The ground under Irwindale is just one giant sand wash
Being so close to the mountains that sand is also occupied by some rather large chunks of aggregate that just rolled with the flow
They have numerous neighbors that specialize in the processing of such material for their use in concrete and asphalt products. I attended the "Last Blast" at the previous Irwindale in the late 70s. Many racers that evening expressed their displeasure with "MILLER BEER SUCKS!" added to their race car graphics
Few years later their sits a Miller brewery.