A Devonian Primer for Teachers and Students

New York’s Finger Lakes region has some of the best Devonian aged (416 to 359 million years old) rocks in the United States, and their exposure as a result of gorges and erosive waterfalls bring tourists of all ages and from all over the world. In addition to being beautiful, however, this area is also geologically intriguing to scientists, and has been the focus of many research endeavors. This is because ‘many of these rocks contain abundant fossils’ and ‘these rocks are among the most important in the world for studying events that occurred during Devonian time. Indeed, because of the exceptional exposure of the rocks in this region of central New York, much of the basic work on defining geology in North America in the nineteenth century was done right here.’ The origin of these fossiliferous Devonian rocks is the focus of this primer.

Deep Time: In order to understand anything about the Devonian age of New York, we must put it into context with the rest of the Earth’s long history, using the concept of Deep Time. Deep time is the idea that the Earth has been in existence for 4.6 billion years, and throughout this time it has undergone many transformations. The way it appears today, with continents, mountains, oceans, and glaciers, is a reflection of how it has changed in the past. Unlike human history, where we think of events as having happened hundreds or thousands of years ago, in geology we must think in millions and hundreds of millions of years ago.

Geologists have made a timeline of events, which we call the Geologic Time Scale, that help us understand the order of important geological events relative to one another in the billions of years of Earth’s history.

The Devonian is the time (416 – 359 million years ago) when fish with jaws began to diversify and control the waters. However, what we see in the rocks in central New York are mostly benthic (or bottom-dwelling) organisms like clams, brachiopods, crinoids, etc. This is because it is much easier to preserve shelled animals than it is to preserve fleshy animals (see Taphonomy).

How do you know how old it is?
Answering this frequently encountered question in geology requires two separate steps. They correspond to two different ways that we express how old something (or someone) is in our everyday experience. When we ask how old an object or a person is, we can answer either with a number or by comparison to something (or someone) else. Thus, you might say “I am older than my brother” or “my friend’s car is older than mine.” This is called relative dating, because the age of something is stated relative to the age of something else. We can also give an age in numerical units, such as days, months, years, etc. So your response might be “I am 21 years old’ or “my friend’s car is one year old.” This is usually called numerical or absolute dating.

Relative Dating: In geology, depends on 2 assumptions and 1 observation
Assumption 1. Superposition. Rocks that formed from sediment (mud, sand, gravel) are called sedimentary rocks. This is the kind of bedrock you will find around central New York. Such ricks are usually seen to be arranged in stacks of layers (strata); these stacks are commonly called stratigraphic sequences. When we look at a stack of sedimentary layers, we can ask which layers are older; that is, which formed first? By reference to our common experience with such things as stacks of magazines on the living room floor, we can suppose that, in the absence of evidence to the contrary, the oldest layer in a stack of rocks is at the bottom, and that the youngest is at the top. This principle of geological reasoning is called superposition.

Relative Dating: In geology, depends on 2 assumptions and 1 observation
Assumption 1.

Superposition. Rocks that formed from sediment (mud, sand, gravel) are called sedimentary rocks. This is the kind of bedrock you will find around central New York. Such ricks are usually seen to be arranged in stacks of layers (strata); these stacks are commonly called stratigraphic sequences. When we look at a stack of sedimentary layers, we can ask which layers are older; that is, which formed first? By reference to our common experience with such things as stacks of magazines on the living room floor, we can suppose that, in the absence of evidence to the contrary, the oldest layer in a stack of rocks is at the bottom, and that the youngest is at the top. This principle of geological reasoning is called superposition.

This is a simple stratigraphic column depicting a typical shallowing upward sequence that is seen in the Middle Devonian. In the above image, the shale was deposited first, followed by the siltstone, and then finally the limestone. The locality in which these rocks were deposited was a shallow sea that experienced changing water depths. This column shows a time when the sea got progressively shallower.  This is similar to the localities from which Fossil Finders samples were collected.

Observation 1. Succession of fossils. When we examine sedimentary rocks, we often find that they contain fossils. Many places in central New York, where the Fossil Finders Project samples are collected, are rich in fossil shells. Fossils are the remains or traces of organisms from the geological past that are preserved in rocks. We think that fossils were produced by living organisms because they resemble organisms alive today. When we look at fossils in stacks of sedimentary rocks from many places, we notice that different kinds of fossils occur in different layers and that the order of the various kinds of fossils from bottom to top is always the same. This is called biological succession.

Assumption 2. Correlation. When we look at fossils in stacks of rocks in different places, we make the reasonable assumption that, in the absence of evidence to the contrary, layers containing the same fossils in separate locations are similar in age. This is called correlation. The consistency of biological succession in different places gives us the confidence that this assumption is a reasonable one.

These assumptions and this observation allow us to construct series of fossils that occur in different layers of rocks. As we travel to more and more places, correlating stratigraphic sequences of biological succession as we go, we construct a grand series of fossils and their associated rocks, oldest at the bottom and youngest at the top. They are then named for places at which rocks of that particular age were first well studied, and represent the interval of time during which a particular set of organisms existed. This series of names is the Geologic Time Scale, which we mentioned earlier. Numerical Dating: How we assign time in years across the Geologic Time Scale.

The numerical ages of rocks in the Geologic Time Scale are determined by radiometric dating, which makes use of a process called radioactive decay – the same process that goes on inside a nuclear reactor to produce heat to make electricity. Radiometric dating works because radioactive elements decay at a known rate. They act like ticking clocks, and let geologists measure how much time has passed since those elements were sealed into a particular mineral in a rock. Radiometric dating provides the numbers of years that are found on the Geologic Time Scale. These numbers are revised occasionally, as better radiometric methods are developed or new datable rocks are found.

