Where to find rudistids in New York

Sidney Horenstein attracts a lot of attention when he goes for a walk. Most people take a second look when they see this bulky, professorial man surrounded by 24 eager fellow travelers with their faces a couple of inches from the facade of the Sherry Netherlands Hotel, exclaiming with wonder over some minuscule holes in the stone.

Even in New York, where one comes to expect street theater from the average pedestrian, this kind of behavior hardly goes unnoticed.

What Sidney Horenstein and friends are looking at does pass unnoticed, though. Here in the lap of luxury, embedded in the French limestone -- wrapped around Sherry Netherlands guests spearing shrimps with their forks in subdued dining room elegance -- is a slice of earth history stolen from an ancient sea floor.

In the stone, easily visible to the naked eye, are cross sections of snails, small scallop shells, large clamshells: not faint afterimages of forgotten creatures, but the creatures themselves, trapped 40 million years ago and perfectly visible today. These remains pock the entire surface of the stone and provide enough fascinating reading to keep these sidewalk geologists riveted to the wall, oblivious of the snickering comments of passing pedestrians.

By the time they are through accompanying Mr. Horenstein on a walking tour he calls "The Fossils of Fifth Avenue," they will have seen: 100 million-year-old sea lilies from Spain scattered around Tiffany's window; remarkably dramatic goniatites (365 million-year-old relatives of the chambered nautilus) curled up in the polished marble of the ultramodern Tishman Building; huge snails embedded in Texas limestone adorning the stairway of a Bun 'n' Burger; limestone from Trieste freckled with rudistids (clamshells from an era when a whole group of clams went crazy and started to imitate coral); sea moths and snails from a tropical seabed now on the side of a Rockefeller Center building; a slice of coral reef from a sea that once submerged Missouri now gracing the cornices of Saks Fifth Avenue; and petrified sponge from an ancient British reef.

Mr. Horenstein, a geologist from the American Museum of Natural History, says these fossils are "pages of earth history" brought here from around the world by man and scattered among the city's own geological products. These scattered pages are, he points out, just the latest chapter in an entire volume of petrified biography folded into the geology of this city.

Mr. Horenstein is an excellent guide to help one dig into this historical record of ancient New York. "The Fossils of Fifth Avenue" is only one of many walking tours he conducts around the city for the curious, the studious, and the random lover of New York City topography. Other tours include a ramble through the northernmost tip of Manhattan (probably the only unspoiled wilderness on the island); a hike along the imposing Palisades, looming with primordial splendor over the Hudson River; and fossil-hunting expeditions through less explored regions of the city.

Sitting in his laboratory-style office on the specimen-crammed fifth floor of the museum, he roams easily through the billion years of New York City history that make these tours interesting, artfully setting the stage for a few days of walks he and I are to take back into time. He lounges back in a swivel chair surrounded by boxes of stone artifacts and copies of the newsletter he sends out to a small but earnest readership, occasionally getting up to pore over a geological map of the city or pull out a dusty 1843 volume entitled "Survey of the Island of New York."

He makes a strong point of showing that all the evelutionary processes that shaped this region are still going on.

"If you can imagine a motion picture starting 4.6 billion years ago," he explains in his most patient, Mister Wizard manner, "we are just one frame somewhere in the middle. The only thing we can count on is that all of the changes in the past are only a prologue to future change. The earth is just too dynamic for anything to remain the same."

If the next billion years are anything like the last billion, New York has a pretty exciting future ahead of it. Since the primordial bedrock of the city was formed from sands at the bottom of an ancient sea, the region has gone through several geologic "events" that have dramatically changed the place from an ocean floor to a string of volcanic islands, to a mountain range, to the southernmost ridge of the polar cap, to forested wilderness, to the cityscape we see today.

The entire earth mass has risen and fallen in unexplained upheavals. The Hudson River has changed its course several times. A huge lake system has broken through The Narrows here and formed many of the rivers and streams currently infiltrating the city. Tundra, boreal forest, pine forest, and deciduous forest have replaced one another in the wake of the Ice Age a mere 6, 000 to 14,000 years ago.

