A tall island in the sea, a geologic bone, the sea stack represents rock harder than what once surrounded it and has been eroded away. In some instances, this harder material was originally a volcanic intrusion into sedimentary strata. The sea stack—which in desert country might be called a chimney, pinnacle, needle, knob, horn, or pillar—rises in the ocean, and is often favored by cormorants, pufﬁns, and gulls for nesting and whitewashing with clamorous possession. The stack has been whittled by waves in a cycle that may begin as a cliff cave at sea level, cut away to be an arch at sea, and then twin stacks, then one, before weathering to a stump that subsides in time to be a reef below the waves. Sea stacks that abound on the West Coast (Oregon’s Haystack Rock is technically a sea stack) shouldn’t be confused with another resistant geologic remnant, the haystack, which is found in karst terrain more common in the East.