People, as curious primates, dote on concrete objects that can be seen and fondled. God dwells among the details, not in the realm of pure generality. We must tackle and grasp the larger, encompassing themes of our universe, but we make our best approach through small curiosities that rivet our attention--all those pretty pebbles on the shoreline of knowledge …
—Stephen Jay Gould, Wonderful Life
The pebbles are prettier on the other side of Lake Michigan. Even amateur beachcombers know that the Dunes-area shoreline of Indiana and Michigan yields better rocks than Chicago's gravelly man-made beaches. Over there the pebbles washing ashore have richer colors--deep scarlets and jadelike greens, eggshell whites and midnight blacks, buttery golds and chestnut browns. And their sizes and shapes are more varied--some are small enough to set in rings, others large and flat enough to skip in broad, skimming arcs over calm shoreline waters.
We do most of our beachcombing in the vicinity of Union Pier, Michigan, whose shoreline offers a rich array of gleaming granites, basalts, sandstones, and shales. With children of our acquaintance we've been known to cart away sand pails full of pretty pebbles, and often tucked away in these collections are "small curiosities that rivet our attention." They are dull gray or tan disks, no bigger than a child's fingertip and round and flat as a button, with small holes in the center, suggesting petrified Cheerios. They are not rocks, for they are too consistently round, too symmetrical and too identical in their markings to be pebbles. And in many the center holes are shaped like stars. Few amateur beachcombers seem to know what they are--even those of us who regularly visit the beaches and collect enough to make necklaces or bracelets of them. Children who collect them have called them "Indian beads"--without knowing perhaps that archaeologists digging up Indian burial mounds have found that, indeed, several tribes collected or made jewelry of these natural oddities.
What are they? To geologists and paleontologists these fascinating beads are as common as dirt. They know them as fossils of crinoids, strange creatures that throve in the midwest 500 to 300 million years ago--long before the Indians, long before the lake, long before the glaciers came that formed the lake, long before the dinosaurs. Crinoids were plantlike marine animals that lived in vast meadows rooted to the bottom of the warm, shallow coral seas that washed over Chicago (and large portions of the earth) as long as half a billion years ago--roughly the last 10 percent, by the way, of the earth's lifetime.
As the dominant form of marine life in the Silurian period (380 to 350 million years ago), crinoids are also an important part of our present. Pulverized into the limy muds at the bottom of those coral seas, their remains are a significant constituent of the limestone bedrock on which much of the midwest rests. Every skyscraper built in Chicago reaches down to support itself on crinoid-rich limestone. The same limestone has been taken from quarries and used to build railroad embankments, lake retaining walls, and handsome graystone homes. Look hard at the three-flats in Wrigleyville and you'll find some of these curious beads.
Amazingly adaptive, crinoids are survivors. They haven't lived in this area since it was the bottom of a saltwater sea, but of 5,000 identified crinoid species, some 500 thrive today from the Arctic Ocean to the South Pacific, from Greenland to the Caribbean. Crinoids survived the cataclysmic extinctions that mark major geologic eras, including the great Permian extinction of 250 million years ago, which wiped out practically everything (perhaps 96 percent of all species then living, according to a recent National Geographic), and the better known Cretaceous extinction of 65 million years ago, which did in the dinosaurs.
As Stephen Jay Gould writes of similar fossils found in the Burgess shale quarry in the Canadian Rockies, crinoids were once "grubby little creatures of a sea floor" hundreds of millions of years old. "But," he says, "we greet them with awe because they are the Old Ones, and they are trying to tell us something."
Most of us can hear their message only indirectly, through the translations of scientists who have painstakingly collected, studied, and put together the story of their life and times. What these scientific translators tell us--about the life of crinoids, their death, their endurance as fossils, and their ubiquitous presence not only on Dunes beaches but also in our graystone buildings and common concrete sidewalks--is the story of an ancient earth that humans have occupied only briefly. It seems a fitting subject for contemplation while restlessly seeking peace, sun, or adventure some afternoon on the vast and often isolated beaches of the Dunes.
The buttonlike crinoid fossils that come ashore may seem extraordinary in a handful of common pebbles, but they are the least interesting-looking part of a full crinoid. The buttons are like vertebrae, pieces of the long stalks that held up the crinoids' strange, magnificent heads, called calyxes. In some forms the calyxes looked like flowers, as suggested by the popular name "sea lilies." Others had calyxes shaped like ragged-edged tulips or intricately carved goblets. Some smaller species look like miniature palm trees. Others suggest surreal, scaly chicken feet reaching out of the sea bottom. Some look truly ominous, like a crusty octopus when the arms (sometimes called pinnules) are open, or like an alien creature when shut, like the pods in the original Invasion of the Body Snatchers.
A few complete crinoid fossils from the area are in collections kept at the Chicago Academy of Sciences and the Field Museum of Natural History. While they are generally inaccessible to the public, one can arrange to see them.
