20 May 15. During the several days that we had an opportunity to observe the bison, there were very few in which we didn't get to watch them wallowing in a patch of bare ground with plenty of loose dirt. Occasionally they would be rolling around in the dirt for the better part of 5 minutes, rocking back and forth, pausing for a few seconds to a minute, then resuming their rolling and kicking up the loose dirt as they enjoyed their "bath." It is really quite entertaining to watch, and we generally stopped to watch each time we saw it happening. This cow was apparently having the time of her life rolling and kicking the dirt high into the air. Look carefully and you will see the dirt flying off her legs and reaching high enough to blur all the light green trees top center in the photo.
I've adjusted the image for max tonality and added a small amount of micro contrast adjustment. Nikon D300s; 18 - 200; Aperture Priority; ISO 200; 1/400 sec @ f / 8.