Secrets in Sedementary Rocks

[This is a follow-up post to the first Geology post from a few weeks ago.] 

These rocks need names.  Each of the big limestone boulders in the circle in our outdoor classroom has something unique to offer the curious eye.  I'm sure by now most students have decided which rock is their favorite to sit on when they visit the outdoor classroom.  For now, I'll number the rocks 1-8, but hopefully students will come up with actual names for them. 

Rocks 1-4, left to right in the picture.

If you follow the main path into the circle, rock number one is on the left.  Then continue counting around the circle.  The fence is between 4 and 5.  Rocks 7 and 8 are the last ones, and they have been placed right next to each other.

Stand in the center of the circle and look around at the rocks.  What are the first things you see?  I see scrape marks, those lighter marks on the rocks where some serious equipment carried them from somewhere else in TN to here.  Next I notice that the rocks look layered.  These rocks are sedimentary rocks, formed over thousands of years as water dropped off layer after layer of tiny mineral particles (more below).

Let's take a look at a few specific rocks in more detail and see what they have to tell us.

Rock #1:
Most of the rock does not contain visible fossils, but there is a gigantic nautiloid fossil that is fairly easy to locate on the surface of the rock.  Hard structures of ancient creatures formed fossils easily, and nautiloids had hard, segmented structures that left fossils like the one to the right of my finger below.  Nautiloids were predators, and where there are predators, we know there has to be prey.  Nautiloids' prey organisms were soft-bodied, so they didn't tend to leave fossils.  The amount of rock that contains a nautiloid is small compared to the rest of the rock because nautiloids were fairly rare in their ecosystem.

Rock # 2:
This rock has a fossil from an organism called a bryozoan.  Bryozoans are filter feeding animals that live in colonies.  They are much like corals, though they are not closely related to corals.  It's common for organisms to be similar and yet unrelated.  For example, penguins and dolphins have many similarities (shape, color, swimming style), yet they are very different types of organisms: penguins are birds and dolphins are mammals.  In the same way, bryozoans and corals are similar in shape and habit yet unrelated.  Bryozoans were much more common when these rocks were formed (300-500 million years ago!) than corals were, and there are several bryozoan fossils on our rocks.

T-shaped Bryozoan fossil on Rock 2.

Rock 3:
Find the crystal-lined hole in Rock 3.  You have found a geode-like structure.  Geodes can form in different ways, but it is likely that this one formed when part of the original sedimentary rock dissolved and washed away to leave a hole, or cavity.  Then over thousands of years, water seeped through the cavity.  Water usually has dissolved minerals in it, and sometimes those dissolved minerals crystallize into solid minerals.  If you have an older faucet at home, look for a whitish crust around it - the white crust is crystallized minerals from the water that is carried through the pipes.  I suppose if the conditions were right your faucet could be covered with pretty crystals in a few thousand years as more and more minerals are deposited on it.  The same process that makes the crust on your faucet made the geode you see in this rock.

A geode-like cavity in Rock 3.

Rock 7:
Rock 7 is my favorite for looking at the layers of sediment that formed the rock.  For some reason, this rock was placed on its side, so the layers are more easily visible.  Flowing water makes sedimentary rocks.  Fast-moving water picks up larger, sand-sized sediment and carries it until the water slows down.  Then the sediment falls out of the water and settles on the ground in a flat layer.  Slower-moving water picks up smaller particles, like the particles that make up mud.  When slow water stops moving, it drops its particles onto the ground.  If the speed of the water above the ground changes through the decades, the sediments that settle onto the ground will have different textures.  The darker, smoother bands in Rock 7 were formed from slow-moving water's particles.  The lighter, rougher bands were formed from faster water's particles.

Layers in this sedimentary limestone reveal the speed of the water that deposited the particles.

There are lots more treasures to find on these rocks, so look carefully.  There are many more fossils, interesting rock layers, geode-like structures and other strange features.  I would like to thank Mr. Smail for teaching me about our rocks.  Feel free to email him if you have more Geology questions!

