Geologic Time

Unless otherwise noted the artwork and photographs in this slide show are original and © by Burt Carter. Permission is granted to use them for non-commercial, non-profit educational purposes provided that credit is given for their origin. Permission is not granted for any commercial or for-profit use, including use at for-profit educational facilities. Other copyrighted material is used under the fair use clause of the copyright law of the United States.

1. Uniformitarianism

The behavior of physical, chemical, and biological systems remains the same throughout time.

(“The present is the key to the past.”)

Where stream currents move sand in one direction the resulting ripples are asymmetric – steeper on the downstream side.

Sand on the bed of the Suwannee River

Upper Paleozoic sandstone from Lookout Mountain

Where waves move sand back and forth in shallow marine environments the ripples are symmetric rather than asymmetric – both faces are steep.

Shallow marine sand near Panama City, FL

Lower Paleozoic symmetric ripples from near Chickamauga, GA

Upper(?) Precambrian symmetric ripples from near Silverton, CO. For several reasons the only moving water in the vicinity (the waterfall to the right) could not have made these ripples. Why not? We can more deeply consider the principle of uniformitarianism here as well. Do you think these ripples could have been formed on a nearly vertical face, where we see them today? Why not?

Mudcracks in a claypit near Andersonville, GA

Early Paleozoic mudcracks in a roadcut near Ringgold, GA

As wet mud dries it cracks into polygonal pieces and the edges curl. These are called mudcracks. The lithified versions below lead us to a simple version of uniformitarianism – if it looks like a mudcrack it probably is.

Periarchus pileussinensis from near Roberta, GA The fossil at right looks like a sand dollar, even though it does not look exactly like any living sand dollar. The place it was found has been called by local residents “sand dollar hill” because they can plainly see what this is, educated or not. If it looks like a sand dollar then it must be one. Why are they so abundant around Roberta and Macon, and Wrens – 10’s of km from the coast? (Relatives of sand dollars cannot live for even a few minutes in fresh water – they must have salt water.)

2. Principle of Inclusions

If one solid object is enclosed entirely within another then what is enclosed is older than what encloses it.

~1 cm

Here is a specimen from our teaching collection. It is a fiddler crab that has been preserved by embedding it in a solid plastic block. How did the people who prepared the specimen get it inside the block?

Coin is 2.5 cm in diameter

Here is a fossil crab that I found weathering out of a solid block of limestone near Gainesville, FL. Same question – how did it get inside a solid rock?

Here are three specimens of the same species of fossil in three different stages of preparation. We will look at them one at a time, starting with the one at the top.

Here is a specimen that has just begun to be removed from the rock. The black you see is an organic stain that affects limestone exposed to moisture and light. The black was all that was visible of the fossil when the rock was found. It’s hard to tell much about the fossil except that it is round (and flat, seen from the right side). But if you look carefully you’ll notice some structure to it. It is breaking in some spots, differently – into larger pieces – from how the surrounding rock breaks. It also has some tiny granular or bumpy texture, particularly obvious near the bottom right. It’s not much, but someone with a trained and experienced eye can make a shrewd guess about what species it is from this little bit of evidence and knowledge of where it was found. (Near Roberta, GA in Crawford County.)

Here is a specimen that has been removed completely from the rock it was found in, but has not been cleaned fully yet – a step that takes more time and care. The same granular texture is apparent in several places (black arrow, for example) and some radial structure is also apparent near the center (red arrows). Notice that there are five similar things – the best exposed one near the bottom of the picture. What is this thing?

This specimen started out in the same condition as the others but it has been extracted from the rock and cleaned to expose the structure better. Unfortunately it has not been cleaned carefully and the granular/bumpy texture has been destroyed. Still, it should now be very obvious what the thing is – a sand dollar. What is a sand dollar doing 40 miles west of Macon, and how did it come to be embedded in solid rock?

I should say here that as someone with a well trained and experienced eye, able to spot a tiny part of a fossil exposed on a rock face, it is frustrating to think about how many fossils are in the rocks that I cannot collect because I cannot see any of them at all – they are completely enclosed and not visible. Just whacking the rocks with a hammer will often expose fossils on the inside, but almost as often will shatter them to pieces at the same time. I have only half of one of the rarest fossils I ever found because it was in a quarry wall and the excavator had scraped the other half off and taken it to be turned into cement or ag lime or road gravel.