And then man has put incredible finishing touches on nature's handiwork, leveling hills, redirecting rivers, reclaiming seabed, rejoining islands that had been sliced by sea waters, burying the numerous streams and rivers that used to feed the forest here under layers of gravel, concrete, and city life, and transforming the wildlife that inhabits the city.

Much of this evolutionary history is visible today around the city, neatly concealed in full view, like Edgar Allen Poe's "Purloined Letter," completely overlooked by the millions who walk over, under, around, and through it every day.

Few people sitting in Central Park, for example, realize they are on a small hunk of what used to be the New Jersey Palisades, deposited here by the glaciers , and that they are gazing at an undulating rise of seriated bedrock which is just one frozen cap of a wave from the sea of rock beneath the park. (This sloping outcrop of rock drops sharply off; and, if the soil and city were washed away, it would be exposed as the tip of an ancient mountain range that stood here 200 million years ago.)

Here in the park, the patient earth-erosions, gashes left by the glaciers, and outcroppings of bedrock, whose glaciated surfaces bear the original traces of earth movements in this region, are all still in place. Gullies, rises, twists, and tortures of the land are pretty much as they were several hundred years ago. Although the place has been manicured (all the existing soil and vegetation were stripped away and new earth was brought from New Jersey to play host to an entirely redesigned ecostructure), it remains faithful to old New York as it looked to early settlers.

When the layout of New York City was conceived in 1807, a gridlike street pattern was laid out over the existing forest and farmland, and the hills were leveled and the valleys raised. As the city spread northward, the only part of Manhattan that survived this topographical denuding was Central Park.

"Look over this way, and you see old New York," Mr. Horenstein advises, pointing to a section of the park just beyond the stone wall where we are sitting. Across the street from us is the imposing museum building and the city streets; under us is the rumble of a subway; above us is a huge elm tree, which has grown in the layer of soil over the train station.On the other side of the wall where we are sitting is a sheer drop of a couple dozen feet and the abrupt boundary of the park.

"There used to be a lake down there," he explains, "-- Butterfly Lake." Now there is a horse path, undulating terrain covered with trees and rocks.

"When the designing of Central Park began," he continues, as we make our way along one of the numerous paths that curl through the park's variegated terrain, "the region of the park supported 300 species of plants and trees. The landscapers eventually planted over 2,000 new and old varieties. Today, over a hundred years later, only 300 species have survived. This is a different 300, but it's interesting that the same number remain.

'That tree is a living fossil," he says, pointing to a ginkgo tree, a species that started to grow in the days of the dinosaurs and was found on every continent 100 million years ago. It began to die out and became almost extinct, except for a few specimens kept alive in Buddhist temples, where the species was eventually discovered by a Dutch doctor in the 1600s, after which it was redistributed around the globe.

This one stands in a valley that used to be a branch of the Saw Mill River. Today, the only water flowing here is from a broken water main that for several years has fed a stream trickling down into Central Park's largest lake.

"In the 1880s, some man about town, who thought our lot would be improved if every bird mentioned in Shakespeare were in Central Park, had 40 pairs of birds brought here from Europe. Only one pair took. It was the starlings, a bird that has caused widespread hostility throughout the entire country. He was just a misguided do-gooder."

Today, he complains, misguided and unthinking people are reshaping the park and other natural places in the city in other ways. Foot traffic over parklands is causing erosion and -- worse -- soil compaction, which prevents moisture from seeping into the ground, thus weakening the trees. Standing under the canopy of a cut-leaf beech, in shade which feels like the inside of a Chautauqua tent, he points to the evidence of ubiquitous tree ailments caused by this weakening process.

"All of this is natural, though," Mr. Horenstein says reflectively, "because it is brought about by man. And man is natural."

Man's "natural" activities are steadily altering the parklands of the city, intermingling imperceptibly with the work of other natural forces. All the squalor and untidiness of the city seep steadily into the park, invading the ecosystems and damaging the physical beauty of the place. More deliberate man-made changes are less obvious. But Mr. Horenstein has a good eye for distinguishing between man-made and earth-caused phenomena.

He is quick to distinguish the boulders brought here by park planners from the glacial "erratics."

"Erratic means wandering," he comments, "so these are wandering stones, carried by the ice floes from one place to another."

