All rocks in the Monument are sedimentary (made from sand/mud/etc.) except for the bottom-most layer, which is metamorphic. Picture the changes to this place by using the rock layers. This landscape has seen floodplains, desert sand dunes, intertwining ("braided") streams, beach sands, woodland streams, and lakes. And, thanks to shifting of the earth's crust, the land slowly moved from near the equator to its present location.
How to Make a Rock Layer
All rock layers have a pre-rock environment. For example, some layers of rock are made of sand from ancient dunes (an example of sedimentary rock). Other layers were once magma within the earth's crust (like how some igneous rocks form). Some used to be other rocks that were squeezed and heated so intensely that even their chemical structure changed (metamorphic rocks).
Top-Down, or Bottom-Up?
The oldest rock layers are at the bottom because they were laid down first. The landscape changed across the eons, adding more layers on top of older ones. The youngest rock layers are on top of the rest. Think of adding to a pile of laundry. You put some clothes down first, and as you add more clothes on top, the weight presses on what's below. The top of the pile is whatever clothing you added last. To grasp how the pile of laundry (or rock layers) changed over time, it makes sense to start at the bottom and see what got added above.
We'll read this diagram - and scroll the rest of this page - from top to bottom because that's how we read pages. But, to understand the geologic story in order, we should consider reading from the bottom up. Starting at the top with most recent rock layers, the diagram shows that the Morrison layer formed in ancient lakes and forests. Continuing down in layers, you can see the Entrada was once a sandy beach. Below that, the Kayenta is revealed as braided streams. Then, the Wingate sandstone was once a vast sandy desert. Lower still is the Chinle layer, which was once a floodplain. At the very bottom, the Precambrian layer was once the core of a mountain range.
Burro Canyon Formation
140 million years old
100 feet thick
Sandstone, conglomerate, and green mudstone
This layer consists of stream and floodplain deposits. It's most easily seen on Black Ridge. Petrified wood and dinosaur bones have been found in this layer in areas outside the Monument.
150 million years old Brushy Basin Member: 305 feet thick, Red, green, purple and gray mudstone and bentonites with occasional sandstone lenses
The uppermost part of the Morrison is composed of stream and floodplain deposits and layers of volcanic ash that spewed out of ancient volcanoes west of here. Salt Wash Member: 93 feet thick, Thick channel sandstone with minor red to green floodplain mudstone
An ancient delta extended deep into a prehistoric lake. The main stream channels were able to expand across the area and became this part of the Morrison. Tidwell Member: 134 feet, Interbedded sandstone & green mudstone with thin limestone lenses
This layer was formed as a delta built out into a shallow lake.
The Morrison formation is known worldwide for containing abundant dinosaur fossils, including what you find at nearby Dinosaur National Monument. It is illegal to remove fossils and artifacts from public lands, but if you find anything let a ranger know. Every piece of ancient evidence helps put together the story of this place as it changes over time. Here, the Morrison rock layers are seen mostly in the upper elevations along Rim Rock Drive and as you hike Black Ridge.
160 million years old
31 feet thick
Thin red mudstone, sandstone, and green shale
Lake and stream deposits mark the transition from inland sand dunes to stream deltas. This entire layer is exposed on the roadside at Artists Point overlook.
165 million years old
151 feet thick, Salmon-colored eolian (wind-deposited) sandstone with white bedded unit on top Board Beds Member: Thinner, lighter colored rock Slick Rock Member: Wide, smooth and wavy rock, often with oddly shaped holes in it. This is the same layer that makes most of the arches at Arches National Park and the nearby Rattlesnake Arches in McInnis Canyons National Conservation Area.
This is the smooth, more rounded layer that sits next to or above much of Rim Rock Drive. The Alcove Nature Trail, across from Saddlehorn Visitor Center, ends in an alcove of Entrada sandstone. The Saddlehorn rock formation itself is composed of Entrada sandstone.
190 million years old
76 feet think
Sandstone with minor shale and conglomerates
This is the typical "cap" rock on many of our monoliths and named formations, such as the thin, flat, top layer of Independence Monument. Because it is harder than many of the other layers and forms a moderately flat surface, much of Rim Rock Drive, including Saddlehorn Campground, was constructed on this layer.
200 million years old
329 feet thick
Wind-blown ("eolian") sandstone with large, sweeping crossbeds
This is the thickest layer that forms many of our steepest cliffs. It often has darker and lighter colored stripes across it. All three tunnels in the Monument pass through this layer.
210 million years old
88 feet thick
Red mudstone, shale, conglomerate, and thin limestones
This dark brick-red layers is often seen at the bottom of cliffs, underneath the lighter colored Wingate sandstone. This layer washes away ("erodes") more easily than the Wingate, which can cause large chunks of upper layers to tumble down. "Fallen Rock" seen from Rim Rock Drive fell because of the Chinle formation eroding faster than the layers above.
1.7-1.5 billion years old
Basement rocks, composed of gneiss, schist, and granites
This dark, grayish/purplish layer can be seen at the bottoms of almost every canyon. Rim Rock Drive passes through this layer near both the East and West entrances.