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August 28, 2023

Geologic maps are a fundamental tool for earth scientists to assemble and communicate geologic information. Under the umbrella of geologic mapping are subcategories of maps that emphasize different types of information, like surficial geologic maps, which are important to understanding Yellowstone’s geology.

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from James Mauch, geologist with the Wyoming State Geological Survey.

It’s hard to overstate the importance of geologic maps to the modern understanding of Yellowstone’s geologic history. After all, it was detailed maps of volcanic rocks on the Yellowstone Plateau that led to the recognition of Yellowstone’s three most recent caldera-forming eruptions. These were classic bedrock geologic maps, meaning they primarily depict rock units and geologic structures that help geologists determine what’s happening underground.

Cutbank exposure near West DuNoir Creek, Wyoming, illustrates the differences between bedrock and surficial geologic mapping
A cutbank exposure near West DuNoir Creek, Wyoming, illustrates the differences between bedrock and surficial geologic mapping. On bedrock maps this location is mapped as the Eocene (55–34 million years old) Indian Meadows Formation, which corresponds to the bedrock visible in the stream bed. On surficial maps this location is mapped as Pinedale-age (22,000-13,000 years old) glacial till, as seen in the 1.5 m (5 feet) thick package of unsorted sediment in the cutbank. Photo by James Mauch, Wyoming State Geological Survey, on September 13, 2022.

Less widely recognized, but similarly important, are surficial geologic maps, which show the distribution of the materials and deposits that are at or within a few meters (yards) of the ground surface. In many landscapes around the world, including Yellowstone, bedrock is only occasionally exposed at the ground surface. Instead, there is often a veneer of unconsolidated material that has weathered in-place (residuum) or that has been transported from somewhere else (sediment). While bedrock maps typically omit this veneer in order to show the underlying, consolidated rock units, the unconsolidated material at the surface holds its own information and is the primary subject of surficial geologic maps.

What can we learn from surficial geologic maps? From a practical perspective, surficial maps provide fundamental information about the earth materials humans are most likely to interact with—that is, those near the ground surface. For instance, surficial maps are used to site roads and pipelines, highlight areas of slope instability, and predict the movement of shallow groundwater contaminants. For scientific purposes, surficial geologic maps give insight into a landscape’s recent geologic history. Information about earth-shaping processes like glaciation, earthquakes, and river incision are all recorded in surficial deposits.

Reading a surficial map is similar to reading a bedrock map in that polygons symbolized with colors and patterns refer to different map units and show their distribution across the study area. Surficial geologic units are typically classified according to a basic deposit type, then subdivided by texture, composition, or age. Points and lines on the map highlight other surficial features important to the geologic interpretation, such as glacial erratics, former lake shorelines, and fault scarps.

There is a strong history of surficial geologic mapping in the Yellowstone region. In the 1970s, Ken Pierce, Gerald Richmond, and Henry Waldrop published 18 surficial geologic maps at 1:62,500 scale that cover Yellowstone National Park. These maps were an enormous undertaking and required many summers of fieldwork in the Yellowstone backcountry. Furthermore, the work laid the foundation for Ken Pierce’s pioneering research on the Quaternary (the past 2.6 million years) geologic history of the Yellowstone landscape and the complex dynamics of the Greater Yellowstone Glacial System.

Geologists today are building upon this legacy. The Wyoming State Geological Survey is nearing completion on a long-term effort to produce 1:100,000-scale surficial geologic maps that cover the entire state, and the agency has recently published several maps in the Greater Yellowstone region. More broadly, surficial geology is undergoing a modern renaissance thanks to the proliferation of lidar data, which gives geologists a detailed view of the ground surface beneath the tree canopy. The recent availability of lidar in Yellowstone has already led to recognition of some spectacular surficial geologic features that were previously unknown, such as landslides and fault scarps, and it promises to fuel research for years to come.

Correlation of Map Units chart from the Surficial Geologic Map of Yellowstone National Park
An example Correlation of Map Units chart from the Surficial Geologic Map of Yellowstone National Park (U.S. Geological Survey, 1972) that demonstrates a typical classification scheme for surficial units. For this map, the units are classified by basic deposit type (horizontal axis) and subdivided by age and glacial interval (vertical axis).

More than a niche offshoot of geologic mapping, surficial mapping is a fundamental part of characterizing a landscape, understanding its recent geologic activity, and ensuring human industries on Earth’s surface are safe and sustainable. Early surficial geologic mapping in Yellowstone was the nucleus for our current understanding of this landscape’s rich Quaternary geology, and ongoing mapping efforts are poised to continue filling in the pages of this unique chapter of geologic history.

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