Goblin Valley State Park in Utah presents one of the most distinctive landscapes in the American Southwest, featuring a valley floor densely populated with thousands of strange, mushroom-shaped rock formations called “goblins” or hoodoos. These features, ranging from small knobs to formations several yards high, create a unique, sculpted appearance. This landscape is the result of a multi-stage geological process spanning approximately 170 million years.
The Source Material: Deposition of Entrada Sandstone
The foundation for Goblin Valley’s strange figures is the Entrada Sandstone, a rock layer deposited during the Middle Jurassic Period, roughly 170 million years ago. At that time, the region was not the arid desert seen today but a transitional coastal environment. This area sat near the margin of an ancient shallow sea, leading to a mix of environments like tidal flats, beaches, and coastal sand dunes.
The Entrada Sandstone reflects this mixed origin, consisting of fine-grained sediments, including sand, silt, and clay. Unlike the massive sandstone cliffs seen nearby, the Entrada here is characterized by alternating, poorly cemented layers of siltstone and shale. This weak internal stratification made the rock susceptible to later shaping into the goblin forms. Over millions of years, the weight of overlying sediment compressed and cemented these fine grains into the soft rock that formed the valley floor.
Tectonic Forces and Exposure
Before erosion could begin its work, massive geological forces acted upon the deeply buried Entrada Sandstone. This process was tied to the regional uplift of the entire Colorado Plateau, which began tens of millions of years ago. A specific feature known as the San Rafael Swell, a massive dome-shaped uplift, raised the horizontal rock layers in the Goblin Valley area. Without this significant vertical movement, the Jurassic-age rock would have remained hundreds of feet beneath younger deposits.
This uplift placed stress on the Entrada Sandstone, causing it to flex and fracture. This resulted in a network of vertical joints running through the rock layers. These fractures were inherent weaknesses created by the movement of the Earth’s crust, not by erosion. These zones provided pathways that allowed water and air to penetrate the solid rock and begin the weathering process.
Differential Weathering: Sculpting the Goblins
The final and most visually dramatic stage is the sculpting of the rock through differential weathering and erosion. Differential weathering describes the uneven breakdown of rock layers that possess varying degrees of hardness or resistance. In Goblin Valley, this process exploits the soft, poorly cemented nature of the Entrada Sandstone and the pre-existing vertical fractures.
Water, freezing and thawing cycles, and wind are the primary agents of this ongoing erosion. Water seeping into the joints widens the cracks, leaving isolated columns of rock. During cold periods, water expands as it freezes, exerting powerful pressure that pries the rock apart from within, a process called ice wedging. This attack from all sides causes the rock to round into the characteristic spherical shapes, a phenomenon known as spheroidal weathering.
The distinctive mushroom shape of the goblins is preserved by caprock protection. A harder, erosion-resistant layer of sandstone sits atop the softer siltstone and shale layers below. This cap shields the softer material beneath it from the impact of rain and wind abrasion. As the surrounding, unprotected soft rock erodes quickly, the protected column narrows, creating the pedestal neck and the large, rounded “head” of the goblin. The shaping process is continuous, meaning the thousands of goblins seen today are constantly shrinking and will eventually erode away completely.