State by State
North Dakota
- 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
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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