Wallowing is where deer bathe and roll around in mud. They create wallow sites in wet depressions in the ground, eventually forming quite large sites (2-3 metres across and up to 1 metre deep). These sites tend to be used by most members of the herd at some stage, although access can be hotly contended on some occasions. It is not uncommon to see deer walking around caked in mud.
McDowell, R.W. (2009) The use of safe wallows to improve water quality in deer farmed catchments. New Zealand Journal of Agricultural Research 52: 81-90.