Beach Stripe

by Carl Strang

 

Yesterday I described my return to check on a seedling growing near the town park at Culver, Indiana. On the way to the seedling I noticed a black line of material that had been sorted out from the beach sand at the edge of Lake Maxinkuckee.

 

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I don’t remember seeing this at Culver, before, but it reminds me of similar deposits at Illinois Beach State Park that are of finely ground iron minerals. Here’s a close-up of the Culver stripe.

 

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I had no trouble finding a bottle cap in the litter along the lake edge. Sad, that, but it provided the tool I needed. I used it to scoop up some of the black material and carry it back home.

 

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Notice how the mainly quartz sand is revealed beneath the thin layer of black particles. The next stage was to dry out my sample, then see if it would respond to a magnet. My hypothesis was that it would prove to be magnetite or hematite, both black iron minerals, possibly from chunks of banded iron formation in Canada that were picked up by the glacier, deposited in Culver, broken into tiny pieces by glacial and/or wave action, and finally distributed along the beach.

 

The black powder was strongly attracted to a magnet. Here is the sample after I poured it onto a white card and dragged a magnet beneath it. The sand was left behind.

 

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A close-up shows the particles aligning themselves with the magnetic field.

 

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The particles were not themselves magnetic; they didn’t stick to iron. So, I conclude that they are not magnetite but rather bits of hematite, an iron mineral very common in banded iron formations, and deposited billions of years ago in Canada (in this case; Culver is on the route of the glacier’s Saginaw lobe). During the early days of life on Earth, oxygen was increasing in the atmosphere through the work of stromatolite-forming, photosynthetic cyanobacteria in shallow seas. Iron was abundant, dissolved in the water, but it reacted with the oxygen to form the hematite, which precipitated out to produce iron formation deposits that in recent times have been our important sources of iron ore.

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