I think what you are looking at is the Nulhegan Basin. Google for "Nulhegan Basin geologic history". One source: "The Nulhegan Basin was formed when a pool of magma formed within existing metamorphic rock. The magma cooled into a relatively soft granite rock called quartz monzonite. Once erosion wore away the cap of metamorphic rock, the softer monzonite eroded more rapidly than the surrounding metamorphic rock. This resulted in a relatively flat, circular area, roughly 10 miles in diameter, surrounded by hills. " (https://www.geocaching.com/geocache/GC4J611_the-nulhegan-basin?guid=035bf082-3180-40c9-8190-095bf75b5da1)