Back in the days before Powder River coal flooded down the Fort Worth and Denver (BN), the ballast came from a cinder cone pit at Twin Mountain, New Mexico on the C & S. Being nothing more than volcanic pumice or scoria, it would actually float away from the roadbed during a hard rain. When the coal started, it quickly pounded it down to dust. I wonder if the rock from Darling is similar--it looks the same. These days, BNSF ballast comes from Pedernal, New Mexico and Davis, Oklahoma. It doesn't float!