The islands are mainly low-lying except for some sharply rising sandstone hills on Mainland, Rousay and Hoy (where the tallest point in Orkney, Ward Hill, can be found) and rugged cliffs on some western coasts. Nearly all of the islands have lochs, but the watercourses are merely streams draining the high land. The coastlines are indented, and the islands themselves are divided from each other by straits generally called sounds or firths.