Islay Hill is a volcanic cone in central California on the southeast side of San Luis Obispo. It is the southernmost of the Nine Sisters, a line of volcanic mountains and hills extending from Morro Bay to this point. The chain is extinct, though possibly dormant, and Islay Hill stands out within it because the other eight formations are volcanic plugs rather than cones. Its importance lies chiefly in this geologic role, as it marks one end of one of the best-known volcanic alignments in the San Luis Obispo area.