To the Editor:
People who want to experience Chicago’s bedrock without getting their feet wet, as inevitably happens when exploring Morgan Shoal, should head a few blocks south to hike along the beach between LaRabida Hospital and 67th Street. This fascinating beach, believed by some to be paved with concrete, is in fact an emergence of Chicago’s Silurian bedrock deposited on the primeval ocean bottom between 375 and 400 million years ago. This rock is not concrete, nor is it limestone, which is almost pure calcium carbonate. It is, in fact, dolomite, a kind of limestone containing a high concentration of magnesium carbonate. Take a magnifying glass — you will be able to see fossils of primitive sea creatures, not corals because they hadn’t evolved yet, but trilobites, crinoids (relatives of sea stars) and simple mollusks all embedded for hundreds of millions of years in the fossilized slurry of the primitive marine environment.
This bedrock is relatively close to the surface in the South Shore neighborhood. In the late 19th century it was quarried by the gravel company of Dolese and Shepard for constructing buildings, roads and laying railroad track beds. A glass plate photograph in the Special Collections Department of the Harold Washington Library taken around 1890 shows two people standing on a wooden walkway above a quarry pit on South Exchange Avenue, then Railroad Avenue, between 74th and 75th streets. And they didn’t get their feet wet at all!
Frances S. Vandervoort