State by State

North Dakota

  • Specimen Number


  • 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

    Link to State Geological Map from USGS.  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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Specimen stone for North Dakota.
Closeup of North Dakota. Outline of North Dakota. Geol Map of North Dakota.