dimple the eastern Washington landscape, so-called Mima mounds, where geologists have proposed multiple hypotheses in order to explain their puzzling origin. They seem to be relic features not being formed today, so I favor the periglacial origin, similar to the mechanism that creates patterned ground in Arctic regions.
Wide view of Mima mounds in Turnbull National Wildlife Refuge. Click on image to embiggen.
The low areas between the mounds often contain vernal pools of shallow water in late winter and early spring. Click on image to enlarginate.