Join College of Southern Idaho geology professor Shawn Willsey as he investigates and explains a spectacular geologic structure hidden in plain sight in Ogden Canyon in northern Utah.
Ogden Canyon’s Z-Fold, A Geological Marvel
Nestled along Utah Highway 39 in the heart of Ogden Canyon lies one of the most remarkable yet overlooked geological structures in the western United States. Known as the Z-Fold, this massive rock formation near Pine View Reservoir is objectively cool to look and fascinating to geologists like professor Sean Willsey who ventured to the location to provide an in-depth explanation of this stunning feature formed by forces of nature over millions of years.
From a vantage point just downstream of Pine View Reservoir, the Z-fold emerges as a breathtaking display of Earth’s geological history. Spanning roughly 1,400 feet from the canyon floor to the ridgeline, the structure is part of the Humbug Formation, a series of sedimentary rocks laid down approximately 340 million years ago during the Mississippian period. These ancient layers, visible in drone footage of the canyon, tell a story of immense tectonic forces that folded and faulted the landscape into the dramatic shape we see today.
To understand the Z-fold’s formation you have to way back to around 340 million years ago when sediments began accumulated in horizontal layers. These layers remained relatively undisturbed until the Cretaceous period, about 120 million years ago, when a major east-west compressional event called the Sevier Orogeny reshaped the western United States. This tectonic upheaval caused the once-flat sedimentary layers to buckle, forming a series of folds known as anticlines (upward arches with older rocks at the core) and synclines (downward troughs with younger rocks at the center).
As the Sevier Orogeny persisted, the compression intensified. The rocks didn’t just fold, they began to break. A significant thrust fault, known as the Willard Thrust, located just east of the Pine View Reservoir, played a pivotal role. This fault pushed the folded rocks, rotating them dramatically resulting in the striking Z-shaped structure where the central limb of the fold was overturned, placing younger rocks beneath older ones in a configuration that defies their original stratigraphic order.
This Z-fold, composed of an overturned anticline and syncline pair, is a testament to the colossal forces that shaped the region. In the upper part of the formation, near the ridgeline, the rock layers appear nearly horizontal. As you descend toward the canyon floor, the beds steeply rotate, forming the overturned limb that defines the Z-shape. At the base, the layers curve back to a near-horizontal position, completing the fold’s dramatic silhouette.
What makes this geological wonder even more extraordinary is its scale. While small-scale folds are common in rock outcrops, Z-fold of this magnitude are quite rare. Heads up the next time you are driving on Highway 39, take a moment to slow down and scan the rocky slopes near Pine View Reservoir and see this incredible geological for yourself.
