The sac was a strange shape, and very difficult to get in to. You can see the anchor connected to the right side.

Since finding this, I've found a couple of similar egg sacs without the surrounding dirt on the inside of a large crack on a fallen tree. Inside the crack I also found a large female Eratigena duellica. It's possible that the egg sacs were going to be covered later, but the spider that laid them was eaten before that could happen.