On a clear, calm day near 49th Street Beach, you might spy what looks like an angular rock just breaking Lake Michigan’s surface about 600 feet from shore. That’s the iron boiler of the Silver Spray, a double-decker wooden steamship that sank more than a hundred years ago—its sparse wreckage (mostly the boiler and propeller) a popular swim and dive spot for Chicagoans in the know. Of the estimated 6,000 shipwrecks that litter the Great Lakes, this unassuming iron debris is the city’s closest.

The Silver Spray, née Bloomer Girl, was built in 1894 and serviced Milwaukee before moving its home port to Chicago. On July 15, 1914, the 109-foot excursion vessel set out to pick up 200 University of Chicago students for a tour of the Gary, Indiana, steel mills. On its approach to dock, the ship ran aground on Morgan Shoal, a kind of rocky outcrop rarely found in Lake Michigan (as the lakebed is mostly sand and mud).

The captain and six crew members were on board; an Irish stew was stewing. The captain instructed his crew to “man the lifeboats” and strike for shore but said he would stay with the ship. “There was a note of hunger in his voice,” reported the Chicago Examiner the day after the grounding. “Not a man moved save the cook, who stirred some spice into the stew.” But someone had to notify the students that there would be no tour that day, so one of the ship’s owners and a rower abandoned ship.

News spread that the Silver Spray was sinking, but the ship was merely stranded; Morgan Shoal is under about 20 feet of water at its deepest. For the time being, the salivating sailors were in no danger. An aviator who led dirigible balloon tours from nearby White City Amusement Park flew a load of passengers over the ship, offering to throw down a rope. Without a bucket to ride up, the crew declined but offered a bit of stew if the tourists wanted to come down. The blimp continued its tour of the South Side.

Later the Silver Spray’s twin ship, the Mineral City, arrived, attempting and failing to tow it off the shoal. An unusual fog cloaked the lake as the stuck ship slipped into the first of several nights, crewed and stranded just offshore.

The wreck of the Silver Spray began not on that mid-July day but at least 420 million years earlier, during the Silurian period of the Paleozoic era, when Illinois was under a shallow tropical ocean near the equator. Morgan Shoal was once a coral reef.

According to a Field Museum survey, the 32-acre shoal is made of Silurian dolomite, a type of rock containing the calcium carbonate of ancient corals, sponges, and seashells. “It’s probably very similar to the limestone blocks that rim Promontory Point,” says geophysical sciences assistant professor Clara Blättler, who studies dolomite from all over the world, including samples more than two billion years old. Limestone is calcium carbonate, while dolomite (also called dolostone, which was named Illinois’s state rock this past summer) is calcium magnesium carbonate, explains Blättler.

The fossil-rich bedrock of northeastern Illinois is dolomite, which was covered by clay and silt carried from glaciers during the last ice age 20,000 years ago. The ice melted into Lake Michigan between 14,000 and 12,000 years ago. But in a few places, the bedrock was exposed, like Morgan Shoal and Stony Island, a rocky elevation located around Stony Island Avenue and East 92nd Street—the site of a former nineteenth-century dolomite quarry.

Today the shoal’s crevices, shallow waters, and shipwrecked remains offer a complex habitat that supports a diverse ecosystem, including insects, mollusks, crustaceans, and at least 15 species of fish. And boats continue to run aground there.

After three days, the captain and crew finally agreed to be ferried back early in the afternoon by “life savers” from the Jackson Park Station—part of the United States Life-Saving Service, established in 1871 to rescue shipwrecked sailors. (In 1915, the service became the US Coast Guard.) Soon after 4 p.m. on July 18, 1914, the Silver Spray finally sank. Shortly after running aground, 14 tons of coal had been removed from the boat, lightening its load but making the ship more vulnerable to Lake Michigan’s whims. The waves took their toll, dashing the boat to pieces against the rocky shoal.

“With greedy eyes and twitching fingers the ‘buzzards of the beach’ gathered from far and near to wait the wreckage,” reported the Chicago Examiner. Wreck-revelers collected the timbers washing ashore and built bonfires on the beach, dispatching the Silver Spray one last time in a piecemeal pyre.

The dolomite problem

Dolomite is a very unusual rock, says Clara Blättler, an assistant professor in geophysical sciences. It’s very common in the distant geological record, she says, but much less so in the past 100 million years, give or take. It’s a long-running geological mystery called the dolomite problem: “Why we see it in all these ancient deposits,” says Blättler, “and why it’s not very abundant in analogous environments today.”

Dolomite likely begins as limestone that chemically reacts with some source of magnesium, but the process isn’t well understood. “This is a question geologists have been asking for decades,” says Blättler. Maybe it happens gradually over time; maybe water flowing through it accelerates the process. But why does it matter?

Carbonate rocks—limestones and dolomites—“are the ultimate place that CO2 goes after it comes out of volcanoes.” Carbon dioxide in the atmosphere contributes to global warming, but eventually, over long geological cycles, it reacts with calcium and magnesium and forms these carbonate minerals. “We’re trying to use those rocks to understand what was going on in the ancient carbon cycle,” says Blättler, and how this natural process evolved over time and shaped changes in climates. The chemistry of dolomite formation is an important, yet still mysterious, part of that story.