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Lecture 1:

Listen to part of a lecture in a geology class.

Okay. Before we began, I want to remind you that our field trip to Bryce Canyon National Park is this weekend. Remember, the bus leaves early, 5 a.m. so don’t forget to set your alarm clocks. I think you are all gonna enjoy getting out of the classroom and actually seeing some remarkable geologic phenomena.

Now, while we are there, I want you to pay particular attention to two things, one obviously would be the sediment layers making up the rocks, since we’ve spent so much time on sedimentary rocks. Bryce Canyon is a great place to see how millions of years have turned layers and layers of tightly packed sediment, mud particles, sand, remains of plants and animals, into rock.

But you are also gonna see some fascinating rock shapes, formations that are the result of the weathering and the erosion processes that occurred at Bryce Canyon.

There are two main processes that are important.

The first one is a weathering process called frost wedging. Frost wedging is a process that widens cracks in rocks in the winter time. It begins with warm air or daytime sun melting the snow. As the snow turns into water, it seeps into the cracks that occur naturally in sedimentary rocks. At night, this water freezes in the cracks, but when water freezes, it expands quite a bit, which means that it pries cracks open, gradually making them wider, and breaking up the little bits in the process. Now, this thaw-freeze cycle can happen as many as two hundred times in a single year, so that makes it the most important weathering process at Bryce Canyon.

The other key process is runoff, which is an erosion process.Runoff takes place in the summer, the parks in the deserts the grounds very dry. When it rains in late summer, the ground is too hard to absorb the water, so it runs off. And as it runs off, it carries away the gravel, the broken bits of rock created by frost wedging in the winter. So runoff is the main erosion process that alters the rock landscape in the park.

And because these processes have occurred over thousands of years, some of the results can be pretty dramatic, like the giant corridors or passageways that are developed within the rocks. These passageways are known as slot canyons.

Here’s an example of one, not from the park we are going to. This one is actually in Australia, but the scale is typical.So these huge spaces started out as small cracks throughout the sedimentary rock. Then thanks to millions of cycles of frost wedging and runoff, what used to be one big area of rock is now sort of two smaller areas of rock with a corridor in between. We’ll have a chance to walk through some like this. These slot canyons are great places to explore, but let me just say for any of you who aren’t from around here, if you ever go on your own, make sure you check the weather forecast first, a sudden heavy rain can cause a flash flood in the slot canyon, so you want to know when it’s save to explore them.

Fortunately, it will be dry this weekend. Now, these deep narrow slots are pretty common. You might even have two of them very close to each other with only a thin wall of rock in between.Of course, frost wedging is still at work, so it starts wearing away at the front of the thin wall until you get a hole, I mean, a hole all the way through the wall, front to back. And this hole gets bigger and bigger. Once it’s at least one meter in diameter, it’s called a window. And eventually the weight on top of it is just too much. So the roof caves in and only the sides, sometimes it’s just one side is left standing. These sides, which look a lot like columns now are called hoodoos. Here’s a photo of something we’ll be seeing.

One of the things that makes Bryce Canyon unique is that it has more hoodoos than anywhere else in the world.Yes? Margo.

Why is it so lumpy-looking? You’d think it’d be smoother.

Well, remember these are sedimentary rocks. So they have layers. Some layers are mostly limestone. And limestone erodes pretty quickly in the presence of any kind of acid. Now Bryce Canyon is in a very unpolluted area, but even there the rain water has a little carbonic acid in it, which causes the limestone to erode. But other layers are made up of different types of sediment, which aren’t so vulnerable to acids, so they don’t erode as quickly.

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