1 cm

Not all rocks break equally however. For some fossils the best way to collect them is to break the rock they are in and you don’t even have to see the fossil first. Some shales in the western Appalachians are loaded with plant fossils like the fern shown here. You can sit in a coal mine and tap pieces of shale on the sides, forcing them to split along the thin layers that characterize shale. (Remember shale?) If you’re luck you find some nice plant fossils inside. (If you’re very lucky you might find some animal fossils as well.)

The point of all this is to make sure you realize that fossils occur inside of solid rock. Even the ones I have collected lying loose were there because they had weathered out of the nearby rocks. I know this because I could find the rocks they came from, still containing visible parts of the same species of fossils. This, to my surprise, is not widely known. I’ve had modestly heated exchanges with people who insist that the fossil oysters they’ve seen in the bed of the Suwannee River grew attached to the limestone “back when the ocean covered this area”. No, I point out – only half the fossil is outside the rock, the other half is still inside. It is only partially weathered free. My advice is to wait a year or two and see if it comes all the way out. Or else bring a hammer and chisel next time. “But how did they get inside of a rock”, I’m asked. Exactly the question I’m asking you. It was a question that bothered Nils Stensen (aka Nicholaus Steno – his Latinized name) after he made an interesting observation.

For decades, maybe centuries, Europeans had found “tonguestones” lying loose or embedded in the rocks in their regions. (for a explanation of the name google images of tongues.) Everyone, including Stensen, assumed they were some sort of crystal, like the other crystals they found inside of rocks.

Stensen was an anatomist in the service of the Medici family in Florence. He belonged to a group of scientists that had been formed (and funded) by the Medici in honor of Galileo. When rumor came to town of a “sea monster” that some fishermen had caught in the Mediterranean and brought to shore its head was ordered brought to Florence for study, and Stensen was able to study it. The first thing he noticed were the teeth, and not just because it is always prudent to notice the teeth of a shark first. Steno immediately realized that they were glossopetrae – tonguestones, just like the ones that came out of rocks far from the sea. How did shark teeth get into such an unexpected place? Answering this question had three interesting effects: 1) Stensen realized that it had to be a multi-step process. The shark tooth first had to settle into loose sediment, then be completely covered, and then the sediment had to solidify around it. This was the first record of a European scientist reasoning out a series of events in the formation of a rock. 2) Stensen began to explore other ways of arranging events in Earth history into a sequence. All the other basic principles – superposition, cross-cutting, etc. can be traced to his work in this area. He effectively founded geology as a rational science. 3) Once he realized the implications of his work Stensen abandoned science and became a successful catholic priest. The reason: he had proven that (if reasoning were a valid way to learn the truth) a creator could not have made the rocks first and then created sharks, as Genesis claims – the sequence is exactly backwards. Stensen’s faith won out over his rationality and he followed that path.

(“shark fish head”)

(“some shark teeth”)

Stensen’s drawings were made to illustrate the similarity between glossopetrae and shark teeth.

Here’s a shark tooth mostly excavated from limestone (from Morocco) for comparison.

Applying the principles of uniformitarianism and inclusions we reason: 1) That thing embedded in limestone in the desert of Morocco looks

for all the world like a shark tooth, so that’s what it is.

2) Given that a solid tooth cannot be inserted completely into a solid rock, the rock around it must not have been solid when the tooth was emplaced.

3) The tooth is necessarily older than the rock, despite the claim of the writer(s) of Genesis to the contrary.

(People have been burned at the stake for saying things like that!)

Inclusions don’t just appear in sedimentary rocks. The large gray thing in this quarry wall is a piece of the same rock that surrounds the granite that is being mined. A piece of it fell into the magma before it cooled and was therefore completely surrounded by granite after final crystallization of the magma. The xenolith (“alien stone) is an inclusion and so must come from a rock older than the granite. The vertical dark gray streaks are springwater coming from cracks in the rocks.)

Granite quarry near Elberton, GA

3. Principle of Original Horizontality

Sedimentary rocks and extrusive igneous rocks are formed in layers that are essentially flat.

The layers in Grand Canyon are very obvious and also very obviously virtually flat.

