Polygons in Utopia Planitia (EDM - Absolute Natural Colors; credits for the additional process. and color.: Dr Paolo C. Fienga - Lunexit Team)

On the Surface surrounding the "Scalloped Depressions" there are several large Boulders (see this EDM). Sources for these rocks may include Ejecta from nearby Craters, Volcanic Floods, or Boulders emplaced by Glaciers or Periglacial Processes.
Also on the surrounding Surface is a Polygonal Pattern of Fractures. This is commonly associated with Scalloped Terrain, and indicates that the Surface has undergone stress, potentially caused by Subsidence, Desiccation, or Thermal Contraction.

Variations in the sizes of the Polygons seems to be partly dependant on their location with small Polygons appearing within the Scallops and larger Polygons appearing outside the Scallops on the Surface of the Mantle. Several cracks cut through the side of the Scallops indicating that they must be at least as deep as the Scallops. The Polygons may have been present previous to the erosion of the Mantle. The formation of some Scalloped Depressions is believed to be an ongoing process today.