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.