Fossils themselves usually cannot be dated radiometrically. We must, therefore, combine information from fossils and radiometric dates from rock layers above and below the fossils to answer the question “How do you know how old it is?” Using these techniques over many years, geologists have determined that almost all of the rocks at or near the surface in central New York formed during the Devonian Period, 416-359 million years ago.

How the Bedrock Formed?
Now that the age of the rocks in central New York has been established, how they came to be there may now be discussed. Scientists have reconstructed the paleogeography of New York by studying Devonian-aged rock samples throughout the world in order to piece together the puzzle of what the Earth looked like during that time. To do this, they use the theory of Uniformitarianism, which states that the present is the key to the past. This means that geologists use laws that exist in the natural world today to explain the natural world of the past. For instance, geologists know that in rivers and oceans today, different areas of the water body have different levels of energy. There are places of high energy (i.e. rapids in rivers and coastlines in oceans) that are able to transport large sized sediment, and places of low energy (i.e. deep ocean without current activity, river eddies) that can only transport very small sized sediment. This process of sorting sediments that happens in water bodies today is assumed to have happened in ancient water bodies, as well, because of the theory of Uniformitarianism. Therefore, geologists can reconstruct the relative location and depths of water bodies during a particular geologic interval, like the Devonian. In environments where the shallow water is filled with marine organisms, like today’s coral reefs or central New York’s Devonian sea, the shallowest water allows deposition of fine to medium sized grains composed of bits of organic material. This is called limestone. As the sea gets deeper (lower energy), smaller particles can be deposited in the forms of silt (siltstone) and mud (shale), respectively.

How the Bedrock Formed?
Now that the age of the rocks in central New York has been established, how they came to be there may now be discussed. Scientists have reconstructed the paleogeography of New York by studying Devonian-aged rock samples throughout the world in order to piece together the puzzle of what the Earth looked like during that time. To do this, they use the theory of Uniformitarianism, which states that the present is the key to the past.

This means that geologists use laws that exist in the natural world today to explain the natural world of the past. For instance, geologists know that in rivers and oceans today, different areas of the water body have different levels of energy. There are places of high energy (i.e. rapids in rivers and coastlines in oceans) that are able to transport large sized sediment, and places of low energy (i.e. deep ocean without current activity, river eddies) that can only transport very small sized sediment. This process of sorting sediments that happens in water bodies today is assumed to have happened in ancient water bodies, as well, because of the theory of Uniformitarianism. Therefore, geologists can reconstruct the relative location and depths of water bodies during a particular geologic interval, like the Devonian. In environments where the shallow water is filled with marine organisms, like today’s coral reefs or central New York’s Devonian sea, the shallowest water allows deposition of fine to medium sized grains composed of bits of organic material. This is called limestone. As the sea gets deeper (lower energy), smaller particles can be deposited in the forms of silt (siltstone) and mud (shale), respectively.

The red box in the above depth gradient image may be a sample of what geologists see, millions of years later, in a rock outcrop along the side of the road here in central New York. The stratigraphic column we used to discuss stratigraphic sequences could have been formed by a sea similar to the one in the above image. For there to be shale at the base of the column, the location in which the column is situated must have been under deeper water at an earlier time. Then, the water must have gotten shallower over that area to yield silt deposition (creating siltstone). Finally, as it is seen in the current water level, limestone was deposited as the see became even shallower at the point where the column is situated. This scientific principle is known as Walther’s Law of Facies. It states that the vertical succession of rocks (seen in the stratigraphic column) reflects a lateral change in the environment (seen in the image of a depth gradient above). Therefore, we can interpret the stratigraphic column to represent a time when, at the location from which the stratigraphic column was drawn, the initial water depth was deeper than the final water depth in the stratigraphic sequence represented by the column. This is known as a shallowing upward sequence, because the rocks depicted in the stratigraphic column show a shallowing of the water in which they were deposited. This is how geologists use rocks to begin to paint a picture of the ancient water body that was in central New York during the Devonian Period.

To understand the nature of the reconstructed water body, geologists turn to the fossils (or lack of fossils) to understand the paleoenvironments (paleo=old). The presence of fossils in a rock indicates that the water body in which the fossils lived could support life; relatively high amounts of oxygen and other nutrients had to be present. The absence of fossils may indicate a lack of one or more of those nutrients. Again, scientists use uniformitarianism to understand the nature of the environment. If the fossils found in the rocks are the ancestors of animals today that only live in marine waters, then the fossils most likely lived only in those environments, as well.

Using rocks and fossils, geologists determined that during the Devonian New York was covered with a shallow sea. The sea crept up onto land from Devonian ocean masses and extended over much of what is now the US. Fossil and other evidence shows scientists that the ocean was located in a subtropical environment, making conditions appropriate for corals and other warm, shallow, marine life.

Scientists have determined that around 415 million years ago (the beginning of the Devonian Period), sea level began to rise. The area that is now the northeastern United States, including New York, was near the equator, and the oceans were warm and tropical. Marine live thrived on the sea bottom, and the skeletons of these marine organisms piled up on the sea floor. As we discussed earlier, some of these skeletons, made of calcium carbonate, formed medium size grains of lime mud that eventually became limestone.

As the high mountains to the east of this sea continued to erode, huge quantities of gravel, sand, and mud flowed off the land into the shallow sea. These sediments contributed to the deposition of sediments which we know see as siltstone and shale in central New York. The blackest mud accumulated in water that had little or no oxygen, creating very dark shales with few or no fossils preserved in them.