Mapping mayhem: How to solve a Yellowstone National Park puzzle with unknown pieces

The making of geological maps is almost as old as recorded human history; The oldest geological map – found in Egypt – dates back to 1150 BC

Since then, geologists have created countless maps of areas around the world. These maps help people locate critical resources, better understand the landscape, and make predictions about how our environment might change.

Mapping is a dynamic practice, and as more tools become available, more maps need updating—including in Yellowstone National Park. An example of this near Mammoth Hot Springs is Mount Everts, named for an early American explorer of the region famous for getting lost in the Yellowstone wilderness in the 1870’s. He survived to tell the story.

Mount Everts is divided by two geological maps, one to the north covering the Montana portion and one to the south covering the Wyoming portion.

Geologists noted that the Montana side of Mount Evert has been mapped as Archean metamorphic rocks – ancient material that was buried and subjected to great pressures and temperatures before being brought back to the surface by tectonic activity. The Wyoming side has been mapped as Cretaceous sedimentary rocks 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 at the outcrops near Gardiner, Montana, Montana State University geologists speculated that the entire mountain was likely composed of Cretaceous sedimentary rocks as indicated on the Wyoming map, but they had to be sure they were accurate updated the Montana side of the map.

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To better understand how the problem was addressed, let’s take a trip into the field together!

Yellowstone geological map
These simplified geological maps show the difference in mapped rock units from the current, large-scale geological maps dividing Mount Everts and those converging along the Montana-Wyoming border. Geologists working on this project spent most of their time collecting data to update the northern (Montana) portion of the Mount Everts map, which is currently mapped as “Archean Shale and Hornfels” but turned out to be sedimentary rock . (Courtesy of Yellowstone Caldera Chronicles)

Before we leave, we need some background information. For this mapping project, Montana State geologists knew that the area contained either sedimentary rocks or metamorphic rocks. That said, they were looking for the telltale signs of the two: while sedimentary rocks show individual pieces of somewhat rounded sediment pressed together in no particular order, metamorphic rocks often show elongated grains that line up with one another and form an inner tissue. The geologists also consulted older maps and tool logs to see which rocks to look for.

With all gear packed, it was time to head to the field. Geologists spent three and a half days observing rocks on the northern part of Mount Evert. Over this time they determined that the rocks were actually sedimentary and appeared to be composed primarily of two distinct units: a dark colored sandstone with embedded mudstone and persistent volcanic components, and a light gray sandstone with embedded siltstone and mudstone and a very occasional thin bed of coal . To ensure that the line between these different rock units was accurately updated, a GPS device and an iPad were used to retrieve the precise location of the contact between the two rock types and transfer this information to the geological map.

Now that we had the information we needed, it was time to head back to the office.

In the office, the task was to determine the names of the observed rock units and to make corrections to the geological maps. To identify the rock units, field notes were compared to existing data on geological units known to be in the area. In that case, it’s fortunate that Yellowstone has long been an area of ​​geological interest, so there’s plenty of information to refer to.

Ultimately, the dark colored sandstone was identified as the Landslide Creek Formation and the light gray sandstone as the Everts Formation. With this information and the data from the field, the contact lines between the units have been shifted to reflect the finds on the mountain.

Yellowstone rock formations
These are photos of the two predominant rock types found on Mount Everts: the Everts Formation (courtesy Natali Kragh) and the Landslide Creek Formation (courtesy Emma Kerins). Note the difference in scale between these two units, indicated by the pencil and field book.

Now you’ve seen what it’s like to identify a mapping problem and get into the field to fix it—without all the hiking, of course.

It might seem like a weird mapping error, but this type of problem is not uncommon.

Montana geologists have identified a number of problems at geological map boundaries in the Yellowstone region. These problems are often caused by different geologists mapping different areas and having different interpretations. But this invaluable early mapping laid the foundation for how we understand the geological history of the Yellowstone region. Map refinement, such as that carried out at Mount Everts, will help improve our understanding of this geological history.

And there’s a lot of geological history in and around Yellowstone National Park!

Yellowstone Caldera Chronicles is a weekly column written by scientists and staff at Yellowstone Volcano Observatory.

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