To travel through Arches National Park is to view a 200-million-year slice of history through rocks. Four strata make up the iconic rock formations of the park. As we travel through time from the oldest rock strata to youngest rock strata we see Navajo Sandstone, the Dewey Bridge Member of the Carmel Formation, the Slick Rock Member of Entrada Sandstone, and the Moab Member of the Curtis Formation. All four layers were deposited during the Jurassic Period.
Navajo Sandstone runs from the Visitor Center to The Windows Section. At one time, this layer was a field of dunes. Time and force compressed the dunes, and calcium carbonate cemented them together. Petrified Dunes viewpoint is a great place to see this layer.
Ancient tidal flats from a long gone shoreline make up the Dewey Bridge Member. This is the chocolate colored base of most of the park's towers, pinnacles, and walls. This muddy sandstone erodes irregularly, giving it a "wavy" appearance. This layer often erodes faster than the Entrada Sandstone above it, creating top heavy "mushroom" rocks. Balanced Rock is a great example of this.
The Slick Rock Member of Entrada Sandstone represents coastal dunes. Created around 140 million years ago, this is the layer that contains most of the park’s arches. This layer has only had exposure to erosion for the past 2 million years. But, this layer is prone to vertical fracturing and its joints weather easily. Water seeps into fractures and dissolves the calcite cement holding the sandstone together.
This is the beginning stages of the making of arches. As the sandstone erodes, openings form through fins. As time goes on this opening can become an arch. Prominent features in Entrada Sandstone include Delicate Arch, Landscape Arch and the Three Gossips.
The Moab Member of the Curtis Formation is made of marine modified dunes, and is often characterized by cross-bedding and a light color. It is found at the tops of walls and fins in the Devils Garden and also the upper part of Delicate Arch.
This sequence shows the rock layers from youngest (top) to oldest (bottom). For clarity, some layers have been included that are not found within the park, but are visible from locations near the park boundary, such as the visitor center.
|Name of Rock Stratum
|Millions of Years Ago
|Abajo, Henrys and La Sal Mountains
|1.6 to 66
|66 to 144
|Cedar Mountain Formation
|Morrison Formation, Brushy Basin Member
|144 to 208
|Morrison Formation, Salt Wash Member
|Morrison Formation, Tidwell Member
|Curtis Formation, Moab Member
|Entrada Sandstone, Slick Rock Member
|Carmel Formation, Dewey Bridge Member
|208 to 245
|Cutler Group, Undivided
|245 to 286
|Honaker Trail Formation
|286 to 320