He himself is somewhat of a wanderer. His quarterly publication, New York City Notes on Natural History, ambles interestingly through such subjects as "Cretaceous Age Deposits," "Underground Water in Brooklyn and Queens," "Urban Entymology -- The Cockroaches," "Grand Central Terminal," "Landfill Map of New York City," "New Jersey Phytosaurs," and other arcane scientific and natural lore.

He is an unflagging walker who takes 10-mile hikes with his two daughters, as well as his regular tours of the city, during which he is likely to fill the time with minute observations on everything from the errors of construction in 1950s buildings, to the legendary significance of a tree, to the habits of the miner worm (an insect so tiny it can burrow in the middle layer of a nearly transparent three-layer leaf without disrupting the surface).

He almost always stops to take note of a tree that has taken root in a small pocket of soil someplace, or one that has wedged its roots down into the bedrock and spread the rock apart with steady force. He gets excited when he talks about the new layer of vegetation growing on a garbage dump on Rikers Island. And he constantly remarks on the returning evolution of the natural things in the city.

"I'm always thinking about what this city would be like if everyone abandoned it and gave it back to nature," Mr. Horenstein muses.

He is something like a kid who started with a nature collection and wound up collecting the entire city.

On another day trip, striding tirelessly up a steep path, he proudly shows off the Inwood section of Manhattan, the slender northern tip of the island that still has some interesting exposed geological formations available for study.

"Rocks originally deposited horizontally at the bottom of a sea were upended by enormous forces that contorted and crystallized them," he observes, pointing to a huge slablike outcropping of rock and adding that the three-dimensional formation of the rock is sometimes difficult to grasp.

"What looks to us like a two-dimensional surface is really a cross section of layered rock." If you catch this layered rock in the right position, you can see the centuries of evolution that formed it.

Up in this part of the city there are huge deposits of Inwood marble mingled with Manhattan schist. The marble is much softer and always occurs in the valleys (because the softer stone gives way to erosion and creates depressions in the earth), while the much more durable schist inhabits the ridges and high places of the earth. Throughout this entire section there is endless evidence of these rock formations, especially in Inwood Hill Place, a densely wooded park of deciduous forest that is much like it was three centuries ago.

Up here is the only spring left in New York with a substantial flow of water. There are ancient Indian caves formed by landslides on the side of a heavily wooded ridge. Across from the caves are Indian garbage mounds, known as "middens," where one can easily find oyster shells caught, disembodied, and discarded over 200 years ago.

Tulip trees 100 years old, the tallest trees east of the Rockies, tower overhead, their straight trunks shooting up to hovering crowns that permit only a dappling of light to drop to the forest floor.

"This is Manhattan!" he exclaims quietly a couple of times, as if he's still pinching himself to see if it's all a dream.

From the top of this ridge, one can see what New Yorkers erroneously call the HArlem River, really a shipping channel cut beside the course of the old riverbed in the 1920s. (The old Harlem River course is now dry land covered with apartment buildings and belonging to the Bronx, having been severed by man from Manhattan.) Along this watercourse is a railroad track with crushed limestone from the Catskill Mountain quarries, bearing numerous fossils. Above this railbed on a shaded roadside is a humble Buddhist cemetery, hardly recognizable except for the modest mounds of stone and the tiny ceremonial monuments.

Also from the top of this ridge, one can look across the water to a hidden jewel of evolution in New York City, under the Henry Hudson Bridge, behind the Spuyten Duyvil train station. A barely discernible patch of water behind a solid wall of ditch weed, this little-known patch of antiquity sits beside a heavily used train station surrounded by parked cars. But the dense grass shields it from prying eyes. Mr. Horenstein picks his way gingerly through the grass to show off the beauty of the place.

Here in the shadow of a huge arching bridge is a spring-fed pond, 20 feet across filled with primitive life, a sort of ecological soup stock, breeding insects, bullfrogs, gold fish, and other less visible creatures. Irides cent blue darning needles and dragonflies hover on the surface of the pond, impervious to the roaring traffic overhead; a flame-orange patch of goldfish feeds quietly just beneath the surface; and primordial-looking tadpoles scatter the loam on the pond floor in search of the abundant insect life that thrive there.

What if everybody did abandon the city, as Mr. Horenstein muses? Looking around this busy pond, one realizes it would be a new primeval forest in no time at all.