Monumental Rocks

Instructions: Select each rock formation layer, starting with the oldest rocks on the bottom of the column, moving up the column to the youngest rocks on the top. By the time you reach the top of the column you will have read a geology story that extends for nearly two billion years and continues to be written today.

Introduction

The rocks of Colorado National Monument record a fascinating story of mountain building, erosion, and changing climates as the continent of North America gradually moved northward toward its present position.

You can read that story, chapter by chapter, by clicking on each rock layer shown in the stratigraphic column.

Burro Canyon Rock Formation
Burro Canyon Formation — about 140 million years old

The youngest layer of rock still present in the monument is characterized by sandstone, conglomerate, and green shale that formed from stream and floodplain deposits. It is only found on Black Ridge in the westernmost reaches of the monument.

Morrison Formation
Morrison Formation — about 150 million years old

This rock layer (along with the slightly older Wanakah Formation) was deposited through the accumulation of sediment in shallow lakes, streambeds, and floodplains. The red, green, purple, and gray mudstones and sandstones formed in a temperate climate, at a time when dinosaurs thrived. Dinosaur bones have been found in the Morrison Formation at several locations both within and outside the monument.

Entrada Sandstone Formation
Entrada Sandstone — about 165 million years old

The salmon-colored Entrada Sandstone was deposited by wind, but in a climate not as arid as before. It preserves ancient sand dunes that migrated from the shores of an inland sea located in central Utah.

Kayenta Formation
Kayenta Formation — about 190 million years old

The sparkly tan and red Kayenta forms a protective caprock over the Wingate sandstone cliffs. This stronger layer formed when rainfall was more abundant and streams braided (criss-crossed) over wide floodplains.

Wingate Sandstone Formation
Wingate Sandstone — about 200 million years old

As the continent slowly drifted northward, the climate changed and desert conditions prevailed. The towering cliffs of the wind-deposited Wingate Sandstone preserve the remnants of sand dunes formed in a great sand sea, one much like today’s Sahara Desert.

Chinle Formation
Chinle Formation — about 210 million years old

The lowest and oldest layer of sedimentary rock is the Chinle Formation. These brick-red rocks form the base of the monument’s dramatic cliffs. Comprised chiefly of red stream and floodplain deposits, the crumbly Chinle mudstone records a time when this area was close to the equator.

Black Canyon Group
Black Canyon Group — 1.7 billion years old

The dark-colored rock at the bottom of the canyons is Precambrian in age, about 1.7 billion years old. These rocks were originally sedimentary rocks, but over time heat and compression changed them into metamorphic and igneous rocks (gneiss, schist, and granite).

There is a huge gap in the geologic record at the contact of the rocks of the Black Canyon Group with the overlying red sedimentary rocks. Erosion removed all traces of several ancient mountain ranges, erasing several chapters of our geologic story. This left a gap of about 1.5 billion years! Scientists refer to this gap in the geologic record as The Great Unconformity.