If you drive through central Nebraska and go an hour north of Grand Island, you’ll find Happy Jack Chalk Mine just off Highway 11. 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 -- five million years back. In fact, these mines contain a curious collection of clues that are helping scientists learn more about the last great period of climate change, a time of global cooling and drying that occurred right before the most recent Ice Age.
Jesslyn Weiner has been a tour guide at the mine for five years. She enters 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 gray balloon-shaped rock embedded in the stark white rock of the mine’s wall. There's another one less than a foot away. Weiner points out another one, “And this is just one of the burrows, but it looks like a rabbit. Can you see its ears?”
The burrows are all over the ceiling and walls in the 6,000-square-foot mine. It wasn’t until recently that they became a regular part of the tour. 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 that makes up the mine, called diatomite, but he got a surprise when he walked in: “The minute we walked into the mine we recognized the features on the wall as being fossil burrows. We picked them out right away."
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. Some time later sand washed in and filled in the burrows, helping to preserve them as fossils.
Since 2002, 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.”
Joeckel interrupted, “It sounds like that would be great fun, for me, lying on my back on this little four-wheeled cart. But it wasn’t. It was more like a medieval torture chamber.”
Joeckel and Tucker have been meticulously mapping out the burrows because these burrows are one of the few ancient burrows that have been preserved in the world. Burrows are hard to preserve as fossils because they’re often made in soft soil that erodes easily. In this case, Joeckel and Tucker got lucky that the burrows were made in harder rock.
Looking for the burrowmakers
Burrowing is a very common rodent behavior now, but it wasn’t always. The first thing Tucker and Joeckel wanted to do was try and figure out what kind of animal made these burrows. 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 20-some different body fossils we got from there, six teeth were well preserved. So…not great,” Tucker said.
But, Tucker added, 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 that 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 the disappearance of ancient forests and the formation of open grasslands like the Great Plains.
“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.
The beginning of the Great Plains
Sometime in the late Oligocene, around the time when huge portions of the earth began to dry, forests gave way to open land. Grasses started growing on those open areas in the late Miocene. Scientists know rodents started burrowing around this time, but because burrows are hard to fossilize they haven’t had any direct evidence -- until now.
The burrows at Happy Jack show that complex burrowing behavior that included tunnels and hibernation dens had already evolved in some rodents by the late Miocene. The burrows are also giving us details like what specific kinds of plants made up the first grasslands and how much seasons developed as the climate changed.
Joeckel says it’s hard to find rocks from that specific time period in Nebraska, so these burrows can help fill in those gaps. But there’s a lot more they’d like to know. Joeckel compares it to having a big picture versus a closeup. “We’re looking through a glass darkly at an emerging world. There are a lot of things we don’t know.”
For instance, Joeckel and Tucker want to know how seeds of specific plants responded to the cooling and drying period right before the Ice Age, and they want to know more about how these rodent burrows may have shaped the Great Plains ecosystem as we know it today.
“As is usually the case with these kinds of geological studies, it 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.”