We had a one acre pond dug after moving here, and one fist sized rock found. One! It's a builder's sand resource, down ten feet at least.
Drainage is not a problem...
Mary
Carbaryl kill insects, and is not good to spread around 'just because'. It's
also off the list of approved products for chickens.
It's about fencing, as others have mentioned. Depending on what breeds you have, it's possible that even that 24" decorative fencing might be adequate.
Rocks...