I spent a day last week helping to record this old cemetery. It doesn't look like much because the stones are all natural fieldstones, but the pattern tells the story: pairs of stones (head and foot stones) about six feet apart, with a depression in between. These have been disturbed and are not quite in the right places, but it's clear enough. And sorry about the photographs, but it was harsh sunshine through trees, and all I had was my phone.
Here is a classic grave for this sort of cemetery: two stones seven feet apart, depression in between. The depression is there because amateur gravediggers never got all the dirt back into the grave, just enough to level the surface, so when the coffin collapsed the top of the grave sank with it.
Carved headstones were and still are expensive, so most ordinary folks in the 18th and 19th centuries got either wooden boards or uncarved stones.