A small waterfall at “The Great Unconformity” in Southington, Connecticut.
Anything with a name like “The Great Unconformity” is bound to have people scratching their heads. The term is so fantastically vague that it seems to suggest anything from a little-known literary movement to some sort of historic political upheaval. However, any amateur geologists out there probably know that “The Great Unconformity” is not a radical style of renaissance painting nor an emerging trend in fashion. “The Great Unconformity” refers to a peculiar gap in the geologic record, whereby a layer of relatively young bedrock sits directly upon a layer of much older bedrock with seemingly no transition in between. During a recent outing to a little-known hiking location in Southington, Connecticut, Trails of Freedom took a look at this unusual phenomenon first-hand.
Ordinarily, whenever we find a bare cliff or other exposed rock face, we would expect to see layers of stone deposited in a tight chronological order. To illustrate the way in which we typically expect layers of rock to form, consider an analogy involving magazines.
Say, for example, that you receive the January 2011 issue of your favorite magazine and leave it on your coffee table. Later, you receive the February 2011 issue and put it on top of the old issue. The March 2011 issue gets stacked on top of the two older issues, and so on and so on. Eventually, you receive the December 2011 issue and stack it on top of all the older issues you’ve received that year. What you end up with is a stack of magazines that is “layered” in chronological order, the newest issue on top and oldest issue on the bottom. In between are magazines that each account for a certain interval of time between the latest issue and the oldest issue.
Layered bedrock tends to form in the same way. Every few million years, we see a new layer of rock form on top of the older layers and the result is that, wherever a cross-section of these layers becomes exposed, we can see a rather complete geologic history where each successive layer of rock represents a certain period of time.
In areas where a “great unconformity” is visible, it seems to completely shatter this classic model. The “unconformity” that gives this phenomenon its name refers to the fact that the layers of rock don’t conform to the typical way in which in rock is layered in regular intervals over time. These unconformities are “great” because there is an enormous gap in the geologic record that is unaccounted for by any layers of rock.
Consider our magazine analogy once again. Imagine that you go back to your pile of 2011 magazines and discover that every issue between March 2011 and October 2011 is missing from the middle of the stack. Where did they go? That’s basically the situation that geologists found themselves in when they began discovering rock formations where there was a “great unconformity”. Sometimes they would find a layer of rock resting directly on top of another layer that was anywhere from 150 million to a billion years older.
The Grand Canyon is perhaps one of the famous places in the United States where unconformities in the Earth’s bedrock can be observed.
(Photograph by “Olivier from France”, licensed via CCA2.0G)
Of course, in the case of a stack of magazines, we can perhaps deduce that a friend wanted to read some back-issues and borrowed the missing the months. But for geologists, it was out of the question that somebody simply walked away with millions and millions of years of rock! How could the geologic record simply skip such an enormous length of time?
While “great unconformities” aren’t typical features in the geologic record, we know today that they aren’t especially rare, either. In fact, they exist all over the world in a variety of different rocks of different ages. What makes them seem rare is that the bedrock in which they are contained is usually buried deep beneath the surface of the Earth, leaving the “unconformity” hidden from human eyes. In order to actually see a “great unconformity”, you need to stumble upon some place where the geologic gap is both close to the surface of the Earth and where some type of natural force has cut a trench, chasm, or gorge in the Earth that is deep enough to expose a cross-section of the rock layers. This combination of criteria is not commonly found, so although “great unconformities” may not be all that rare in the big picture, it’s exceptionally rare that you’d find a place where you could directly the observe the phenomenon.
One such place exists along a small stretch of Roaring Brook in Southington, Connecticut where hikers and geologists can observe a gap in the geologic record that spans roughly 170 millions years! But before we take a close look at the nature of this unconformity, let’s learn a few things about Roaring Brook.
Roaring Brook is perhaps a rather deceiving name for this small stream which meanders quietly through the hills of Southington. At first glance, it seems mind-boggling that such a reserved little brook could carve a gorge through the forest more than 30 feet deep in some areas *. Yet the persistent waters of Roaring Brook have probably been flowing along the same watercourse for thousands of years now, and with each passing century the stream sinks deeper and deeper into the bedrock. Thanks to this methodical erosion, we are afforded a rare glimpse into one of Connecticut’s most unique geological features: “The Great Unconformity” in Southington, Connecticut.
