I think this is a Potter Wasp Nest I found on this rock while hiking the Ouachita National Forest here in Arkansas. Wikipedia states that the name "potter wasp" derives from the shape of the mud nests built by species of Eumenes and similar genera. It is believed that Native Americans based their pottery designs upon the form of local potter wasp nests.
Potter wasps are also known as Mason Wasps.
When a cell is completed, the adult wasp typically collects beetle larvae, spiders, or caterpillars and, paralyzing them, places them in the cell to serve as food for a single wasp larva. For example, Euodynerus foraminatus paralyzes the larvae of the poison hemlock moth. As a normal rule, the adult wasp lays a single egg in the empty cell before provisioning it. Some species lay the egg in the opening of the cell, suspended from a thread of dried fluid. When the wasp larva hatches, it drops and starts to feed upon the supplied prey for a few weeks before pupating. The complete lifecycle may last from a few weeks to more than a year from the egg until the adult emerges. Adult potter wasps feed on floral nectar.
Potter Wasp Nest - Ouachita National Forest - Arkansas