The Creek, or the Ubiquity of Evidence of Geological History
Last updated March 22, 2020
The first essay in anthropologist and naturalist Loren Eisley’s 1957 book The Immense Journey is entitled “The Slit.” In it, Eisley describes his exploration of and rumination on a narrow crevasse in the Earth, which reveals evidence of geological history. I recall—when I first read this essay as a young graduate student—thinking that Eisley’s slit was a very special, perhaps unique place, where only someone with his special skills could pry loose both its fossil mammal bones and their significance. As my career as a paleontologist and geologist proceeded, however, I came to understand that such places are not in fact rare, and that virtually every spot on the Earth that reveals even a small portion of the planet’s outer crust is a potential site of the discovery of its immensely long and winding history. The same goes for the history of life. Every organism also contains signs of its long evolutionary history.
Yet perhaps it is the very ubiquity of these opportunities to examine the history of the Earth and its life that discourages most people from noticing and appreciating them. If the signs of something are everywhere, we are prone to tune them out, rather than savoring their significance and value. It therefore seems to me that the time is always right to remind ourselves and everyone around us of how these signs of history surround us every day, and that we can all see and use them to reveal—to ourselves and others—both the manifest fact of that history and our ability to unravel it.
Ithaca, New York is a wonderful place to live if you’re interested in geology. It’s one of the few places I know of in the country where geology is embraced by the local tourism slogan: “Ithaca is Gorges.” In his 1883 book Ithaca and its Resources, Morris Kurtz wrote admiringly:
The bed of the small tributary creek just before it enters the much larger Fall Creek.
“Within ten miles of Ithaca there are one hundred and fifty waterfalls – cascades and cataracts. Found in dark gorges and in beautiful glens, all of them are accessible, each one possessing peculiar features of interest in connection with its surroundings, many with special characteristics which, independent of the rest, attract visitors and captivate them by the beauties and grandeur presented, nowhere east of the Rocky Mountains has nature been more lavish with her gifts of wonder and awe-inspiring scenery.”
All of these gorges show abundant evidence of change, visible to even the casual observer. The sand, gravel, and cobbles in the stream obviously shift and move as the water level varies. The course of the stream itself changes as it finds new channels. Trees fall from the eroding banks. These changes can sometimes be dramatic over short intervals of time, such as during summer rainstorms or spring floods.
Yet, a little closer look at the rocks in these streams suggests that these changes over seasons or years are dwarfed by changes that have occurred over much longer time spans. Making just a few more observations and then asking a couple of critical question—why do these rocks look as they do?, and how did they come to be where they are?—crack open this vault of time and reveal just how much change has occurred in this one small spot of the Earth’s surface. These observations and questions can remind us, even on the most causal stroll or hike, that the signs of geological change—of geological history—are indeed everywhere if we know how to look and ask, and hypothesize and test.
On a recent, sunny, early-spring day, I was walking in the valley of Fall Creek near Ithaca. The small gorge of a tributary creek drew my attention. The tributary was just a trickle, and so I clambered through the ferns and downed hemlock trees into its bed. There among the cobbles, my eyes quickly fell on two rocks, less than three feet apart. One was a round, reddish one, perhaps a foot in diameter; the other was a flat, gray-brown slab, also about a foot across, pockmarked with round shapes. It suddenly occurred to me that the juxtaposition of these two rocks, in this little creek, was a microcosm of all of geology, just like Eisley’s slit.
My two rocks: the reddish granite boulder (left arrow) and the gray fossil-bearing boulder (right arrow).
The pink boulder is granite, of a type that does not occur in bedrock in New York State. So where did it come from? (Its location in a gorge, quite distant from any road or building strongly indicates that humans did not put it there.) Rocks like this are common around Ithaca, and across central New York, especially in stream beds. Some broader geographic exploration, however, will show that the nearest similar bedrock is in southern Quebec, several hundred miles to the north. Which begs the question of how this granite boulder got to this particular tiny stream here. The first hint is its shape, which suggests that it has been rounded and worn by movement over a long period, meaning it has likely been transported. Rocks like this are found around and underneath modern glaciers, and they also show signs of having been moved great distances.
Left: The pink boulder at Fall Creek. Right: Gravel and boulders melting out from the front of a glacier in Patagonia, Argentina.