Before we continue, I want to answer a question that may be brewing in the minds of some readers. If I’ve just told you that “great unconformities” exist in many places, why is the unconformity at Southington called “The Great Conformity”. The name makes it sound as if it is the only example of this phenomenon in the entire world! Pretty confusing, huh? Consider that it’s not uncommon for people to refer to their local bank as “the bank” or their local supermarket as “the grocery store”, even though there are probably dozens of banks and supermarkets in the county. A similar manner of speaking was involved in naming the exposed unconformity in Southington. While there may be other places that unconformities can be observed elsewhere in the United States, the Roaring Brook gorge is the only known place in the entire surrounding region. Thus, the place has come to be known as “The Great Unconformity”, since it’s the only place of its kind in the area.
Along the stretch of Roaring Brook where the gorge is at its deepest (roughly 30 feet), hikers will find that the gorge walls are comprised of two very different types of rock. The lower portion of the gorge wall is made up of what geologists call “Devonian Straits Schist”. Believed to have developed roughly 390 million years ago, this schist is a very grainy, flaky stone with a subtle sprinkling of “glitter” deriving from mica that is sandwiched within. The layer of rock directly above this schist is strikingly different in composition, being known as “conglomerate”. This conglomerate stone is part of a rock layer called the “Triassic New Haven Arkose” and it’s a rather odd-looking type of rock that was deposited about 220 million years ago. The peculiar structure of conglomerate arises when a large volume of ordinary stones (like those you’d find along a river bed or seashore) become buried. Slowly, fine grains of clay, silt, or sand seep into the gaps between the stones. As time goes on, the mixture is eventually subjected to high pressures beneath the Earth’s surface which causes the fine-grained materials to solidify, essentially trapping the larger stones within. The result is a lumpy layer of rock that appears, at first glance, to be little more than a thick band of soil containing ordinary stones. However, closer inspection reveals that this conglomerate is certainly a hardened rock. Although tree roots possess enough prying force to fragment sidewalks and driveways, the trees along the rim of Roaring Brook Gorge cannot penetrate this conglomerate layer and are left to fasten themselves to the relatively thin layer of topsoil that has accumulated on top of the conglomerate.
“The Great Unconformity” can be seen in the walls of the gorge at Roaring Brook. The lower layer of rock is comprised of grainy schist, while the upper layer is lumpy conglomerate.
Despite the fact that the conglomerate of the Triassic New Haven Arkose formed 170 million years after the schist of the Devonian Straits, there isn’t a single layer of transitional rock in between. The mica-laden schist juts from the floor of the gorge directly into the lumpy conglomerate above. There isn’t any other location along Roaring Brook where this unconformity is exposed, and I have yet to find any record of exposed unconformities elsewhere in Connecticut. According to modern geologists, the vast majority of this “Great Unconformity” was dropped a significant ways beneath the surface of the Earth as a result of geologic forces along the Mixville fault. The gorge of Roaring Brook is one of the rare places where circumstances have come together to expose this unconformity at the surface.
Now that we’ve learned what defines a “Great Unconformity”, as well as where we can find a rare, exposed example of this phenomenon in Connecticut, the next natural question becomes how these peculiar geologic landforms came to exist. How can such an enormous amount of time pass -a whopping 170 million years in the case of Southington’s Great Unconformity- without leaving a single trace of rock behind?
The answer to this question is rather complex, so in an effort to simplify the explanation, I have chosen to replace certain confusing terms with more familiar ones that still convey a generally accurate account of how the “Great Unconformity” at Southington’s Roaring Brook may have been created. Readers that possess a more advanced understanding of geology can find links to more advanced reading material online that uses proper geologic terms and provides further detail into the process.
Our journey in understanding the formation of Southington’s Great Unconformity requires that we travel some 400 million years into the past to a truly ancient time in the development of the Connecticut landscape. During this era, the continents weren’t arranged as we know them today. In fact, the European continent (still in a developmental stage known as “Baltica”) had been slowly drifting across the surface of the planet for eons on a crash-course for the Eastern border of North America.
