Welcome! This site is one of five EarthCaches located on the Marias River between the Tiber Dam Recreation Area and Moffat Bridge at County Highway 223. While this series is intended for a younger audience, feel free to use this as a quick guide or learning tool no matter what your age.
For the next EarthCache in the trail, click here.
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Hike to Hoodoo Point
This EarthCache is divided into two parts, a boat landing and an overlook. The landing is located on the left bank of the river in front of the mouth of a coulee. As you float down the river, look out for a big red barn on your right -- you’ll be getting out on the left just around bend. If you’ve come to an island with cottonwood trees then you’ve gone too far.
To get to the overlook, begin with your back to the river and walk up the hill on your left. As you climb up, Be on the lookout for animal tracks, prickly pear, and little pieces of pink granite (where do you think this rock came from? Think back to the earlier EarthCaches). If you’re lucky you may even spot a deer. Keep walking into the coulee. You should hit an old cow path before long. Follow it up away from the river.
Hoodoo Point
Now that you’re up here take a few minutes to catch your breath and enjoy the view. Pay particular attention to the mushroom shaped rock formations sticking straight up out of the ground nearby. These are called hoodoos. These interesting looking formations form vertical columns capped by pieces of darker looking rock. They form because the top pieces of rock resist erosion better than the bottom pieces, protecting them from the the weather just like an umbrella. Over time, any rock that isn’t underneath the natural roof erodes way, giving rise to the hoodoo’s distinctive shape.
An important question to ask here is why the top piece of rock is so much stronger than the one underneath. The most intuitive answer is that they are different kinds of rock, but this isn’t actually the case. Both are actually sandstone. The key difference is the stuff that’s holding it together, what geologists call cement.
Cement is the difference between a pile of loose sand and a piece of hard sandstone. Around here, most of the sandstone is held together by cement made of calcite. This relatively weak mineral is soft and easily dissolves in acidic water. It can support sand grains pretty well if left undisturbed, but when it gets exposed to the elements -- wind, rain, ice -- it can break down. Luckily for us, the protective capstones on the hoodoos are not held together by this material. Instead, these rocks are cemented by a mineral called hematite. Hematite is a form of iron, the stuff we use to make cars, tools, and heavy machinery, so it’s very strong. This material’s resistance to erosion strengthens the rock and enables it to act as a shield for the ground below it. When a bit of regular sandstone is cemented by a special mineral like hematite, we call it a concretion. Without concretions there would be no hoodoos on the Marias.
Let’s look at some hoodoos and see what they can tell us. Start by facing towards the river and looking ninety degrees to your right. See the hoodoos? What do you notice about the height of all their capstones compared to each other? They’re all the same aren’t they? This is because in this case the hematite holding the tops of these hoodoos together has been put in place as one big layer. It was only worn down into the individual capstones later.
A cool way to think about this is to look straight down at the rock beneath your feet, then look along the edge all the way towards those hoodoos. Notice how it transitions from the big slab you are standing on into capstones on the same plane. It’s because it’s the same exact rock. The only difference is that the area with hoodoos has been exposed to the elements for longer, while the piece you’re on top of was protected by the rock and sediment above it until more recently. In time, however, erosion will do its work on the rocks under foot, too. You are standing on the top of a future hoodoo!
This isn’t to say that hoodoos only form in layers like this. Sometimes the concretions form out on their own in any number of shapes and sizes. Over on the Missouri River, there are cliffs that are full of pockmarks where golfball sized concretions just popped out of the sandstone. Here, there’s a really cool example of a much larger concretion that’s all by itself.. To find it, stand out on the GPS point and look out across the coulee. There should be a very big hoodoo about ten feet in front of you. Walk over to the other side of it and you should see a concretion poking out about halfway up the column. Weird!
Despite the best efforts of the protective concretions, hoodoos don’t last forever. As time goes on, the columns can get so thinned out that they collapse, and the tops can get unbalanced and fall over. The only reason we don’t see broken bits of hoodoo bases everywhere is, like we talked about earlier, how prone they are to erosion. Still, you can get a good idea of how many more hoodoos there used to be here by looking down into the coulee. Each one of those big red boulders used to be the top of a hoodoo. They’ve been forming and disappearing for a long, long time.
To claim this cache: Answer the following question and send the answers using Geocache's messaging tool.
Q: Why are the hoodoo capstones stronger than the surrounding rock?
Q: What stops the elements from destroying the bases of all these hoodoos?