The machine being pulled by a tractor on this 1937 cover is a then-newly developed rockpicker. It featured a rotating, inclined, cylindrical cage that allowed finer soil particles to sift out the sides as it moved along a recently spread layer of earth. Oversized rock is retained in the cage. The machine offered savings in time and cost because it could sift material directly from a borrow pit without the delay and expense of putting it through a stationary screen first. That was an improvement over the prior rock removal method, hand labor, which was especially onerous in hot weather. The rockpicker, shown here at work on the channel of the All-American Canal in California’s Imperial Valley, was developed by the Crook Co. of Los Angeles.