Glad You Asked: What is the Story Behind Lake Powell’s Rocks?

by Lance Weaver

Houseboats parked in Navajo Canyon, amidst towering cliffs of Navajo Sandstone.

Nestled within the Glen Canyon National Recreation Area (GCNRA), Lake Powell is a testament to both human ingenuity and the enduring power of nature. The lake fills the entirety of the roughly 170-mile length of Glen Canyon. Sometimes referred to as Utah’s “sixth national park,” this vast reservoir offers a unique blend of recreation and stunning geological beauty. Its sheer size and intricate shoreline distinguish it from any of Utah’s other lakes, while its geological story reveals a complex and fascinating past.

The first recorded European encounter with Glen Canyon comes from the Domínguez-Escalante Expedition of 1776. Just a few months after Thomas Jefferson signed the Declaration of Independence, the two Spanish explorers spent several weeks being led by Native American guides across a narrow passage through the canyon’s nearly impenetrable walls. Almost one hundred years later in 1869, John Wesley Powell, the lake’s namesake, explored the area by boating down the Colorado River and wrote the first known geologic investigation of the region.

Almost another century later, in 1956, construction began for Glen Canyon Dam near the Utah-Arizona border. Designed to regulate and store water from the Colorado River Basin for six states—Utah, Colorado, Arizona, New Mexico, Nevada, and California—the dam also became a vital source of hydroelectric power. Its construction followed years of debate among government agencies, conservationists, and water resource planners, underscoring its enduring role in regional water management and the ongoing discussions surrounding its impact. The completion of the dam led to the formation of Lake Powell, one of the largest reservoirs in the United States.

Simplified stratigraphic column.

Perhaps Lake Powell’s most unique feature is its expansive cliff-lined shores. At full pool, its shoreline stretches greater than 1,900 miles, a figure that dwarfs the entire west coast of the continental United States. This remarkable length is a product of the flooding of Glen Canyon’s intricate network of slot canyons and translates to a myriad of opportunities for exploration in the 96 major canyons and countless smaller coves and inlets. Adding to its allure are numerous natural features found within or near the recreation area, including iconic natural bridges and arches like Rainbow Bridge, Halls Creek Bridge, Broken Bow Arch, LaGorce Arch and many more in Glen Canyon’s Escalante arm. These natural wonders, sculpted by millennia of erosion, punctuate the landscape and provide a glimpse into the canyon’s ancient geological history.

The geology of Glen Canyon is a story spanning hundreds of millions of years, displaying a continuous sequence of rock layers that range from Pennsylvanian (~320 million years old) to Late Cretaceous (~66 million years old) age. The oldest exposed rock layer, the Pennsylvanian-age Hermosa Group, lies at the base of this geological sequence and is exposed on the flanks of the Monument Upwarp on the northeast end of the lake. Above the Pennsylvanian–Permian-age Cutler Group lies the Early Triassic-age (~250 million years old) Moenkopi Formation, which is visible on both the northernmost and southernmost areas of Glen Canyon National Recreation Area. Overlying the Moenkopi is the Middle to Late Triassic-age (~230 million years old) Chinle Formation, known for its vibrant colors and abundant petrified wood.

Perhaps the most impressive geologic unit in the region is the Early Jurassic-age (~200 to 190 million years old) Glen Canyon Group, a suite of sandstones that dominate the canyon’s landscape and borrows its name. This group consists of the Wingate Sandstone, known for its deep red sheer cliffs; the Kayenta Formation, characterized by its ledges and slopes of interbedded shale and sandstone; and the massive Navajo Sandstone, which forms tall cliffs and slickrock canyons. This sequence of sandstones tells a story of ancient deserts with shifting sand dunes in a setting not unlike that of the modern Sahara Desert (see Survey Notes v. 44, no. 3). The entirety of the southern Utah Cretaceous-age sequence (~145 to 66 million years ago) is also visible from the southern and central parts of the lake, offering a complete picture of the region’s Mesozoic Era history.

Following deposition, burial, and lithification of these rock layers, the Colorado Plateau was uplifted, and the Colorado River and its tributaries began to incise or cut down into the rocks to form the impressive collection of canyons that compose the Glen Canyon National Recreation Area. The majority of modern Glen Canyon’s incision occurred within the last 5.5 million years, during a pulse of rapid uplift on the Colorado Plateau.

Broken Bow Arch, Escalante Canyons section of Glen Canyon National Recreation Area.

Part of what makes the geology of Lake Powell so unique is the juxtaposition of the lake’s completely level surface against the undulating regional geology. As one travels up lake from Glen Canyon dam, geologic rock formations of different ages take turns occupying the shorelines. This unique experience is caused by the regional folding of the Monument and Circle Cliffs upwarps that have uplifted the oldest layers to the surface in the northmost sections of the lake, exposing the Pennsylvanian- and Triassic-age units. Conversely, the Kaiparowits basin brings younger Cretaceous- and Late Jurassic-age layers to lake level. Adding to the geological intrigue are prominent geologic features visible from the lake, called laccoliths, caused by magma rising through the subsurface and bending the overlying rocks into unique dome-like structures seen at Navajo Mountain and the Henry Mountains.

Lake Powell stands as a significant testament to the interwoven forces of nature and human activity. It offers visitors a unique way to traverse Glen Canyon and explore the breathtaking landscapes of the canyon and its fascinating geologic history.