV 17 (Seti I) and KV 20 (Thutmes I and Hatshepsut).
There are numerous pieces of chert embedded in the Theban limestone, and this material was used by the ancient Egyptians to make the very stone tools with which they dug the tombs. Many pieces of chert litter the hillsides of the Valley of the Kings and are embedded in the bedrock.
When they have lain unmoved for millions of years they acquire a dark patina from exposure to air and sunlight. Archaeologists know there is no point clearing such a dark-colored hillside because there will be no remains of human activity beneath its surface.