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August 15, 2022

What goes into making a geologic map?  Follow along as geologists investigate a perplexing incongruity in adjoining geologic maps in the north part of Yellowstone National Park.

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Hayley Woodrich, Research Experiences for Undergraduates student, and Natali Kraugh, graduate student, both at Montana State University.

Photograph of the west side of Mount Everts taken from Sepulcher Mountain
Photograph of the west side of Mount Everts taken from Sepulcher Mountain. The linear features seen on the western face are sedimentary rocks running north to south. (Photo by Jess Condon, June 8, 2022)

Geologic map-making is nearly as old as recorded human history; the oldest geologic map—found in Egypt—dates back 1150 BC! Since then, geologists have produced countless maps of areas all around the world. These maps help humans locate critical resources, better understand the landscape, and make forecasts of how our environment may change. Mapping is a dynamic practice, and as more tools become accessible, maps require updating—including in Yellowstone National Park. One example of this is Mt. Everts near Mammoth Hot Springs, named for an early American explorer of the region who is famous for getting lost in the Yellowstone wilderness in 1870, and surviving to tell the tale!

Mt. Everts is currently split by two geologic maps, one to the north covering the Montana portion and one to the south covering the portion in Wyoming. Geologists noticed that the Montana side of Mt Everts was mapped as Archean metamorphic rock—ancient material that has been buried and experienced great pressures and temperatures before it was brought back to the surface by tectonic activity—while the Wyoming side was mapped as Cretaceous sedimentary rock that was deposited in and around a shallow sea over 65 million years ago. That’s over a billion years of discrepancy! Based on a cursory look of the outcrops near Gardiner, Montana, geologists from Montana State University (MSU) speculated that the whole mountain was probably made up of Cretaceous sedimentary rock, as indicated in the Wyoming-side map, but they had to be sure in order to accurately update the Montana side of the map. To better understand how the issue was addressed, let’s take a journey into the field together!

Difference in mapped rock units for Mount Everts, Yellowstone National Park
Simplified geologic maps showing the difference in mapped rock units from the current, large-scale geologic maps dividing Mount Everts and that join along the boundary between Montana and Wyoming. Geologists working on this project spent most of their time gathering data to update the northern (Montana) portion of the Mount Everts map, which currently is mapped as “Archean Schist and Hornfels” but was found to be sedimentary rock.

Before we depart, we need some background information. For this mapping project, the MSU geologists knew there were either sedimentary or metamorphic rocks in the area, which means they were looking for the tell-tale signs of the two: where sedimentary rocks have individual pieces of somewhat rounded sediment that are compacted together in no specific order, metamorphic rocks commonly show elongated grains that align with one another and form an internal fabric. The geologists also referred to older maps and unit descriptions to see which specific rocks they should keep an eye out for.

After all the gear is packed, it’s time to head for the field! Geologists spent three and a half days observing rocks on the northern portion of Mt. Everts. Over this time, they determined that the rocks were indeed sedimentary and appeared to primarily be two distinct units: a dark colored sandstone with interbedded mudstone and persistent volcanic components, and a light gray sandstone with interbedded siltstone and mudstone and a very occasional thin coal bed. To make sure the line between these different rock units was accurately updated, a GPS device and iPad were used to retrieve the precise location of the contact between the two rock types and transfer that information to the geologic map. Now that we have the information we need, it’s time to head back to the office.

At the office, the task is to determine the names of the rock units observed and make corrections to the geologic maps. To identify the rock units, field notes were compared to existing data on geologic units known to be in the area. In this case, it’s lucky that Yellowstone has long been an area of geologic interest, so there is plenty of information to refer to! Ultimately, the dark-colored sandstone was determined to be the Landslide Creek Formation, and the light gray sandstone was identified as the Everts Formation. With this information and the data from the field, contact lines between units were shifted to reflect what was found on the mountain.      

Now you’ve seen what it’s like to identify a mapping issue and head into the field to correct it—without all the hiking of course!

It may seem like an odd mistake in mapping, but this sort of issue is not uncommon.  Geologists from Montana State University identified a number of issues at the boundaries of geologic maps in the Yellowstone region.  These issues are often caused by different geologists mapping different areas and having different interpretations.  But that invaluable early mapping laid the groundwork for how we understand the geologic history of the Yellowstone region.  Refining the maps, as done at Mt. Everts, will help with improving our understanding of that geologic history.  And there is an awful lot of geologic history in and around Yellowstone National Park!

The two dominant rock types found on Mount Everts, Yellowstone National Park
Photographs of the two dominant rock types found on Mount Everts: the Everts Formation (photo by Natali Kragh, May 18, 2021) and the Landslide Creek Formation (photo by Emma Kerins, May 2021). Notice the difference in scale between these two units, indicated by the pencil and field book.

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