Water continues to be the major erosional force, thanks to Pleasant Creek, which flows from Boulder Mountain to the west and joins the Fremont River around the community of Notom to the east. Pleasant Creek originates from a spring on Boulder Mountain, and like many of the bodies of water there, does not remain entirely above ground. Water makes its way through cracks in the igneous rock before returning to the surface as a creek.

Pleasant Creek experiences flash floods during summer monsoon rains, and these floods can overflow the banks, carrying sediments, rocks, trees, and other debris downstream. Clear flowing water has some erosional force, but it is magnified when sediment in the water grinds against and wears down the rock layers in the creek. Scientists estimate the erosional force and downcutting of the nearby Fremont River to be about 30 inches (76 cm) per thousand years, which is a fast rate of erosion for the Colorado Plateau.

Rockfalls are another sign of erosion, and in 2019, about 200 feet of vertical Wingate Cliff crashed to the valley floor, just east of the Field Station, along the trail to the petroglyphs. While walking through this section, please do not stop, as the cliffs still have many vertical fractures, and rockfalls can happen at any time. As Wallace Stegner wrote, “Geology knows no such word as forever.”

You might have noticed the many black boulders scattered throughout Pleasant Creek. Where are they from, and what kind of rock are they? They are basalt and andesite boulders, volcanic or igneous rock. About 20 million years ago, lava came to the surface of the earth and filled in low-lying areas, which are now the high points of Boulder and Thousand Lake mountains. All the other higher areas weathered away, but Boulder and Thousand Lake mountains were capped by erosion-resistant volcanic rock. These mountains were tall enough to have small glaciers on them during the last few ice ages, and as the glaciers melted and refroze, they mechanically weathered the basalt into big, jagged chunks.

Within the last 200,000 to 100,000 years, massive landslides rafted these boulders (often still very angular) into what is now Capitol Reef and deposited them on the valley floors. Thanks to continual erosion and the carving of waterways like Pleasant Creek, some of these valleys are now mesas and terraces.

Visible to the east of the field station, in the gap created by Pleasant Creek, sits another outcrop of igneous the rock, the Henry Mountains. These young mountains are a laccolith. An igneous intrusion of magma came up from the center of the earth but did not break the surface. The softer, sedimentary rock above the intrusion eventually weathered away, exposing the hard, igneous rock. According to the Utah Geologic Survey, “a laccolith is a sizable magma body which attempts to come to the surface through a neck or conduit but spreads horizontally between sedimentary layers on its way up, pushing the host rock layers up into a dome shape before cooling beneath the surface.” Timeframes for the intrusion range from 29 to 23 MYA. Geologists continue to study the Henry Mountains, which were the last mountain range to be mapped in the Lower 48 and are home to a free-roaming herd of bison.