Tribal groups. Obvious as all get out.
They keep coming back with the herds, leaving their mark (due to vegetative decay). Finally the 18 year clock ticks on and then they set up another camp nearby. This goes on for a couple of thousand years you eventually get a build up of rocks (to hold down the tent flaps during bad weather), and more and more decayed vegetative discoloration ~ plus, they sweep the little rocks out of the area.
American Indians customarily would live in their long houses for a period of years, then build new long houses elsewhere and burn the old ones to the ground.
It gets rid of the more aggressive lice and cockroaches.
In more luxurious times the guys living in tents might well have burned the old ones on site every now and then. That'd give you some photoreactive chemistry stirred up to help discolor the ground.
BTW, i'm thinking about things like this constantly these days figuring out what signs the earliest Spanish settlers in Virginia left behind. The written record is "thin" ~ to say the least ~ but the more aggressive Iroquois moved into this region between the Shenandoah Valley and the Algonquian people who lived along the Potomac and the Cheseapeake Bay back in the 1500s.
The Iroquois were always pretty good about making deals with Europeans and actually played the middle man part of traders in the fur trade. Elsewhere Europeans played that role.
What that means is there should be TRACES of extensive Iroquois stockades and long house complexes ~ modified with Spanish Technology ~ like cut stone. So, where are they and what might remain 300 years later.
Remember, the Iroquois wouldn't be living out in the open in flimsy tents ~ if they could help it.