Answer each question based on the following passage. Sometimes we went off the road and on a path through the pine forest. The floor of the forest was soft to walk on; the frost did not harden it as it did the road. But we did nol mind the hardness of the road because we had nails in the soles and heels. Nails bit on the frozen ruts and with nailed boots it was good walking on the road and invigorating. It was lovely walking in the woods. Frozen ruts' means -