Ancient islands


LOGAN, UTAH–Today we explored the area around Promontory, in northern Utah. Among the many beautiful sites were these rocky, faceted hills that several thousand years ago. This particular hill was intriguing although inaccessible. (It is on a vast tract of land owned by the rocket company ATK. Trespassing is discouraged and no doubt at times very discouraging!) You can see this feature on Google Maps at 41° 37′ 21.11” N and 112° 21′ 42.28” W. Note the cone shape of the top that appears to be sitting on a flat layer beneath. That flatness is a beach terrace of ancient Lake Bonneville. It was formed 14,500 to 16000 years ago by wave action eroding away the hillsides, with the pinnacle exposed above water. (I love wave-cut terraces. Wooster Geologists have noted them before in this blog.)

Screen shot 2013-05-28 at 9.57.17 AM

Here is a Google Earth view of the scene from the south and above. You can see the terrace cut deeply into the hill and extending to the sides. The highway below is where we stopped for the top photograph. Now note that curvy structure of rocks in the lower right of the above image. A close-up of it is shown below.

Screen shot 2013-05-28 at 10.00.08 AM

This is a plunging fold. I can’t tell if it is a plunging anticline or syncline because I couldn’t visit it. You can make it out on the lower right of my photograph at the top of the page. These rocks are mantled with sediments from Lake Bonneville. In this case the sediments are coarse sand and gravels because of the lake energy at this shoreline. The exposed rocks are limestones, probably from the Paleozoic.


On the other side of the valley, on the northwest end of the Wellsville Mountains, two terraces can be seen. The top one against the mountain is the 14500-16000 year-old one we saw throughout the Promontory region. The lower one is younger and made after lake levels dropped precipitously following a catastrophic flood through Red Rock Pass in southern Idaho (which I visited three years ago and recorded in this blog). Note on the lower left that it is being mined for sand and gravel. We see this throughout the area because these terrace deposits are so well sorted and useful in making concrete, building roadbeds, and the like. I learned recently that these kinds of deposits are in a category called “alloformations” because they are laid on top and against much older units.

Oh yes, and what else happened in Promontory, Utah? May 10, 1869!


About Mark Wilson

Mark Wilson is a Professor of Geology at The College of Wooster. He specializes in invertebrate paleontology, carbonate sedimentology, and stratigraphy. He also is an expert on pseudoscience, especially creationism.
This entry was posted in Uncategorized and tagged , , . Bookmark the permalink.

One Response to Ancient islands

  1. mpollock says:

    It’s too bad you couldn’t access the site. I bet Dr. Judge would love to take some measurements. Fantastic scenery, by the way. Hope the rest of your trip goes well!

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.