Friday, September 28, 2012
This will be the first of several entries from Minnesota's north shore of Lake Superior. All are based on fairly short visits during a one day drive between Thunder Bay and Duluth. On a long trip dominated by prairies and mountains, this was the day reserved for beaches. As usual, the posts are based on a combination of cursory observations and small amounts of pre- and post- trip web-work. I hope my interpretations aren't too far off the mark - but if you're writing a term paper on the North Shore Volcanic Group or on Lake Superior sediment transport, I recommend you consult more rigorous sources! But feel free to borrow my pictures.
The bedrock along this shoreline consists largely of mafic volcanics over a billion years old, although they are contained in a broader landscape of gabbro and diabase. Glaciers modified the surface much more recently, creating the coarse grained sediment that has subsequently been reorganized to form beaches. The gravel beaches vary from dark gray to red (although red certainly dominates), depending I assume, on local variation in the oxidation of the Proterozoic source rock. In a blog about beaches, I rarely get to use words like Proterozoic, but I have written about Proterozoic beaches previously, like the rippled Belt Series rocks in the riprap at Shipwreck Point in June 2007 and the beautiful Tapeats beaches at Blacktail Canyon (Cambrian beaches composed of Proterozoic sediment) in June 2009.
The beach on Deronda Bay is typical of many beaches along the north shore. It is basically a pocket beach, oriented towards the east and therefore fairly oblique to the coastline. This presumably is a function of the dominant wave action being out of the east. On straight segments of this coastline with sufficient sediment, this would tend to transport material southwest, forming accumulations on the eastern sides of rocky promontories.
The gravel was stacked up into a series of berms, suggesting storms at several lake levels. And the gravel on this beach was uniformly red. I would have looked more carefully at the lithology, but my mineralogy is as rusty as the oxidized volcanics. I only picked up the flatter stones, and that was only to see how many times I could get them to skip.