Grand Canyon, AZ

These rocks in western Maryland are obviously not flat. They were when first deposited, but after lithification something has tilted them into these odd orientations.

Both images from a railroad cut near Pinto, MD

On the previous slide we can interpret a logical sequence of events that have affected the rocks: FIRST: the sediments were deposited as loose material in flat layers. (Because they have marine fossils in them the setting was a shallow seabed, not the side of a mountain.) SECOND: The loose sediments were lithified, giving them enough strength to stand up to whatever force caused them to change their orientation. and THIRD: A compressional force squeezed them from the sides and buckled them into very large (left picture) and rather small (right picture) “folds”. We’ve already talked about what might have caused that folding, so we could elaborate on that if we wanted. (Actually, we could apply uniformitarianism and consider ever older events: the formation of a source rock, the weathering and erosion of that source, the transport of the sediment from the source to here. The nature of the rocks can sometimes even tell us what kind of transport system brought them – stream, wind, glacier, whatever.)

4. Principle of Original Lateral Continuity

Sedimentary rocks and extrusive igneous rocks are formed in layers that do not end abruptly unless something interferes with their deposition or the movement of the lava. (Which should be obvious.) Other rocks also

have an expected lateral extent and that can also be disrupted.

Individual prominent layers in the Grand Canyon can be traced for many miles – the entire length of the canyon in most cases. At the upstream end they emerge from beneath overlying rocks, at the downstream end they end at cliffs from which their original continuations have mass wasted. They can also be matched across the canyon because they were originally continuous before the canyon erosion disrupted that continuity and took the intervening material away to the Gulf of California.

Grand Canyon, AZ

?

Faults also disrupt the continuity of things. In this photo a specific bed is indicated by the blue dashed line. That bed was initially deformed (folded) by a compressional stress, and when it could deform that way no longer it broke, in several places, and the higher resulting pieces were shoved over the lower ones, as the arrows indicate. We recognize faults by exactly this: the lack of continuity of the affected rocks.

Railroad Cut near Pinto, MD

The mafic igneous rocks (red arrows) you see here are called “dikes”. They have been intruded through cracks in the surrounding limestone. (Notice the vertical bedding, suggesting that the limestones were folded after lithification.) Because the dikes interrupt the continuity of the limestone beds they must also have been intruded after the limestone was lithified.

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Here is another mafic dike that disrupts the continuity of an older granite. The lighter colored granite around it would be a continuous mass were the dike not there because it was originally a liquid filling a “magma chamber” underground. The dike disrupts that expected continuity.

Granite Quarry near Elberton, GA

So three different things can disrupt thee original continuity of a rock or other geologic feature:

1) Erosional Surfaces

2) Faults

and

3) Intrusive igneous rocks

5. Principle of Superposition

Sedimentary rocks and extrusive igneous rocks are formed in layers that accumulate in a particular order. Each bed must be laid atop the one immediately below it. Thus the oldest rock in a stack is at the bottom

and they are progressively younger toward the top.

This is applicable even if the beds are subsequently tilted or folded from the original horizontal position though “up” and “down” must be

understood in terms of original horizontality, not present orientation.

Grand Canyon, AZ

Oldest

Youngest

1

2

(2 – lateral continuity)

3

4

5

6

This is both the simplest and the most powerful of the relative dating tools for sedimentary and volcanic igneous rocks. Here we see a simple example from the Grand Canyon. Superposition can sometimes also be applied to metamorphic rocks (with sedimentary protoliths), but the forces that cause the metamorphism also often deform the rocks so intensely as to make it difficult to establish which way was originally “up”.

Grand Canyon, AZ Oldest

Youngest

. c t E 6 5 4 3 2 1

Older mudstone layers

younger mudstone layers

Just to be complete, we can apply this principle at any scale (bottom) and to any pair of rocks, even if they are not directly touching each other (top).

Because we can use the principle of lateral continuity to establish that a bed in one place is the same as the a bed in another place (double ended arrow) we can extend the principle of superposition between those places.

(~200 miles)

These rocks are younger than the ones below them and they are therefore younger than the ones in the Grand Canyon as well.

