Mapping Utah’s Geology
by Grant Willis
In the 1980s geologic mapping was in serious decline—of much concern to oil and gas companies, geologic hazards geologists, and many others. The U.S. Geological Survey (USGS), long the bastion of geologic mapping, had suffered several major budget cuts; many senior mappers retired and younger mappers were laid off—few were ever replaced. Accurate, detailed geologic maps that meet modern standards are essential for land-use, resource-development, and geologic-hazard planning.
Some states, recognizing the large negative impact a lack of up-to-date geologic maps could have on their economies, stepped in and created their own mapping programs—Utah created one of the first. In addition, many state geologists, industry leaders, and others lobbied Congress to fund new geologic mapping. The result was the National Cooperative Geologic Mapping Program, with three parts: FEDMAP—USGS, STATEMAP— state geological surveys, and EDMAP—students and their professors. Now, 23 years later, the results are impressive. Nearly every state and many universities have participated, and the act is reapproved with nearly unanimous support every 10 years. In Utah, this 50:50-match program has funded 168 geologic maps covering about 75 percent of the state (7.5′ and 30′ x 60′ quadrangles and GIS databases).
Soon after STATEMAP started, Utah’s State (geologic) Mapping Advisory Committee (SMAC) recognized that Utah has a unique situation—over 90 percent of our population and growth is concentrated in two areas: the Wasatch Front area and southwest Utah. Of course, these high-growth areas, with their hazard, resource, water, and other geology-related issues, have great need for up-to-date geologic maps. But if all mapping was in these growth areas, then the rest of the state, with equally pressing economic-resource and land-management issues, would go without modern mapping for decades.
SMAC found a compromise—half of the budget would fund detailed (1:24,000 scale) mapping in high-growth areas; the other half would map large rural areas at intermediate detail (1:62,500 to 1:100,000 scale). While not suitable for geologic-hazard technical studies, the intermediate-scale maps meet most resource, recreational, and land-management needs. Now, two decades later, the benefits are clear. We just passed a major milepost in which 75 percent of the state is now covered by intermediate-scale geologic maps. These maps are produced in printed (plot-on-demand) and digital (GIS-geographic information system) formats and are purchased and downloaded by individuals, small companies, large corporations, professors, students, and many government agencies.
Perhaps of greatest use, these maps are posted on an interactive web page (geology.utah.gov/apps/intgeomap) in which the user can quickly view and zoom into hundreds of maps at several scales. Linked GIS data from the 30′ x 60′ series provides a detailed description of the geologic unit. With the addition of our latest 30′ x 60′ map, the Ogden quadrangle, we now have a continuous strip of intermediate-scale maps from St. George to Logan, plus much of the Uinta Basin, Colorado Plateau, and western Utah. In these areas, with the click of a mouse you can get a description of the geology of your neighborhood, work site, research area, or favorite hiking area.
This Survey Notes issue features two of our recent 30′ x 60′ quadrangle maps. The Ogden article tells how some of its complex geology was deciphered, and about the contributions of the new map. The Markagunt gravity slide article adds another chapter to one of the most fascinating geologic discoveries in Utah in several decades—the world’s largest known land-based landslide. This story was unraveled through probing investigations conducted during the Panguitch 30′ x 60′ quadrangle mapping project.
The Topographic Quadrangle Series
Up until the early 1960s, the first step in detailed geologic mapping was usually to make your own topographic base map, which could squander half of your field time before you even drew a geologic line! Fortunately, we now have quality USGS topographic maps on which we build our geologic maps. The USGS produced crude topographic maps of most of the state as early as the 1880s, but it soon shifted to the “series” concept—standard quadrangles at standard scales with standard features (quadrangles follow lines of latitude and longitude and are not rectangles). The first reasonably accurate topographic maps were the 30′ x 30′ series (1:125,000 scale) produced from the 1890s to early 1920s. Following World War II, partially driven by the Cold War, they went on a topographic mapping blitzkrieg! Over the next 40 years they produced almost 2,000 Utah topographic maps, first mainly in the 15′ (1:62,500) and 1° x 2° (1:250,000) series, but gradually shifting to the 7.5′ (1:24,000) series.
The 7.5′ series is now the base for most modern detailed geologic mapping. Utah is divided into 1,512 7.5′ quadrangles (plus a thin sliver of 40 more along the Nevada border); each is about 58 square miles (2 square miles larger near Arizona/smaller near Idaho). Geologic maps have been completed for about 40 percent of these (the first batch in 1952). Unfortunately, we struggle to keep up—methods and expectations have changed so much that fewer than half meet modern standards. Our primary regional maps are done in the 30′ x 60′ series (about 1,850 square miles each); about 34 of 46 quadrangles have a reasonable geologic map, with a few more covered by older “temporary” maps. Today, with nearly all map assembly being done on computers, we can map at any scale appropriate for the area of interest. We also include as much detail as possible even if at “normal” scale the map looks too busy (yes—many map users zoom in far beyond the intended scale!).
In an ironic twist, we have now almost come full circle—just like in the old days, we now often make our own topographic base maps! But now we use USGS digital files of topographic map components, including a ground elevation model. In a matter of minutes we can create a customized topographic map of any area at any scale, with elevation contours at any spacing—in a way, the concept of “scale” has almost become obsolete. Base maps continue to improve—we eagerly await LiDAR (laser-sourced) ground control for our next base maps, which will allow production of even better geologic maps (but, unfortunately, make old maps seem even less accurate).