currently little year-round rain in this part of Egypt, but there are occasional flash floods that hit the valley. These floods dump tons of debris into the open tombs. The quality of the rock in the Valley is inconsistent, ranging from finely grained to coarse stone, the latter with the potential to be structurally unsound. The occasional layer of shale also caused construction (and in modern times, conservation) difficulties, as this rock expands in the presence of water, forcing apart the stone surrounding it. It is thought that some tombs were altered in shape and size depending on the types of rock the builders encountered. Builders took advantage of available geological features when constructing the tombs. Some tombs were quarried out of existing limestone clefts, others behind slopes of scree, and some were at the edge of rock spurs created by ancient flood channels. The problems of tomb construction can be seen with the tombs of Ramesses III and his father Setnakhte. Setnakhte started to excavate KV11 but broke into the tomb of Amenmesse, so construction was abandoned and he instead usurped the tomb of Twosret, KV14. When looking for a tomb, Ramesses III extended the partly excavated tomb started by his father. The tomb of Ramesses II returned to an early style, with a bent axis, probably due to the quality of the rock being excavated (following the Esna shale). Between 1998 and 2002, the Amarna Royal Tombs Project investigated the valley floor using ground-penetrating radar and found that, below the modern surface, the Valley's cliffs descend beneath the scree in a series of abrupt, natural "shelves", arranged one below the other, descending several metres down to the bedrock in the valley floor. Hydrology The area of the Theban hills is subject to infrequent, violent thunderstorms causing flash floods in the valley. Recent studies have shown that there are at least seven active flood stream beds leading down into the central area of the valley. This central area appears to have been flooded at the end of the Eighteenth Dynasty, with several tombs buried under metres of debris. The tombs KV55, KV62, and KV63 are dug into the actual wadi bedrock rather than the debris, showing that the level of the valley was five meters below its present level. After this event, later dynasties levelled the floor of the valley, making the floods deposit their load further down the