The material mass of the city — the roads, the bridges, the buildings — must come from somewhere. For Los Angeles, that somewhere has often been Irwindale. Over millions of years, the San Gabriel Mountains and their largest river deposited a massive pile of rocks and gravel ideally suited for construction underneath the inland city. And so, as Los Angeles has risen upward and outward, Irwindale has sunk downward. On this flat, 20-mile, morning bicycle tour, we will peer down into the City of Irwindale’s finest craters and visit some former pits that have been (mostly) filled and transformed.