Grand Canyon, AZ

Canyonlands N.P., UT

This can be extended yet farther, both geographically and temporally (in terms of time). Bryce Canyon is ~100 miles away from Canyonlands (and only about 50 from Grand Canyon, actually). There the upper rocks at Canyonlands are overlain by even higher (and therefore younger). rocks

Grand Canyon, AZ

Canyonlands N.P., UT

Bryce Canyon, UT

The rocks in the northern part of this area (inset) have been folded, as indicated by the wavy color bands on this geologic map. The rocks in the southern half have not been folded. The folded rocks do not simply end, to be replaced by the others, they continue beneath them, as indicated by the dashed lines.

This feature – folded rocks beneath flat ones, is called an angular unconformity. A Scottish angular unconformity was very important in the early development of Geology and so it is worth working out the correct sequence of events here, and pointing out the logical principles used in determining that order before we go to Scotland in a few slides. The folded sedimentary rocks must have been deposited as ~flat layers (original horizontality) and lithified. They must then have been deformed or folded by a force acting as indicated by the arrows (lack of original horizontality). Then they must have been eroded to a nearly flat surface (Original lateral continuity). Before the (still) flat rocks in the south were deposited atop them (superposition).

5. Principle of Cross-cutting Relationships

Any geologic feature that “cuts across” (or just “cuts”) another feature is younger than what it cuts.

To say that one thing cuts across another implies that it disrupts the

original continuity of that older feature, so this is clearly related to the principle of original lateral continuity.

The erosional surface at the Grand Canyon (including the canyon itself) must be younger than the rocks it cuts into.

(There is a subtle angular unconformity visible in the ellipse. The flat black bed that is obvious is sitting above tilted layers of red material below. Can you see it?)

Grand Canyon, AZ

Similarly, the mafic dike must be younger than the granite because it cuts across it.

Granite Quarry near Elberton, GA

In fact, not far away in the same quarry we can see another mafic dike (the thinner one) that has been cut by the dike on the previous slide – shown again here. The offset of the thinner dike indicates that it was cut and offset by the thicker one. The order of

formation of the rocks is indicated.

1

2

3

Granite Quarry near Elberton, GA

The limestone must be older than the dikes that cut across their bedding.

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Granite Quarry near Elberton, GA

Remember that we’ve already established that the dark rock is younger than the surrounding granite because it is included within that granite. You can arrive at the same conclusion by noticing that the granite cuts across the inclusion in several places.

Railroad Cut near Pinto, MD

The faults must be younger than the rocks they broke. (They must be younger than the folding too. Since they were not folded into the same pattern as the rocks they must not have been there when the rocks were folded.)

8. “Deep Time”, Principle of Fossil Succession, and the Geologic Time

Scale

Fossil species occur in the same vertical order wherever they are observed.

This is clearly a special case of superposition – it implies that different

fossil organisms lived at different sequential times, coming into existence and becoming extinct in a particular order that is recorded

everywhere in the same way.

The first geologic time scales were proposed in the mid 1700’s and were based strictly on superposition and rock type. They were very simple:

Quaternary (Fourth) Tertiary (Third)

Secondary (Obvious, eh?) Primary (First)

They fell apart for a variety of reasons, the most obvious of which was that “Primary” rocks were seen in many places to crosscut (and therefore be demonstrably younger than) secondary rocks. At the same time (late 1700’s) it was becoming obvious that the Earth is a very old place.

Deep Time

“Schistus” – sandstone and mudstone layers, slightly metamorphosed. (Bedding nearly vertical and eroded ~flat)

“Old Red Sandstone” – sandstone. (Bedding tilted 30-45° and eroded)

Recent Sand on Beach (Bedding horizontal And being deposited)

SICCAR POINT, SCOTLAND

Background image from

Monroe and W

icander “The Changing Earth”

Where does the sand come from to make the modern beach? (Weathering and erosion of the rocks in the cliffs.) Where did the sand come from to make the Old Red Sandstone? (Weathering of the Schistus and other older rocks.) Why aren’t they Flat and continuous? (Original horizontality and lateral continuity) (Because they spent a long time being folded and eroded.) Where did the sand come from to make the Schistus? (Older rocks that we can’t even see.) Why aren’t they flat and continuous? (They also spent a long time being folded and eroded, before the Old Red Sandstone was deposited!) Maybe those “older rocks” were sandstones too. Where did that sand come from? (Even older rocks.) And so on. (ad infinitum? Is the Earth infinitely old, recycling itself over and over through repeated folding/uplift/erosion cycles?)

