Read the inventory and the character of the thing becomes clear. Mackaye Harbor Rocks. Danger Rocks. Kanaka Bay Islands. Bakers Reef. Unnamed Rocks off Lime Kiln Point. Site after site is a reef, a stack, or an islet too small and too exposed for anyone ever to have built on it, which is precisely why it survived intact while the shorelines around it filled with houses. The pieces lie off Lopez, San Juan, Orcas, Shaw, Stuart, Sucia, Matia, Clark, Fidalgo, Lummi and Eliza Islands, spread through San Juan, Skagit and Whatcom counties. A handful of names in West Sound on Orcas - Skull Island, Victim Island, Massacre Bay Rocks - record something the charts decline to explain. The Bureau of Land Management administers the whole assemblage as part of the National Landscape Conservation System.

Not every piece is a rock. Some of the monument's most significant sites are headlands that mattered to anyone trying to get a ship through these waters alive. Cattle Point, at the southern tip of San Juan Island, carries its light above the meeting of Haro Strait and the Strait of Juan de Fuca, where tide and wind can make a mess of a calm afternoon. Turn Point, on Stuart Island, marks the elbow of Boundary Pass with the Canadian shore close enough to read. Patos Island keeps its light at the northern edge of the group. These are working aids to navigation with a century and more of service behind them, and they are also, now, protected relics - a rare case of the federal government conserving its own infrastructure as heritage.

The proclamation names archaeological sites of the Coast Salish peoples among the resources the monument exists to protect, and that is not incidental language. These waters were - and are - Coast Salish waters, worked for thousands of years for salmon, halibut, shellfish, camas and reef-net fishing grounds whose locations were held as property and passed down. The small islands and beach fronts that nobody wanted to develop are exactly the ones most likely to still hold an undisturbed record of that. Protecting a quarter-acre islet sounds like bureaucratic fussiness until you understand that the fussiness is the point: what survives on these fragments cannot be replaced, and a single afternoon with a shovel can end it.

Biodiversity is the third pillar of the designation, and here the arithmetic works in the monument's favour. Bare, predator-free rocks are the scarcest real estate in the Salish Sea for nesting seabirds; a reef with no raccoons on it is worth far more to a colony of glaucous-winged gulls or pigeon guillemots than a hundred acres of forested shoreline. Much of the monument is that kind of land. The designation came after sustained local pressure - islanders organised specifically for it, which is not how every monument comes about. The result is scattered, unvisitable in most of its extent, and almost impossible to photograph as a single subject. It is also, acre for acre, about as efficient a piece of conservation as the Antiquities Act has produced.