How to Read the Rocks Around Our Pond

The rocks around our pond have stories to tell if you know how to listen.  Thanks to Mr. Smail, our high school Geology teacher, for showing me how to read our rocks.  Here are a few of the amazing things I learned from him about the flat rocks that border the pond:

Rock # 6, my favorite.

ROCK FORMATION:

Our rocks are ANCIENT.  Middle Tennessee rocks were mostly formed 300-500 million years ago, during what is called the Paleozoic Era.  During that time, life was growing mostly in the oceans and just beginning to expand to land.  The area that would one day be called Tennessee was covered with a shallow ocean at the beginning of the Paleozoic Era.  Many ocean organisms in the Paleozoic Era produced hard shells made of calcium carbonate.  When those organisms died, their shells accumulated, compacted, weathered and eventually formed limestone, which is the type of rock we have in our outdoor classroom.  Many of today's ocean organisms also have calcium carbonate shells, so they are just starting to form limestone of the distant future.

Here's a map to the rocks around our pond.  The map has numbers so you can find the things I'll talk about here.  Feel free to print the map and draw or write your observations on it.

Map of rocks with numbers (zoom in to see numbers).

Many areas of our rocks are smooth limestone, like almost all of rock 19.  The smooth limestone was produced when the water above the forming rocks was calm.  The calm water washed tiny, tiny bits of shell onto the ocean floor that built up and solidified into smooth limestone.  Before the tiny bits turned into stone, they would have felt like smooth mud.  On several rocks, it appears the mud dried and cracked before turning to stone.

Rock 13 with smooth rock made from dry, cracked smooth mud.

Some areas of the rocks have larger particles and lots of fossils (see below).  These formed when the water was more turbulent and washed larger particles onto the ocean floor.  Rocks 1 and 2 are mostly made of larger particles.  If you've walked on a beach made of very rough sand, you know what sizes of particles formed this rough limestone.

Most of our rocks are made of the rough limestone mostly covered in a smooth layer of limestone.  Rocks 15 and 16 are different.  They were likely made when very turbulent water mixed big pieces of smooth limestone with large shell bits that cemented into rough limestone around the smooth limestone bits.

FOSSILS:

The signs of ancient organisms in rocks are called fossils.  Since limestone is made from the shells of ancient organisms, you can expect to find LOTS of recognizable fossils in our rocks.  The most common fossils in our rocks are shells of ancient clam-type organisms.  We have a few whole shells (Rock #2) and lots of c- or j-shaped fragments or pieces of shells (most rocks).  We also have fossils of long, segmented organisms called nautiloids.  Nautiloids were relatives of modern-day squids, and like squids, they were predators that chased down their prey.  There are always fewer predators than prey in an ecosystem, so it makes sense that there would be fewer nautiloids than clam-type shells.  Rocks number 6 and 17 have nautiloid fossils.

C- and J-shaped shell fossils in Rock #6.

Gorgeous nautiloid fossil in Rock 6, surrounded by shell fossils.

The fossils above are body fossils, or actual fossilized body parts of ancient organisms.  Another type of fossils, trace fossils, are fossilized evidence that organisms were present, like footprints or trails.  The light squiggly lines in many of the smooth limestone areas are trace fossils of burrows or trails left by soft-bodied organisms like worms.  Soft body tissues cannot form fossils, but we can learn a bit from trace fossils about ancient soft organisms.

Lighter squiggles are trace fossils, evidence that soft organisms were once present here.

NOTE TO TEACHERS:
Mr. Smail would be happy to meet your class at the outdoor classroom to answer questions if he is available - just email him.  Also, below is another map with a key to where you can find some of the features I mentioned above.  I thought you might want the students to make their own, so I didn't include it above.  Also, I'll discuss the big boulder rocks in a later post.

Map of some of the fossils and rock features around our pond.