Dawn of the Devonian
Feb. 3, 2005
Updated by Robert and Johanna Titus
You have, no doubt, driven down Rte. 23 on your way towards the Rip Van Winkle Bridge, and passed one or more vans parked along the large limestone outcrops that are commonly seen all along the highway there. Along the road, large numbers of students can be seen pounding with hammers on the rock ledges. These are geology field trips. Students and faculty come from colleges all over the northeast to visit these rocks.
The location is one of the great geological tourist traps of our part of the country. Between now and early autumn scores of these visiting vans will appear here. The outcrops stretch for a mile or so along Rte. 23, but there is one location which is even better than all the rest. That site is the exit ramp where drivers leave Rte. 23 and get onto Rte. 145 west. They may be headed for Leeds, but they are passing some very good geology.
The location displays what geologists call an angular unconformity. If you stop there you will see one set of stratified rocks dipping steeply to the right. These strata are alternating horizons of sandstone and black shale. They make up a very small part of what is called the Normanskill Formation. That rock unit dates back to a time called the Ordovician and that makes them about 450 million years old. These strata were deposited in what was probably some very deep, marine waters, but that is not our story today.
Above the right-dipping strata are some left-dipping beds. Left and right, these contrasting inclinations form the “angular” part of an angular unconformity. The overlying strata are called the beds of the Rondout Formation. That unit is Silurian in age and that makes it merely about 420 million years old. That leaves about 30 million years missing and that is the “unconformity” part of our angular unconformity.
The sedimentary rocks of the Rondout are called dolomites. Dolomite is a calcium/magnesium carbonate. It used to be a different rock, something commonly known as limestone. That’s important as limestone only forms in a tropical climate. These sediments accumulated on an ancient mudflat, at the edge of an ancient tropical sea. The sediments made up the upper reaches of the high tide level, we call that a “super-tidal” setting. The ocean waters only reached this high up at the highest of tides. You would have to travel to the Persian Gulf to see something like this today.
The Rondout is important to geologists as it forms the first and oldest unit of the Helderberg Sea. This is a thick sequence of limestone that makes up the Helderberg Escarpment which looms as a great ridge above Albany. The Helderberg strata make up a lot of the bedrock in this area and so geologists pay a great deal of attention to it.
At the Rte. 23 outcrop, the Rondout is capped by some badly distorted strata. They were deposited as flat sheets but subsequently they came to be deformed as great crustal stresses folded them up. You can easily see this at the outcrop. The top of the folded sequence marks what many geologists call the very end of the Silurian time period. Above this horizon is the very first stratum of the next time unit, the Devonian, dating back to about 419 million years ago. This is a very important horizon as the Devonian is the most important unit of time in the entire Catskill region. Every bit of bedrock from here to Syracuse and beyond is Devonian. That’s a lot of rock and it all started right here. This horizon of rock takes us back to the dawn of the Devonian and the very dawn of the Catskills themselves. That makes it important. But, before the Devonian, there was the Silurian.
Sunrise, July 14, 419,632,751 BC – The air is absolutely quiet. Not the slightest of breeze can be felt. The landscape is similarly still. All around us to the east is a monotonous flatness. It’s not that the land is smooth; it’s a very rough mudflat. Every few feet there is a low tilted ledge of rock just poking above the surface. These ledges vary in thickness; some are just a few inches thick, while others can be a few feet thick. Each ledge is composed of sandstone; each has a bleached creamy yellow appearance. Each is broken up by fractures that pass perpendicular to the rest of the rock. The ledges all seem to be broken up into rough cubes.
In between each set of ledges is a small “valley;” all of these are filled with a litter of dark gray pebbles, mixed with a darker, almost black, earth. Here and there, some very small puddles can be seen in the valleys. The edge of each of these is rimmed with a white substance that appears to be salt. These are, indeed, saltwater pools, and they have been evaporating in recent times.
This is the most dead-looking of landscapes. There are no plants here, not a blade of grass nor even a lowly weed. There are no insects on the ground, nor are there birds in the sky. A careful check of the “soils” would not even reveal any worms.
And it is, of course, absolutely quiet. There is no wind and there are no animals to break the silence. It is a silence that is almost impossible for us to imagine for we are visitors from a noisy world.
But this actually is our world, and it’s a very familiar part of our world. Someday this very location will be the intersection of Routes 23 and 145; this will be the exact place were an exit lane connects the two highways. People will call this the town of Leeds. But, at the time of our visit there is no sound of traffic. This is not the noisy world of today, but the almost silent world of the late Silurian time.
But we are time travelers and we have our left foot in the Silurian and our right one is in the modern world. We look to our left and see a Silurian sunrise, to our right the highway traffic of Rt. 23 zips by.
But our interest and presence in the modern world fades; it is the Silurian vision that captivates our mind’s eyes. There and then, on the distant eastern horizon, the sun is about to rise. A gray light first appeared, and then slowly turned red, and now it is brightening with ambitions of an intense yellow. The exact moment of sunrise is always magic. The brilliance of the sun just creases the horizon and, quickly, the rest of the great orb ascends into its rightful position in the sky. The movement is smooth and surprisingly quick. It is the only time of the day when you can really sense the movement of our great nearby star.
For almost four and one half billion years the sunrises have been appearing on this horizon, but the sun does not show its age. This Silurian time period has been a fine era for sunrises. What with all the flatness of the landscape extending off to the east, nothing ever gets in the way of a view of the rising sun. And now the sun is gaining altitude and its light shines across our flat lands to far behind us.
We turn and gaze westwards and there, before us, is a very broad and very smooth sandy mudflat. On the distant western horizon, the disk of a gorgeous full moon is just setting. Full moons are like vampires; they do not like to be out during the day. They always set just as the sun comes up.
And way out there, where the moon is setting, is the flat horizon of a distant ocean. It is nothing like the great Atlantic of today. There is no chop to the water, not even waves lap up against this shore. But it is a sizable body of water and it will, someday, be called the Helderberg Sea. It is, as far as we can observe, a very shallow water ocean. Today, with no wind, there are no waves or substantial currents, and its clear, aqua-colored waters lie still on the horizon.
Our nearby mudflat is composed of white, fine-grained limy sand and its smooth surface almost glistens. It was recently soaked under a very high tide, but those waters are receding rapidly. Mudflats should be dotted with snails, slowly meandering about, but this one is not. It seems to be just as dead as the flat landscape to our east.
With the rising of the sun, a little breeze is at last generated. It is a very warm wind; these are the tropics and the temperature will be very hot later in the day. It was like this yesterday and it will be like this tomorrow. Change comes very slowly in the Silurian.
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