Around Moab UT you can run down sand dunes and you can hike on rocks that were sand dunes in the past. You can take a face-plant on one, as my son did, and not get hurt but you better be very careful not to fall on the other. I recently spent a week in Moab UT with my family and while there I took many pictures of geological formations and the flora and fauna of the area. Over the next few months I will be sharing some of those images as part of a Natural History Photography series. These posts will include less commentary than usual. I would remind you that clicking on the images will open a higher-resolution version of the image. If you are interested in the full resolution images (6000 pixels wide) please email me.
Just above Moab UT to the east is a famous mountain biking and 4×4 area called the slickrock region. The slickrocks are in fact, petrified sand dunes (sand dunes that have solidified into rock) which have been revealed when the material above them eroded. Above is a picture of some of the slickrocks looking toward the north. In the far distance some of you might be able to see the heads of the Three Gossips formation or Courthouse Rock in Arches National Park.
The image above is one of my favorite pictures of a rock that I took while in the Moab region. This mound was right by the side of the road and I thought captured the characteristics that I was seeing in the sandstone rocks formations of the area. The man layers of sand are obviously sloping in different directions. You can look at this and just imagine a sand dune being eroded partly and then a new sand dune moving along from a different direction and piling on top of the old dune. These formations are difficult to image having been formed under water, especially when the types of fossils found rarely in them suggest a terrestrial formation. The current background I am using for the blog is from a similar formation found in Zion National Park and shows the same sort of layers that meet and different angles.
Here is a close up of a mound that was just to the right of the one I showed in the prior image. The contact points between one eroded sand dune and the layers from an overlying sand dune are often very sharp. In the larger version you can trace the layers and see that unlike layers that form at the bottom of a lake, these layers are not all parallel nor do they continue but often meet each other. Imagine a large sand dune and you push a bit of sand down from the top, as the sand run down the dune it spreads out and eventually thins to the point that it stops flowing. As a dune advances it can experience hundreds and thousands of this little slumps as well as the wind-blown layers that organize the sand grains very precisely by size.
The image above shows the fossilized sand dunes as they are expressed in Arches National Park. What I like about this image is the small pinnacle of rock in the upper center. It is at least 100 feet high and represents the remains of what was a layer of rock that once covered this entire area. That sandstone has eroded leaving the more erosion-resistant fossil sand dunes that we see today. In the upper left are more of that rock that once lay over this entire area. It is in those sandstone rocks that the arches and other monuments of Arches National Park are found.
Hi! I commented another place, but this is the article I stumbled upon. I’m trying to figure out how sand dunes get fossilized in the first place. I feel like I’m missing something obvious and wikipedia has no answer for me, haha. How do that many sand dunes get covered in stuff (is it dirt, limestone, etc), in order to get fossilized? How does that much dirt move fast enough to cover them without the top parts of the sand dunes getting blown away?
Great pics! Again, I feel like I’m missing something obvious. I’m not a formally-trained geologist but I did play with dirt, water, and rocks a lot as a kid and I know this would be a pretty fantastic feat.