Nesting In Colonies

I've delayed long enough in introducing the Willow Creek anticline. So far, I've said only that we found the babies on the Peebles ranch in the Two Medicine formation. Well, within a geological formation the rock beds are often bent into twists, hills, valleys and folds, and the vocabulary of geology has numerous terms to categorize these structures or features. One of these terms is "anticline." The geological feature in which we found the first nest is an anticline named in traditional fashion after a local watercourse, Willow Creek. This is a good-size stream when it's not dry, with some trout in it and banks covered with just the sort of tangled willow thickets that grizzly bears love. That first nest and all our later discoveries came from the anticline itself, or just off the edge of the anticline.

An anticline is a wrinkle, or fold, in the earth—a little hill. In talking about the growth of the Rocky Mountains, I described how the thrusting of the mountains created a foredeep, or dip, in front of them. Similar processes account for the anticline, but on a smaller scale. The

Anticline Syncline Monocline

Above: Geological activity in the area of the dig caused a wrinkling of the rock beds laid down earlier by sedimentation. An anticline is a kind of hill; a syncline, a dip or valley; a monocline, a single slope.

Below: A cross section of the anticline outlines its shape before erosion. Various fossil sites (some of them mentioned in later chapters) are indicated on their different levels, or horizons.

Anticline Hidden Below Monoclinal Dip

foredeep might have extended 100 or 200 miles in front of the mountains. The anticline proper is only about one square mile. The entire area that we covered in the dig as time went on was only two square miles. Still, the principle is the same. When geological forces such as the movement of mountains or earthquakes or volcanic activity cause the skin of the earth to bend and break, many small dips and hills are formed. A dip, or concavity, is called a syncline. A hill is an anticline. One slope of a hill (when the rest is missing) is called a monocline.

The hard thing for someone not versed in geology to grasp is that you can't see these hills on the surface. Above ground, the Willow Creek anticline is just a piece of sorry pasture land with a lot of scarring and erosion, hills and gullies. There are areas of barren green mud-stone and reddish sandstone. Where there is enough dirt to support plants, you find short grass, prickly pear and wild honeysuckle. There are black widow spiders, a few scorpions, Richardson's ground squirrels, and badgers, which eat the ground squirrels. None of this, and particularly not the hills and rises that you have to walk over, bears any necessary relation to the shape of the actual anticline. The surface is made up of temporary accumulations of dirt, momentary features of the landscape. They may be gone in a hundred or a thousand years. Geologically, they are no more significant than the cracks and flakes of a bad paint job on a clapboard house. The structure of the house is hidden. And that's what the anticline is, part of the underlying structure of the land.

To find the anticline you must locate the layers of rock underneath the surface, which are exposed in some places through erosion, and follow these layers to see what transformations they have undergone. You must determine how they are shaped and how they are tilted. The Willow Creek anticline is not easy to follow. I sought out two graduate students in geology, Will Gavin and John Lorenz, to study the geology of the Two Medicine formation and the Willow Creek anticline to give us a kind of geological map of the territory.1 They found that after the rock beds had buckled to form an anticline, they had also been bent and twisted so that the hill forming the anticline now curves around in a crescent shape and is in places so jumbled that it's all but impossible to figure out which rocks were originally on the bottom and which on top. Furthermore, the whole top of the anticline has been scooped out by erosion.

The anticline is about 160 feet thick, and it has seven distinct, fossil-rich rock layers. Just as the Two Medicine formation is broken up into large layers of shale and mudstone, the Willow Creek anticline, which is just a thin slice at the top of the Two Medicine formation, dating from about 80 million years ago, is itself sliced into these much thinner layers, each of which has characteristic rock and fossils. The layers are called fossil horizons. One horizon, for example, shows a lot of small streams, and another a large lake, each with a different kind of dinosaur represented in its fossils. Like the larger layers of a geological formation, the horizons of the anticline show a progression in time, with the bottom horizon being the oldest and the top horizon the most recent. It was from a spot on the bottom horizon, in the scooped-out center of the anticline, on a pebble-strewn nub of dirt and rock, that Bob and I dug up the first nest of baby maiasaurs. And it was on this same horizon that, over the next few years, we were able to find eight more maiasaur nests.

THE SUMMER OF 1979 was our first full field season. We camped that year not on the Peebles ranch, but on the banks of the Teton River on land owned by A. B. Guthrie, the Pulitzer Prize-winning novelist who wrote The Big Sky. We got to know Guthrie, who's usually called Bud, in a roundabout way.

The town nearest the Peebles ranch is Choteau, which has a campground, restaurants, clothing stores, and gift shops for tourists.



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