Hickman Bridge is located in Capitol Reef National Park in Utah. The bridge is a prominent landmark, standing over 125 feet high with a span of 133 feet. To understand how this impressive structure was formed, it is necessary to examine the material from which it is composed. This analysis reveals the ancient geological history captured within the rock itself.
Identifying the Primary Rock Type
The rock unit that forms Hickman Bridge is a type of sedimentary rock known as sandstone. This formation originates from the Jurassic period, spanning roughly 201 to 145 million years ago. Geologists identify the specific layer forming the bridge as the Navajo Sandstone, a vast deposit dominating the scenery across the Colorado Plateau. Hickman Bridge is classified as a natural bridge because it was formed primarily by flowing water erosion, unlike a natural arch which is generally formed by weathering and gravity.
Distinctive Properties of the Rock Layer
Composition and Structure
The Navajo Sandstone began as an accumulation of wind-blown sand, creating one of the largest ancient dune fields, or ergs. The rock is predominantly composed of well-sorted, fine-to-medium-sized grains of quartz, which were rounded by continuous wind action. The characteristic massive, sweeping curves visible in the rock, known as cross-bedding, are the fossilized remnants of these ancient migrating sand dunes.
Durability and Color
The strength and durability of the sandstone come from the cementing agents binding these quartz grains together. The cement is often a mixture of silica or calcium carbonate, which fills the pore spaces and creates a rigid, enduring rock. The rock’s typical color ranges from white to light tan or pink, a hue often caused by trace amounts of iron oxides, such as hematite. This combination allows the rock to withstand weathering forces that crumble softer surrounding layers.
How the Natural Bridge Was Formed
Initial Erosion
The formation of Hickman Bridge required the durable Navajo Sandstone to interact with the force of moving water. The process began as water, likely from an ancestral stream, exploited existing weaknesses in a thin, vertical wall of rock called a fin. These weaknesses were typically structural fractures, or joints, that formed perpendicular to the canyon walls. The water flow concentrated along these joints, slowly dissolving the cementing material and carrying away loose sand grains in a process known as fluvial erosion.
Tunneling and Span Creation
As the water found a path through the fin, it began to undercut the rock, creating a small tunnel. The erosive action was accelerated by the presence of softer layers of rock or shale directly beneath the more resistant sandstone layer. Over time, this tunneling action enlarged the opening. The resistant, strongly cemented sandstone layer above remained intact, spanning the gap and forming the bridge structure. The resulting 133-foot span is a testament to the rock’s structural integrity.