During the Pleistocene, a tongue of the Cordilleran ice sheet called the Puget Lobe pushed south out of British Columbia and down this basin, reaching its maximum extent near present-day Yelm. It carved Puget Sound itself along with most of the region's recognizable landforms, and when it retreated it left behind everything it had been carrying. As Nick Zentner of Central Washington University's geology department puts it, "Canadian rocks [are] strewn all over the Puget lowland, stretching from the Olympic Peninsula clear over to the Cascade Range." A glacial erratic is simply a rock that does not belong to the bedrock beneath it. Each one is a shipping label, and with enough petrology you can read the origin address.

Seattle sits on the evidence. Roughly eighty per cent of the city's soil is glacial drift, most of it Vashon till laid down during the last major glaciation, and nearly every one of its famous hills is a landform the ice made. Beacon Hill, First Hill, Capitol Hill, and Queen Anne Hill are drumlins - elongated mounds streamlined beneath moving ice. Magnolia and West Seattle are drift uplands. Residents climbing those hills on foot or in low gear are climbing glacial sediment, shaped and left in place while the ice was still on top of it. Within that till, genuinely large boulders are the exception. Anything more than three metres across counts as rare, which is part of why the biggest erratics have become local landmarks rather than simply large rocks.

The Seattle boulder has a neighbourhood named around it - Wedgwood Rock, in the Wedgwood district, one of a small handful of truly large erratics in the Puget Sound area. Nineteen feet at its tallest and hundreds of tons in mass, it is the kind of object that seems to violate the rules of the street it sits on. Its gneiss matches the rock of Mount Erie on Fidalgo Island, 55 miles north, which means the ice moved it that distance at minimum and possibly carried it from much farther before that. Standing next to it, the arithmetic is difficult to process: the thing beside you is heavier than the houses on the block combined, and something once carried it like grit in a boot.

The largest erratics are not all in cities. On Whidbey Island, in the second-growth forest of Saratoga Woods Park, the Waterman erratic stands about thirty-eight feet high with a circumference of roughly 135 feet - a greenschist monolith with chlorite and epidote in its mineralogy, marking it as low-grade metamorphic rock formed under pressure a long way from here. There is no plaza around it and no plaque on the street. You walk in through the trees and it is simply there, mossed over, considerably larger than anything in the forest around it, doing nothing at all and having done nothing at all for fourteen millennia.

Erratics are more than curiosities. Match a boulder's rock type to its source outcrop and you have a data point on where the ice came from and how it flowed; plot enough of them and the flow lines of a vanished ice sheet start to appear on the map. That is the premise behind the Washington glacial erratics project at the University of Washington's Department of Earth and Space Sciences, which catalogues these boulders across the region. They are scattered from the Olympic Peninsula to the Cascade foothills, some in parks, some in yards, some in the middle of nowhere. Every one is a piece of somewhere else, delivered by an ice sheet that has been gone since before anyone built a road here.