The Apache in this image are softening their horse’s hooves. In the summer heat, when the hooves get very dry, they can crack quite severely and cause the horse to go lame. But as they’re taking it easy, one of them sees this cavalry hat floating toward them in the water. Obviously, it indicates that there are soldiers in the area so it’s just a matter of time before the Apache are found. They’ll have to pack up and get out of there.