By far the larger and more imposing collection is at the Field Museum, which has a particularly striking four-foot specimen that is complete from top to bottom. It had feathery, billowing pinnules extending from the calyx that were thick enough in spots to suggest rows of "beads" like those that make up the stalk. Another impressive item at the Field Museum is a piece of limestone that holds a tangled swarm of crinoid fossils; they appear to have been dumped from a can of scaly prehistoric worms.
The Academy of Science's attic drawers contain some 2,000 crinoid specimens --some bits and pieces, and some fossils of complete crinoids. They are under the care of Ron Vasile, collections manager and archivist, who as a northwest-side kid fell in love with museums and the fossils and bones in them and, as he says, "never outgrew the shit." Most of the complete crinoids in the Academy collection look like tulips with a slender coiled spring for a stalk. The stalks are made up of the beadlike vertebrae, stacked like dimes in a roll, and they have rootlike tendrils at the base, which the animal used to attach itself to the seafloor.
In the corner of the Academy's attic lies a gravestone-sized hunk of limestone from the Thornton quarry near Halsted and the Tri-State Tollway. It is evident from this slice of prehistoric life that crinoids were not alone in the Chicago area some 350 million years ago. The warm, shallow Silurian seas that engulfed the area were crawling with a diverse array of strange marine invertebrates, most of which are now extinct.
Among the most lurid of these prehistoric Chicagoans were trilobites and cephalopods. The trilobites, ancestors to insects, had three-part bodies, armored heads and tails, and three pairs of jointed legs on which they scurried around the seafloor. Judging from their fossils they looked like egg-sized beetles. Ron Vasile thinks the trilobite was the model for the hideous creature in the movie Alien. Though trilobites were once the dominant form of marine life, they did not make it through the Permian extinction.
Cephalopods, like large squids encased in shells shaped like ice cream cones, were ancestors of the modern nautilus, with air chambers that enabled them to hover at varying depths in the water. Some fossil parts suggest that ancient cephalopods got as large as 20 feet.
Understanding how these strange animals got here--how the seas got here--requires a sense of the mind-staggering enormity of geologic time. Compared to the history of the earth, human time, in the words of writer John McPhee, "is much too thin to be discerned--an invisible mark at the end of the ruler." If all of the earth's history were compressed into human time, McPhee writes, we would see before our eyes "Sea levels . . . rising and falling hundreds of feet, ice . . . come pouring over continents and as quickly go away. Yucatans and Floridas would be under the sun one moment and underwater the next. Oceans would swing open like doors, mountains would grow like clouds and come down like melting sherbet, continents would crawl like amoebae, rivers would arrive and disappear like rainstreaks down an umbrella, lakes would go away like puddles after rain, and volcanoes would light the earth as if it were a garden full of fireflies."
People aware of geologic time have a pleasant detachment about themselves, a sense of quiet wonder, patience, and curiosity that refuses to take anything too seriously. When asked to recount his titles, for instance, Dr. Mark Reshkin told us he was "professor of geology and environmental affairs at Indiana University Northwest in Gary," and then added as an afterthought: "I am also a vice chancellor of the university." He paused briefly. "But you don't need that." He paused again. "And I don't either."
Reshkin, an expert on the "recent" geology of the Lake Michigan area--the last million years or so--gives this quick glimpse at what is known about the region's geologic history:
"The earth is about five billion years old, but the earliest we know anything about this region is about two billion years ago, during Precambrian time. This was a mountain range about the height of Mount Everest. We know this because the steel mills have one-mile-deep disposal wells for acids produced during the steel-making process. At the bottom of these wells is granite, an igneous rock that is in the core of mountain ranges. Its presence this close to the surface of the earth indicates mountain-making activity.
"About 600 million years ago, during the first half of the Paleozoic Era, wind, water, and ice wore the mountains down into a gently rolling landscape which was then covered by seas. These warm, shallow seas covered what is now Illinois and Indiana, but probably not Wisconsin." For about 250 million years a succession of seas washed over the area--the Cambrian sea, the Ordovician, the Silurian, and the Devonian--leaving sands, clays, and muds that eventually became sandstone, shale, and limestone.
Another expert in the area's geologic history, Dr. Tom Guensberg, professor of earth science at Southern Illinois University at Edwardsville, said the seas spread over the area at a time of mountain building, during which ocean floors were pushed or fluffed up, displacing seawater over vast amounts of North American land. As the seas advanced, they brought with them some of the earliest forms of multicellular life--a plethora of marine invertebrates including corals, sponges, and various "spiny-skinned" creatures called echinoderms. The phylum of animal life known as Echinodermata today includes such creatures as starfish and sea urchins. Some 350 million years ago the most prevalent echinoderms were crinoids.
The oldest crinoid fossil, Guensberg said, is from the Cambrian period about 530 million years ago. But as a class Crinoidea flourished in the Ordovician period, 450 to 380 million years ago. By the time the Silurian sea spread into the midwest from the Gulf of Mexico (380 to 350 million years ago), there may have been as many as 10,000 crinoid species, according to Guensberg.
