Landforms and Geological Features
of the Southern Colorado Plateau

by Riley Cameron

The purpose of this report is to summarize what Riley learned about the geology of northwestern New Mexico and northern Arizona during a family trip there from October 6 to 13, 2007. His Dad “led the field trip”, “taught the course”, took most of the pictures, and wrote this paragraph and an outline for the report. Riley learned the material and he could identify all the features mentioned below in the field. He wrote the report following the outline and copied the drawings from various sources.

Colorado Plateau

The Colorado Plateau is located in northern Arizona, northwest New Mexico, southwest Utah, and western Colorado. It’s a 5,000 to 7,000 ft high plain underlain by flat lying sedimentary rocks. Common landforms are plateaus, canyons, mesas and related landforms, with rare volcanoes.

Canyons:

Grand Canyon

The flat land surface at the top of the Grand Canyon is the Colorado Plateau. The Colorado River cut the Grand Canyon and the river still runs through it. In the upper parts of the Grand Canyon the rocks are sedimentary and flat lying. We hiked through the Kaibab (ki-bab) Limestone and Coconino ( coco-neno) Sandstone.

Canyon de Chelly

Canyon De Chelly (shay) is also in the Colorado Plateau. Three types of sedimentary rocks are found in the canyon. Conglomerate is a rock with a bunch of little pebbles in it; sandstone is really hard and dried sand formed into a rock; shale is a soft, dark, dirt rock.

Stratigraphy is the order that rocks are laid in. In Canyon De Chelly, the Chinle Shale is the youngest, then Shinarump Conglomerate and the oldest is De Chelly Sandstone. Chinle shale is very colorful kind of redish brown, and it’s also found in the Painted Desert part of Petrified Forest. The Shinarump Conglomorate is brownish. The massive cliffs are reddish De Chelly Sandstone. It has prominate cross-beds (drawing, photo 1, photo 2) indicating it formed in sand dunes. Like those of the Sahara Desert. De Chelly Sandstone is also also in Monument Valley.

Mesas and Related Landforms:

A mesa is formed in flat lying rocks. At the top of a mesa is hard layer of rock and the bottom of the mesa is soft. It looks like a table and a table in Spanish is mesa.

Black Mesa, Arizona

Sentinel Mesa, Utah, Monument Valley

A butte is almost the same but a butte is more square shaped.

Merrick Butte, Monument Valley

A spire/monument is a long peak sticking out of the ground, and it looks like a skinny rectangle. A sliver is the smallest and it’s tiny.

Spires and monuments, Monument Valley

Carving of Monument Valley

In the beginning of monument valley it was just a plateau. Because of erosion little canyons formed in it and they got so big, that all left of the mesa is buttes, monuments, and slivers. The cliffs at the top of the buttes, monuments, and slivers are just made up of De Chelly sandstone. Below the sandstone is soft shale (Organ Rock Shale).


Petrified Forest

The petrified wood is found in the Painted Desert part of the Chinle Shale. Chinle Shale contains volcanic ash which contains silica (Si). The ash and silica dissolve in the ground water. The silica in the water replaces the organic material in the wood. Overtime the silica crystalizes to very fine grained quartz (SiO2). This type of very fine grained quartz with color band of white, red, and gray is called agate.

Volcanic Features:

Rarely do volcanoes occur in the Colorado Plateau, but there are a few. A shield volcano is a volcano with diameter about 5 to 25 miles and the height about several 100 to several 1,000 feet. It’s formed from fluid lava, usually basalt. It has a flanks that slope at low angle. The only shield volcano on the Colorado Plateau is Mount Taylor.

Cinder Cones (photo) are formed from basaltic ash and pebble size fragments. They are several hundred ft high and hundreds of ft in diameter. They have very steep sides!

A volcanic neck happens when there is erosion. When so, the volcano plus some soft shale is removed by erosion. The feeder pipe is really hard so it stays.

Shiprock, New Mexico – a volcanic neck
Agathla Peak, Arizona, near Monument Valley – a volcanic neck.