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.