A combination of downwarping
and burial by glacial sediments has rendered the frac sand source units in the area of southeastern Minnesota westward of Olmsted County nearly Inaccessible and, therefore, not economically recoverable at current prices.
The rocks of the Arunta Block are up to 2 billion years old and are believed to represent a former accumulation of lavas, volcanic ash beds and minor sediments which have been intensely metamorphosed by downwarping
to depths greater than 10 km within the crust, producing metamorphic alteration up to granulite facies.
Processes such as downwarping
and pediplanation are scantily discussed as is the coverage of the climatic, aspects.