Three hundred million years, leaning

The Flatirons stand on the east face of Green Mountain, five of them, numbered First through Fifth from north to south. The geometry is plain: parallel slabs of Fountain Formation sandstone, tipped to roughly fifty degrees, all leaning the same way toward the flatland. Boulder’s street grid runs right up to their base and stops.

The rock is old — close to 290 million years, laid down as sediment shed off the Ancestral Rockies, then lifted and canted during the uplift that raised today’s Front Range. Green Mountain tops out at 8,148 feet. Climbers once called these the Chautauqua Slabs; the name Flatirons came later, for the shape of a smoothing iron.

At their foot sits Chautauqua. The park opened on July 4, 1898, when a few thousand people came up from the plains for the first season, and it never closed. It is one of the last of its kind still running — the only Chautauqua west of the Mississippi in continuous operation, a National Historic Landmark since 2006. The auditorium is the original 1898 wood-frame hall. Cottages still line the lanes below the grass.

The meadow is the foreground. It runs uphill from the cottages to the Chautauqua trailhead, where Boulder’s trail network begins and the city gives way to open space. This is home ground — the view I have walked past more than any other, worked from the red-rock overlooks and ridgelines that ring the slabs.