Bill...
Gotcha, but wouldn't someone have to "dig out" the pits? This was the only digging action I could think of that would be consistently linked with monumentation efforts.
I've heard my parents and family talk about dugouts on the farms when they were young. Typically they were excavated in low spots or anywhere that water would naturally collect, for watering the stock or to create a reservoir for the irrigation pumps.
In your case it may be that there was a series of dugouts that have been silted in, or washed away by flooding.
An invaluable resource for any surveyor working with old records is a book which came out a few years ago called "Home Ground". It is a dictionary of historic, regional, and/or colloquial geographic terms.
from p.116 "Dugout. The term dugout is mostly used in Canada and by people on the prairies in the United States. Equivalent to the farm pond or rancher's tank, a dugout is a small, on-farm reservoir dug into the ground and located in a particular watershed area. Prairie farmers look for land with sufficient drainage area that slopes toward a central point in a field; this is a perfect place to build a dugout. The water held there is for farm maintenance-livestock or crop-and is usually sufficient (an average size is two hundred by seventy feet on the surface,with a twelve- to sixteen-foot depth) to withstand drought conditions"