State by State
- Specimen Number
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."
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.
This erratic glacial boulder could be the oldest rock in the entire collection, possibly as old as 2.7 Ga.
Thorson collected and stacked boulders just like this one when working as a full-time farmhand in north central North Dakota.