Fossil Succession and the Time Scale

Cenozoic

Mesozoic

Paleozoic

“Precambrian”

Present ~66.5 MA ~248 MA ~542 MA ~4600 MA

The Time Scale from your book

Cenozoic

Mesozoic

Paleozoic

“Precambrian”

Present ~66.5 MA ~248 MA ~542 MA ~4600 MA

Know this part.

Cenozoic

Mesozoic

Paleozoic

“Precambrian”

Present ~66.5 MA ~248 MA ~542 MA ~4600 MA

“Whole” or “Perfect” Animals

“Middle” Animals

“Ancient” Animals

“Before the Cambrian” No Animals

The names of the Eras (and Eons) reflect the kinds of animal fossils found in them: “ancient (paleo)” “middle (meso)” and “whole” (ceno). (Note that theparts of the Time Scale, and “Time Scale” itself are proper names and should be capitalized.)

Mesozoic Rocks (Far Away)

Paleozoic Rocks Precambrian Rocks (Deep in the Canyon)

Grand Canyon, AZ

Cenozoic Rocks (High Point on Colorado Plateau at Brice Canyon, Utah)

Mesozoic Rocks (Canyonlands, Utah)

Paleozoic Rocks Upper Grand Canyon/Lower Canyonlands)

Precambrian Rocks (Deeper in the Grand Canyon)

Paleozoic Rocks

Mesozoic Rocks

Unconformity, like the one at Siccar Point.

CENOZOIC

MESOZOIC

PALEOZOIC

“PRECAMBRIAN”

“PERFECT ANIMALS” – things Easily recognized – closely related to living species “MIDDLE ANIMALS” – things that look somewhat like living species but not quite such familiar versions of them. “ANCIENT ANIMALS” – things that, for the most part, don’t look much like living species. Similarities are vague, in general. Rare fossils of very simple organisms

We recognize these various rocks by the fossils in them.

The observation that there is an invariable vertical order in which fossils occur is called the principle of fossil succession.

CENOZOIC

MESOZOIC

PALEOZOIC

“PRECAMBRIAN”

Very few fossils of very simple organisms. Unicellular bacteria in lower part. Mostly the same in upper part, with more complex algal and protozoan cells in addition to algae. Some very simple (and very odd) multicellular things in the very highest beds.

CENOZOIC

MESOZOIC

PALEOZOIC

“PRECAMBRIAN”

1mm

Trilobites

Horn Corals

Stemmed echinoderms

Brachiopods

Twiggy bryozoans

Vertebrates: Odd Fish Amphibians

Plants: Spore-bearers (Fern relatives)

Honeycomb Corals

Scale Bar is 1cm unless otherwise labeled

CENOZOIC

MESOZOIC

PALEOZOIC

“PRECAMBRIAN”

Land Vertebrates: Amphibians Primitive Reptiles

Land Plants: Spore-bearers (Fern relatives) Few Conifers

CENOZOIC

MESOZOIC

PALEOZOIC

“PRECAMBRIAN”

(Note the complex internal partitions)

Ammonites

Oysters and Clams

Sea Urchins and relatives

Scale Bar is 1cm unless otherwise labeled

Vertebrates: Large swimming relatives of dinosaurs

CENOZOIC

MESOZOIC

PALEOZOIC

“PRECAMBRIAN”

Land Communities were dominated by conifers and these guys:

“Sue” at the Field Museum of Natural History in Chicago Photo by Chelsea Carter; used with permission

CENOZOIC

MESOZOIC

PALEOZOIC

“PRECAMBRIAN”

Scale Bar is 1cm unless otherwise labeled

Clams, scallops and oysters

Nautiloids – note the simple internal partitions

Sea urchins and sand dollars

Sea snails

“Modern” corals

Whales

CENOZOIC

MESOZOIC

PALEOZOIC

“PRECAMBRIAN”

Flowering plants and mammals dominate land communities.

CENOZOIC

MESOZOIC

PALEOZOIC

“PRECAMBRIAN”

Here are the fossils you should remember:

Sand Dollars (Cenozoic)

Ammonites (Mesozoic)

Trilobites (Paleozoic)