Tom Slade at Bear Mountain by Percy Keese Fitzhugh

Brent Gaylong could see nothing, but he felt the flat rock on which his foot had pinioned the snake sinking beneath the pressure. As it yielded and settled in the oozy bottom it was necessary for him to press still harder with his foot, or at least to maintain the pressure. There was a tendency to relax this pressure somewhat as the rock sank, for Brent knew not how far it might go down in that unseen muck. Obviously, he could not hold the snake imprisoned except upon a firm foundation. Obviously, also, the harder he pressed the more the rock would go down.

This new and ominous turn of affairs below him was somehow communicated to the winding, clinging, undulating mass that held his leg like a spiral cable. There were pressure and relaxation of pressure in that sinuous body, as if the reptile were preparing for a new stage in the deadly tussle.