Today’s Question:
A STRAIGHT DOPE CLASSIC FROM CECIL'S STOREHOUSE OF HUMAN KNOWLEDGE
Oct. 29, 1982

Dear Cecil:

I'm interested in the "islands" that are commemorated in the Chicago street names Blue Island and Stony Island. I know there really was (and is) a Blue Island, which is that sort of plateau on the far south side where the Morgan Park neighborhood now is. But what about Stony Island? Was there really such a place, or was it the figment of some real estate developer's imagination, like Arlington Heights? If Stony Island existed, where is it and what happened to it?

— M.L., S. Plymouth

Cecil replies:

There really was a Stony Island, and what's left of it is still on the south side, but you'd have to be pretty eagle-eyed to find it. Stony Island was a rocky outcropping that stretched for about a mile and a quarter between Stony Island Avenue (1600 east) and Kingston Avenue (2500 east), from 91st Street to 94th Street. At one time it stood about 20 to 25 feet above the surrounding lake plain. Unfortunately, residential development has almost completely obliterated the original rugged surface of the island, and what might have made an interesting park —  Stony Island offered a compact visual summary of much of the Chicago region's geological history —  is now home to a forest of bungalows.

Stony Island and Blue Island were so called by early settlers because they stood out in such striking relief on the otherwise featureless prairie. The two formations had actually been islands thousands of years earlier, when lake waters covered the area. Of the two, Blue Island was the more imposing, rising 50 to 80 feet above the lake plain and measuring five and a half miles long (running north and south) by a mile wide. (In case you're wondering, Blue Island looked blue from a distance due to atmospheric scattering.)

But Stony Island had a more complicated geological past. Blue Island is a moraine, or glacial dumping pile, and so of relatively recent origin. Stony Island, in contrast, is thought to have started out as a coral reef during the Silurian Period well over 35 million years ago, when much of central North America was covered by the sea. Sedimentation subsequently buried the coral — and everything else in the region —  under a layer of what geologists call Niagara limestone, the bedrock that underlies much of the Great Lakes area. In the case of Stony Island this produced a rock hill called a dome. In more recent times, by geological standards, glaciers scraped and polished the limestone, producing characteristic striations. The glaciers also left massive boulders scattered around the island.

When the last of the glaciers began to melt around 10,000 to 35,000 years ago, the runoff formed Lake Chicago, a crescent-shaped body of water that covered the southern quarter of present-day Lake Michigan as well as almost all of what is now the city of Chicago. At its peak, Lake Chicago's surface was about 55 feet higher than that of Lake Michigan today, at which point Stony Island (though not Blue Island) was underwater. The lake receded in stages, and at length the island broke the surface and a gravel beach was formed around the top.

In 1917 the Geographic Society of Chicago urged that Stony Island be preserved as a park, calling it a valuable geological and botanical asset. Since the island was one of the only points within the city limits where bedrock was close to the surface, it was one of the few places where you could see evidence of glaciation. Two quarries that had been dug into the island at an early date clearly revealed the patterns in the rock. Much of the stone also contained fossils. In addition, pockets of organic matter that had been trapped in the limestone had decomposed into asphalt. These broke open from time to time and the asphalt oozed over the stone surface, making weird shapes.

As might have been expected, nobody paid any attention to the preservationists. In the 1920s, trenches for sewer and water mains were blasted through the limestone, the boulders were cleared away, and the island was graded and paved. Today its existence is largely forgotten. Such is progress.

— Cecil Adams

Send questions for Cecil Adams to cecil@chicagoreader.com or post them on the Straight Dope Chicago Message Board.

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