Most crinoid fossils that wash ashore in the Dunes area or are found in local quarries are believed to be from the Silurian period, from the limestone bedrock known as the Niagaran formation (it stretches eastward to the rim of Niagara Falls).
The abundant meadows of crinoids that grew here then--unlike some later free-floating species--were rooted to the seafloor and swayed slightly with the currents, like underwater sunflowers facing the sun. Their arms, or pinnules, curled upstream from the calyx to trap their food--tiny waterborne particles, like microplankton--as it floated by in the current. The particles were caught on the tips of the arms by strings of tiny, sticky balloons called "tube feet." The tube feet would move the food bucket-brigade style into groovelike canals in the arms, which in turn led to an internal mouth located in the center of the calyx. The animals defecated through an anus located close to the mouth but facing downstream, away from the pinnules and feeding grooves. Because of competition for food from other crinoids, different species grew to different heights.
About 250 million years ago, according to Indiana University geologist Reshkin, the region's land rose above sea level. Exposed to wind and rains, this landscape was carved by erosion, which created a series of river valleys, including one major river system that flowed northward from the area into what is now called Hudson Bay.
Larger changes occurred a million years ago, during the most recent ice age. A series of massive ice sheets or glaciers moved down North America. The latest of these glacial sheets--called the Wisconsin glaciation--began about 70,000 years ago and stretched south to what are now the Ohio and Missouri rivers. Originating in the hills around Hudson Bay, this glacier gouged out and expanded the river valley, including the basin that eventually became Lake Michigan.
This glacier also brought to the region a huge amount of Canadian rock; as the ice sheet repeatedly retreated and advanced, it left the rock in a horseshoe-shaped complex of 200-foot-high ridges, called moraines. After the last retreat, about 14,000 years ago, the moraines started trapping the melting glacial waters between them and the receding tip of the glacier. Lake Michigan began to fill. It took another 12,000 years for the lake as we know it to take shape.
The crinoid remains--fossilized in shale and in the limestone bedrock--were by then part of the vast Niagaran formation beneath the lake and most of the northeastern United States. For the most part this rock was covered by glacial clay and other sand and soils brought down by the glaciers. But wherever erosion or human activity has exposed this marine limestone, crinoid fossils appear.
Russ Jacobson, associate geologist with the Illinois Geological Survey, says that people have been picking up crinoid fossils for at least 12,000 years. Crinoid necklaces and beadwork, he says, have been found in Indian burial sites in Illinois and throughout the midwest.
"You find them along Lake Michigan, but we can find crinoid stems everywhere the limestone bedrock has been exposed. As a kid in Iowa, I found crinoid stems. Most people pick them up when they see them; to them they're really quite interesting. To someone like me, though, finding the whole crinoid is the real challenge."
Many of the best-preserved complete crinoid fossils in the area come from two quarries that were coral reefs during the Silurian period--the Bridgeport or so-called Stearns quarry near 27th and Halsted, now owned by the city (which used it for a time as an ash dump), and the above-mentioned Thornton quarry directly south, owned by Material Service Corporation, one of the region's largest active quarries and sources of building materials. Local geologists working closely behind those mining these quarries in the 1860s discovered fossils of many previously unidentified crinoids, including one now known as Crinus Chicago.
As geologists were making their discoveries, and putting away thousands of specimens in drawers and cabinets in the two Chicago museums, industrialists and tycoons--like the Crown family--were mining the quarries for an assortment of building materials used for roads, railway embankments, agricultural limestone, sand, and cement for a variety of purposes. A brochure published by Material Service says that "as many as 200 trucks are loaded" at the 400-foot-deep Thornton quarry a day, carrying materials as far as 100 miles away.
"Though the quarry has been producing stone since the turn of the century," the brochure adds, there are "sufficient stone reserves for several more generations of quarrying at the present pace."
Maryann Kalin-Miller, a Chicago biologist who leads expeditions to the Indiana Dunes--not far from the beaches where she played as a child, herself a seeker of crinoids--says her tour participants "are positively ecstatic when they find crinoids on the beach and I tell them they are 400 million years old." She herself was delighted to find crinoid remains in the walls of her Lakeview graystone a few years ago. But despite their knowledge of the crinoids' life, death, and stony endurance, neither Kalin-Miller nor any of the crinoid experts we talked with could say why the fossils are washing ashore in the Dunes, or where, exactly, they are coming from.
Are they being scoured off of some ancient reef protruding through the sandy lake bottom someplace due east of Hyde Park, as one naturalist suggests? Are they being eroded away from some of the concrete and limestone riprap being dumped along the shores by humans trying to protect their property from rising lake waters?
Nobody knows for sure. And as beachcombers we're glad that a little mystery remains.
Art accompanying story in printed newspaper (not available in this archive): illustration/Albert Richardson; photos/Mike Tappin, courtesy Field Museum of Natural History.