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Researchers believe that the region stabilized a short time after
the impact, and that a shallow lake formed in the crater moat.
In time, the lake filled with dirt and rocks. The area was leveled
by more than 70 million years of erosion that followed. Due to
the crater’s thick covering of Phanerozoic clast breccia,
only small areas of the central peak and terrace terrane eroded.
The Manson area was covered by several continental glaciers during
the last 2.5 million years, causing further erosion and burying
the region in layers of glacial till. Today, the crater is entirely
covered with glacial sediment and not detectable above ground.
Although groundwater is difficult to find within the Manson Crater,
the structure has brought several benefits to local residents.
According to Dr. Raymond Anderson, Supervisor of Geology and Mineral
Resources at the State of Iowa Department of Natural Resources,
Energy and Geological Resources Division, "Water wells that
penetrate into the brecciated crystalline rocks of the central
peak encounter naturally soft water. On the northeast edge of
the structure, at the town of Gilmore City, gigantic limestone
quarries mine limestone that was lifted up to the current land
surface by the force of the Manson impact."
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