My first day in the field starts with biscuits and gravy at the Trails End Motel. After passing through Sheridan (a town which advertises karaoke, bingo, and tattoos all under one roof), we head up out of the plain and into the Bighorn Mountains. The cut out sides of earth along the switchbacks of the road already hold geological interest—exposed rifts reveal the layers of rock that were lifted up when the mountains formed.
After a short while, the paved routes give way to dirt, and Taylor expertly pilots the Jeep through tight curves and washouts. We’re looking for dikes: vertical sheets of igneous rock which, when sampled, can provide snapshots of geological events that happened 2.6 to 1.7 billion years ago. Though we have a geological map that shows the locations of these dikes, they are by no means obviously apparent. Eventually we get out of the car and tramp through the brush. I ask graduate student Joe Panzik what I’m looking for, and he tells me that I want a rock that is darker grey than granite, with white flecks on the inside when you break off a piece.
As I walk through the pine woods, I can’t help noticing that the air smells like a Christmas wreath, and the sky is huge and blue. A hare bounces through the woods ahead of me, wary of my heavy boots.
At the top of a hill, I see a spiky outcropping of rock poking up through the ridge. It definitely doesn’t look like granite.
These reddish-grey rocks turn out to be the site for the day’s first sampling. We bring the gear up from the road: hammers, a chisel, a drill, a jug of water, and the orienter. The drill is a converted chain saw with a diamond-chip bit capable of cutting cores out of the rock. The water is pumped into the bit to cool it. The orienter determines the position of each core in space: both the north-south direction of the rock and its angle away from horizontal.
After the cores are drilled, oriented, labeled, and packaged, Taylor takes them back to the lab where they will be heated at high temperatures to determine the primary direction each core was pointing relative to the North Pole when it formed. With this data, Taylor can compare events that were happening in this craton (this piece of the earth’s crust) with other cratons around the world. Eventually, he hopes to determine if this crust was shifting independently or if it was part of a larger supercontinent.
But in order to do this he needs lots of samples—preferably from dikes that formed in similar geological events. With that principle in mind, we move on to the next mark on the map, and the next. The last dike turns out to be less useful, as the rock has been changed and shifted by heat and pressure after it was formed. When Taylor cracks off a piece with his hammer, the inside is greenish, indicating fewer magnetic minerals. It’s getting close to 6:00, so we head back to the car and pick a campsite.
As we sit by the campfire, three owls come down out of the trees and swoop over our heads. There is a lot of wildlife here, and Taylor hands me a canister of bear spray—just, he assures me, for my peace of mind.
The first thing I see when I wake up on day two is a mule deer munching grass outside my tent. As I turn on my camera to take a picture, it pricks up its ears and jumps away.
At the day’s first site, we find only piles of rubble—no rock that has not been shifted from its original orientation.
But later in the day we have more luck. By the side of an aborted mineshaft, a long outcrop of rock spans the crest of the ridge like a spinal cord. This is the longest dike I’ve seen so far, and as I take notes in the field book, Taylor tells me to add that this is possibly part of the leopard swarm. The what?
It turns out that swarms (dikes that run in parallel or close to parallel directions) are especially helpful to Taylor, because they are likely to have formed in similar geological events. This dike may be part of the leopard swarm because it runs parallel to another set of dikes that have distinctive white crystal spots. Many similar dikes mean more chances to sample, and a clearer picture of the way the craton was moving 2.6-1.7 billion years ago.
As we start sampling, we realize we need to work fast because a huge thunderstorm is heading towards the ridge. It passes over as we reach the car, speckling the dusty hood with equally leopard-like spots.