Periodically, we see one of the girls lying on her side, foot slowly unclenching, wing outspread in a fan, and her eye slightly unfocused. Then the foot scuffles into the dirt and flings it up and out. She burrows her head into the dirt. Sometimes we see more than one chicken at a time in close proximity doing this.
The dirt bath must be extremely satisfying. That slightly unfocused look is not uncommon, and they seem so at peace. It usually occurs toward the end of the day (just like our baths can), although a mid-afternoon "plunge" is not unheard of.
The first time we saw it we thought the chicken must have been attacked. To our untrained eye our girl appeared to be writhing about in agony. Rushing up to see what had attacked her, the chicken sat up somewhat dazed and appeared perfectly fine. We walked away only to see her resume the same activity. Once we realized there was no pain involved, we understood it was a dirt bath.
They often choose a somewhat shady spot, although I've seen them do it in full sun. A most recent hole outside our porch door is four to six inches deep. While a pleasant bathtub it could also turn into a real ankle-turner. They love it though, so we haven't asked them to switch spots yet.