Beginning about 380 million years ago, during a length of time known to geologists as the “Middle Devonian Period”, Europe closed in on North America and the two enormous land masses began colliding. As the pressure between the two continents fluctuated over the next 50 million years, the Earth’s crust buckled, crumpled, and was progressively thrust upwards to create a great chain of rocky peaks along the Eastern border of North America known as the Acadian Mountains. These grand mountains would have truly dwarfed the mountains of Connecticut with which we are familiar today, being more similar in character to the Himalayas than the Hanging Hills of Meriden or Bear Mountain of Salisbury. These mountains would have been been comprised, at least in part, by the minerals that can now be seen in the Devonian Straits Schist layer at Southington’s Great Unconformity.
So high and so steep were the Acadians that little or no sediment could settle upon them. Instead, millions of years of wind, rain, volcanism, and earthquakes chiseled away at their towering forms, pulverizing the stone into smaller rocks, sand, and dust which washed away to the lowlands. With each passing millenium, the Acadian Mountains grew smaller and smaller. If we could watch this great mountain range in timelapse, beginning roughly 390 millions years ago, we would see the vast, jagged mountains “melting away” under the forces of erosion. They would first assume a profile with softer, gradual slopes. In time, their height would be progressively reduced, until what were once towering peaks would become more like large hills. By the time we reach the latter stages of erosion, the Acadian Mountains had literally been ground down to their bases. All that remained of this great chain of mountains were relatively flat plateaus upon which they once sat. Among these flatlands was the schist that we see today along Roaring Brook. Because this layer of schist had been hidden beneath eroding mountains for 170 millions years, there was never an opportunity for any layers of stone to be deposited on top. Not until roughly 220 million years ago had the mountains flattened to such a degree that a new layer of stone, the New Haven Arkose, would have an opportunity to deposit a thick layer of conglomerate upon the worn stump of the old Acadian peaks.
In this way, a full 170 million years worth of rock is “missing” from the geologic record. As you can see, the “mystery” of the Great Unconformity is not really a mystery, at all. Instead, it’s a portal into the past that allows geologists to better understand the vast and complex history of the New England landscape. Of course, new discoveries always reveal new frontiers; with each question answered, many more new questions emerge. Such is the case with The Great Unconformity at Southington. Indeed, the story of the The Great Unconformity that I’ve related here is a very simplified account of what was certainly a much more complicated process that took place in innumerable stages over a span of time that is nearly incomprehensible to human beings. But while the esoteric details surrounding the birth of this unique formation in Southington, Connecticut may remain outside the realm of knowledge of the hiker or casual naturalist, the Great Unconformity is nonetheless an intriguing place to observe relics of a landscape in perpetual flux. Along the riverbed at Roaring Brook, we may experience the sensation that we are directly connected to an ancient tradition of colossal change that has shaped the place in which we live… a place where vast continents converge and collide, where mountains rise and fall, and where rivers carve ever-deepening footprints through a living landscape.
* Although Roaring Brook is a relatively small stream, it may well have derived its name from the fierce character it assumes during flood conditions. When Trails of Freedom visited shortly after the heavy rains of Hurricane Irene (2011), we observed very recent evidence of a striking increase in water level. The sheer quantity of debris that this little brook had tossed around was quite impressive. The brook had undoubtedly become inundated with rain run-off and it would not be an exaggeration to say that the volume of water coursing through the otherwise tame stream bed may have increased ten-fold or more. Truly, “Roaring” Brook would’ve lived up to its namesake under those circumstances. If one considers that conditions such as these probably occur at least once a year (either during Spring floods or Hurricane Season), it’s not so difficult to believe that Roaring Brook could carve such a deep gorge for itself through solid bedrock over the course of countless centuries.
Mountain Building, Part III: The Acadian Mountains
Paleontological Research Institution
Ask a Connecticut Geologist
Wesleyan University: Earth Science Information for Connecticut Teachers and Students
(“The Great Unconformity” is discussed roughly 1/3 of the way down the page)
Southington Land Trust