The Happy Jack Chalk Mine in Scotia, Nebraska has been around under that name since the 1930s. It’s been an active chalk mine, an abandoned mine, a state-owned wayside park, and recently a privately owned tourist attraction. But the mine’s significance goes way back before its modern history. It goes 5 million years back.
Happy Jack Chalk Mine is about an hour north of Grand Island on highway 11. The mine sits under a hill overlooking the Loup River. Jesslyn Weiner has been a tour guide there for five years and she takes me into the mine through a shack at the foot of the hill.
“We do have lights in here, but this first one’s burned out and it’s not one we can fix. And these are the burrows,” Weiner points to a dark grey balloon-shaped rock embedded in the stark white rock of the mine’s wall. Less than a foot away, there's another one. Weiner points out another one, “And this is just one them burrows but it looks like a rabbit. Can you see its ears?”
At the mine entrance, there are at least four I can see without turning my head. These burrows are all over the ceilings and walls in the 6000 square foot mine. It wasn’t until recently that they became a regular part of the tour. In 1997, Happy Jack reopened as a privately owned tourist attraction. In 2002, the mine’s owners invited University of Nebraska Lincoln geologist Matt Joeckel to visit and give tour guides some background on the mine’s geology. Joeckel expected to talk about the chalky rock, called diatomite, that makes up the mine, but he got a surprise when he walked in.
According to Joeckel, rodents dug the long, nearly vertical tunnels and large chambers into the soft rock in the mine about five million years ago. Sometime later, sand washed in and filled in the burrows, helping to preserve them as fossils. Even though the burrows have always been there, Joeckel didn’t know they were there until that first visit in 2002.
Since then, Joeckel and his colleague Shane Tucker, a paleontologist for the University of Nebraska Museum, have mapped out the locations of all the burrows in the mine. Which wasn’t easy. Parts of the mine are cramped and narrow, and Joeckel is a little claustrophobic. Tucker remembers when they had to start photographing the ceiling. “So we brought one of the museum carts and Matt lay down on his back. He had the light and was looking up,” Tucker said. “And I would push him.“
“It sounds like that would be great fun, for me, lying on my back on this little 4-wheeled cart. But it wasn’t. It was more like a medieval torture chamber,” Joeckel said.
The first thing Tucker and Joeckel wanted to do was try and figure out what kind of animals made these burrows Tucker says they sifted through 500 pounds of sediment from the mine looking for body fossils. What they found fits into the palm of one hand.
“Out of the twenty-some different body fossils we got from there, six teeth were well-preserved. So…not great,” Tucker said.
But the burrows themselves are just as important as body fossils to identify what made them. Tucker and Joeckel made a silicone mold of one burrow to create a plaster replica they could study back at UNL.
“We have an exact carbon copy of what we saw at the diatomite mine. And in some portions of the wall you could see the striations, these paired grooves much more easily,” Tucker said.
Those grooves are evidence of ancient rodents using their top teeth to anchor into the rock and gnawing with their lower teeth to dig. Those grooves and the burrow’s general shape suggest a kind of ground squirrel, like modern prairie dogs, built the burrows. Joeckel says these rodents may have started spending more time underground in response to ancient forests disappearing and open grasslands, like the Great Plains, forming.
“Why would they do that? Well, open grasslands are a somewhat harsh environment. There could be prairie fires. There could be extremes in temperature. So we’re seeing a snapshot of the emergence of the modern grassland environment which is no small thing,” Joeckel explained.
In a way, these burrows are a witness to the last great period of climate change – a time of global cooling and drying, right before the most recent Ice Age. We have general idea of what the earth was like 5 million years ago, but not a good idea of specifics, like what Nebraska was like then. Joeckel says it’s hard to find rocks from that specific time period in the state so these burrows can help fill in those gaps.
“We’re looking through a glass darkly at an emerging world. There are a lot of things we don’t know.”
For instance, we don’t really know how different animals and plants responded to that cooling and drying period right before the Ice Age. The burrows are solid evidence of how one population reacted to that change.
Joeckel and Tucker are hoping the burrows can tell them about small mammal evolution on the grasslands and about the formation of the Great Plains ecosystem as we know it today.
“As is usually the case with these kinds of geological studies, probably raises more questions than it answers. I personally find that comforting,” Joeckel said. “I don’t think I want to know everything. I think I want to be in the business of always trying to come up with interesting ways of approaching an understanding of the history of the planet.”