Abstract The Bridger Formation is located in the Green River basin in southwest Wyoming, and the Uinta and Duchesne River formations are located in the Uinta basin in Utah. These three rock units and their diverse fossil assemblages have great scientific importance and are also of historic interest to vertebrate paleontologists. Notably, they are also the stratotypes for the three middle Eocene North American Land Mammal “Ages,” the Bridgerian, Uintan, and Duchesnean, from oldest to youngest. The fossils and sediments of these formations provide a critically important record of biotic, environmental, and climatic history spanning ~10 million years (49–39 Ma). This article features a detailed field excursion through portions of the Green River and Uinta basins that focuses on locations of geologic, paleontologic, and historical interest. In support of the field excursion, we also provide a review of current knowledge of these formations with emphasis on lithostratigraphy, biochronology, depositional and paleoenvironmental history, and the history of scientific exploration.

Abstract The central Colorado landscape bears a strong imprint of post-Laramide (late Eocene to Quaternary) tectonics, volcanism, climate change, and drainage rearrangement. This field trip will examine the post-Laramide evolution of central Colorado, traversing the Front Range, from the Colorado Piedmont on the east to the upper Arkansas valley segment of the Rio Grande Rift on the west (Fig. 1 ). The first day of the trip will involve a transect from the Denver-Colorado Springs section of the Piedmont across the southern Front Range, South Park, and Mosquito Range to the upper Arkansas valley. On this day we will focus on questions concerning the roles of tectonics and climate in driving post-Laramide landscape changes, examining structural, sedimentological, paleontological, geomorphic, and fission track evidence that has been used to reconstruct post-Laramide history. We will end the day with an initial overview of rift-related structures, sediments, and geomorphology as we enter the upper Arkansas valley. We will spend the second day in the southern portion of the upper Arkansas valley and the adjacent Poncha Pass transfer zone, examining structural and sedimentological evidence for the nature and timing of Neogene and Quaternary faulting and graben formation, and the character of the transfer zone. On our final day we will traverse back to the Piedmont, this time traveling down the canyons of the Arkansas River. We will examine rift-related structures and sediments in the Pleasant Valley graben and at the northern end of the Wet Mountain Valley, and will discuss the history of Cenozoic and earlier faulting in the area, the evolution of the Arkansas River drainage, and its recent downcutting history. We will end the trip with a discussion of the Neogene and Quaternary erosional history of the High Plains and Piedmont, and possible implications of this history for the driving mechanisms of landscape change.

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