Almost certain to be an Iron Stone deposit. That area's shot through with them. Not up to much these days, as they're almost all worked out, and have been for years. Probably back filled as well.
Burliegh House, the local Stately Home used to have, about 15 or so years ago, one or two Mines/Quarries still being worked, around the Back or Wittering/Collyweston/Cottesmore way, though only on a very small scale.
Interesting find none the less.