• Specimen Number

  32

• Description

  Reddish-stained boulder of gray rock identified by survey staff  as a "foliated biotite hornblende granite, possibly a monzonite  with potassium feldspar, hornblendw, biotite, plagioclase and possibly a little quartz."

• Classification

  Igneous. Plutonic. Granitic. Hornblende granite or monzonite.

• Location & Occurrence

  Glacially transported boulder from the Canadian Shield.  Specimen location unknown.

• Geological Age

  Based on the lithological sources areas for the ice-sheet flow-line of the Des Moines Lobe over the Canadian Shield, the survey staff suggested either a date of  2.7 Ga, which is Archaean, or 1.8 Ga, which is Proterozoic. In either case, this is nearly half of Earth history.

• Geoheritage

  This erratic glacial boulder could be the oldest rock in the entire collection, possibly as old as 2.7 Ga.

• Links

  USGS Compilation: Search for North Dakota in upper right.  Link to North Dakota Geological Survey.

• Other

  Thorson collected and stacked boulders just like this one when working as a full-time farmhand in north central North Dakota.

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