In many areas, the sandstone is comparable in texture of the hardened, dried-out brown sugar that you find in the back of your pantry. It’s fairly solid, but can be easily scratched away, returning to granular form. It is this softness that has allowed such remarkably fast progress on the skull.
As an extra bonus, the sand often forms a rind on the surface of the bones that shows a rust-colored stain. When it is encountered, this serves as a warning to preparators that bone is near, and that they may wish to switch to a smaller air scribe or a hand tool. The sandstone is easily removed from the bone, leaving the bone’s intricate textural details intact.
Other areas of matrix include concretions (compact masses of mineral matter) and mudstone pebbles that were carried in with the sand. There are lots of these, but fortunately they do not appear to have compromised the bones in any way.
Article Source: Burke Museum