For these reasons, geologists have long believed that these red rocks were dropped here when central New York was last covered by glacial ice, and lots of other evidence suggests that this was around 20,000 years ago. The red rocks are therefore commonly called glacial erratics. The granite itself, of course, was not formed by the glaciers; it must be much older. Dating of individual mineral crystals inside such rocks (by a method using the natural radioactivity inside the grains) tells geologists who specialize in such things that the rock formed more than 1 billion years ago, as molten magma deep inside the Earth cooled. (Those crystals formed during the cooling, just as crystals form when you make rock candy from sugar dissolved in hot water. The slower and longer the cooling, the larger the crystals.) Calculations based on the size and composition of the mineral crystals in the granite suggest that this formation may have taken place several miles beneath the surface.
How then did this cooled granite get to the surface so a glacier could get at it? It must have been uplifted, by processes similar to those that have made mountains and moved continents over time. Uplifting deeply buried rocks exposes them to the elements: wind, water, and ice. This results in erosion, which may remove thousands of feet of overlying rocks. Our granite was exposed by these same forces.
At some point, freezing and thawing of a granite outcrop in Quebec must have cracked and loosened a piece, and a glacier plucked it from its resting place. A combination of movement under the glacier broke the rock into pieces, and further movement—by ice and flowing water over those several hundred miles—rounded it. At some point it was deposited in a bed of gravel and cobbles in the valley of Fall Creek near what is now Ithaca, until erosion loosened it again and carried it to where I came upon it on that recent spring day.
The gray-brown slab is a very different-looking rock, and would seem to have a very different history. Close examination shows that it is made of very small grains of minerals such as quartz, feldspar, mica, and clay. Grains of this size (which geologists call silt) can only settle out in very still water, suggesting that it was in such a setting that the sediment comprising this rock (called a siltstone) was originally deposited.
Left: The gray-brown fossil-bearing boulder; inset shows details of several fossil crinoid stem segments. Right: A photograph of a thin section of a rock similar to the gray-brown fossil bearing siltstone, viewed through a microscope under polarized light to enhance its compositional variety. All of the grains are extremely small. (Photo courtesy of Ceren Karaca and Teresa Jordan)
Photo of a living stalked crinoid (NOAA; public domain).
What about those shapes pockmarking its surface? They vaguely resemble living things, but we have to search far afield from upstate New York to find something alive today that looks like them. We finally find it in very deep parts of the ocean, such as the Florida Straits or deep trenches in the western Pacific: stalked relatives of sea stars and sea urchins called crinoids (unfortunately the only common name for these animals is the extremely misleading “sea lilies”).
The tiny grains, and the presence in this rock of shapes closely resembling the stem pieces of modern crinoids, can seemingly only be explained by postulating that this gray rock has not always been as we see it today, but was once mud at the bottom of an ocean. Searching in the nearby bedrock reveals that there are numerous layers of similar rock, many packed with such crinoid remains. Clearly, the ocean was here for a long time. Additional research, in New York and around the world, tells us that rocks that contain fossils like this formed around 370 million years ago, during what geologists call the Devonian period (named for Devon, England, where similar fossils can also be found).
How did mud at the bottom of a Devonian ocean get into a gorge on a sunny spring day? The mud must have been buried by more sediment, and then—like our granite—uplifted, and ultimately exposed by erosion. A piece came loose, and was carried by additional erosion to where I found it, less than three feet from the red boulder.
So here, in the bottom of a quiet creek in upstate New York, two rocks came to sit next to each other. They do not speak, or broadcast any information about themselves. But by asking the fundamental naturalist’s question “why are these rocks here?” we elicit their testimony. In seeking to understand how it came to pass that these particular rocks are in this particular place, we are compelled to infer from observations about them that they had this long and complex past history.
Of course, a lot of this story was not reconstructed from just my simple observations in the creek. I was able to draw on decades of work by others from all over the world. But even the parts that involved longer times or larger geographic areas or more complicated analyses relied on the same technique that I applied on the spring day: asking why something is how and where I see it, comparing it to other things I know about, and inferring answers based on the results. Parts of all sciences are, admittedly, sometimes incredibly technical and difficult to understand. But almost always the basic ideas of science are very straightforward. For geology—and its kindred fields of paleontology and evolutionary biology—we infer that the Earth and its life have a long and complex history because that history has left vestiges, traces of past events recorded in the Earth and life. And we can see these vestigial traces almost everywhere around us.
“The truth is,” wrote Eisley in “The Slit,” “that we are all potential fossils still carrying within our bodies the crudities of former existences, the marks of a world in which living creatures flow with little more consistency than clouds from age to age.”