The Ancient Desert

Q. How do we know this was a desert?

A. We can tell that much of this sandstone is made up of dune deposits because it displays enormous cross-bedding. Cross-bedding is the set of swooping lines you can see as you look at an outcrop, and represents the steep sides of the dunes, preserved as the wind drove the dunes across the desert. The constant flow of any kind of fluid can form cross-beds, but ones this size can only be formed by wind.

 

Drawing of how wind blows sand across the backside of a dune and then deposits grains on the steep face of the dune to keep the dune migrating in the desert.

National Park Service

 

Cross-bedding found in the Glen Canyon Formation at Dinosaur National Monument.

National Park Service

As you explore Dinosaur National Monument, keep an eye out for cross-bedded rocks. You might find that they're more common than you think!

 

National Park Service

Q. Where did the animals and plants that lived in this desert get their water?

A. In addition to dune deposits, the Glen Canyon also contains thin, horizontal layers of rock interspersed between the cross-bedded layers. These rocks were deposited in wetter areas between the dunes, which allowed animals and plants to survive, even in the middle of a desert. That doesn't necessarily mean lakes; even a dry-looking spot with water a couple inches below the surface could support plants, as well as animals who knew to dig for it. But there was probably a little standing water as well. It might be strange to picture open water in the middle of a desert, but there is actually a place on Earth right now that might be similar to the environment represented in the Glen Canyon: the Badain Jaran desert in Inner Mongolia. Its dunes are 500 meters high, and in some areas there are small lakes between the dunes. You can check it out on Google Earth: the coordinates of this picture are 39°50'N, 102°30'E.

 

Q. How do we know there was water?

A. The interdunal deposits in the Glen Canyon are generally reddish and are made up of sand, mud, or a combination of both. We think there was open water because of the mud. Mud is so fine-grained that if it never got wet, it would be just like dust. And remember that the huge cross-beds were formed by strong and constant wind; particles that small would have blown away if they weren't wet. So we know there was at least enough water to keep the mud particles stuck to the ground.

 

National Park Service

Q. Where did the water come from?

A. Glen Canyon have found limestone spring mounds. These form when mineral-saturated water flows continuously out of a spring. The minerals gradually precipitate in layers near and around the source, and eventually they form gentle mounds like the ones visible in the photo. Limestone is rather uncommon in the Glen Canyon, and these types of structures are not typical of limestone in general. But impressions of little snail shells preserved in the limestone confirm that there was fresh water here when the limestone was being formed.

 

A paleontologist in the park studies the limestone spring mound layer from which the snail shells above were found.

National Park Service

 

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