My New Favorite Rock

When I lead a geology hike, there are two main concepts that I try to teach. The first is that the earth is always changing, and the second is that every rock tells a story. Last week, I discovered a rock that perfectly demonstrates both concepts at once.

The fossil beds around Hancock Field Station are an ideal place for teaching about changes in the earth. At present, it is an arid scrub land of sagebrush and juniper trees, populated by creatures like scorpions and rattlesnakes. Forty-five million years ago, it was a lush, tropical jungle with gingko, banana, and palm trees, and animals like crocodiles, marsh rhinos, and miniature horses. A series of volcanic mudslides entombed the plants and animals of that time period leaving us with a rich fossil record.

There aren't any fossilized animals exposed near our trails, but the fossils of leaves are easy to find. These leaves are broad and flat because they had to collect as much sunlight as possible from a cloudy and rainy sky. The leaves give us a glimpse into the jungle past but teaching that to students can be difficult. The idea is for the students to figure out for themselves what the existence of broad leaves tells us. Sometimes they get it without much help, but usually it's a question that required being lead to the answer one step at a time.

That brings me to my new favorite rock. On Tuesday morning, I was on a hike with the youngest students I have had this year. They were first through third-graders, which can make large concepts like these even more difficult to teach. Near the beginning of the hike, I stopped the group for a water break where I knew there was a six-inch-long leaf imprint visible in the canyon floor, hoping the students would spot it. One of them did, and I was prepared to do my usual talk about what it meant, but the kids were too excited by the fossil to sit still. So, I told them to see what else they could find in the rocks first.

I have been working at Hancock for over five years, and I thought I knew which fossils were visible on each trail. I didn't think there were any more in this spot, but I wasn't going to stop the kids from looking! I figured they could at least find some interesting bugs. I was surprised when they did, in fact, start finding more fossils. Apparently a flood that we had a few weeks ago had exposed some that were new to me. I saw several nice, flat leaves that I could identify, but then a second-grader brought me a strange, striped fossil. It took me a minute to figure out what I was looking at, but I finally realized that it was a piece of a fern. 

The Fossil Fern

I called all the kids back, and had them sit in a circle. We had a conversation that went like this:

“Take a look at this rock. It has a very interesting story,” I said. “Can anyone tell what this is a fossil of?”
One of the students figured it out. “A fern?”
“Right! Good observation. But, do you see any ferns in this canyon?”
They chorused, “No!” at me, and some of them giggled. There were some hardy grasses and a few bristly desert flowers, but not much else.
“Where do you usually find ferns?” I asked.
I got three answers almost at once: “In a jungle!”
“In the forest.”
“In a rainforest!”
This was almost too easy. When using leaf fossils, I usually have to give the students several minutes and a couple hints before they arrive at “forest,” let alone “jungle,” but all the students knew where ferns come from. “Right.” I said again, “So, what story is this rock telling us?”
“There used to be a forest here,” one of the kids said.
“Right here? In this dry, dusty canyon?” I asked.
“It used to be a really long time ago,” another student said like it was obvious. I paused for a minute to let that sink in, and then we continued up the trail.
With rocks like that out here, the geology concepts just teach themselves!

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