| Just a little north of the mine pit is the "edge
of time". In the photos below the hands are indicating the unconformity
between the Pokegama Quartzite and the Ely Greenstone. It is an
unconformity because of a time difference of about 800 million years.
The Ely Greenstone (a chlorite schist) is about 2.7 billion years old
and the quartzite is about 1.9 billion years old. The quartzite
represents the "basement", the bottom of the iron formation in the
Mesabi Range. Everything from this point southward to Lake Superior was
at one time a shallow sea. Sand at the edge compressed into the
Pokegama Quartzite and stromatolites (a type of blue-green algae)
produced oxygen that reacted with the iron in the water to form the
iron minerals near the shore. |
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