Limestone kiln. There are 3 or 4 individual kilns. Limestone was quarried about 300 yards uphill, delivered via a railcar system, then stacked so that it makes a few arches. Redwood (the old growth forest that used to be here) was placed under the arches and on top, and it burned for 3-4 days. About 70 cords of wood were used for each batch, and 6,000 cords were burned each year. California's natural resources got hit hard back in the day.