Video: Tanya Atwater
Laccoliths are mushroom-shaped bodies with a flat floor and a domed roof. Thus, they appear to have begun forming in the same way as sills; however, as magma continued to intrude, it pushed up the overlying layers rather than continuing to spread out laterally.
Ideal laccolith — modified from image by Gilbert (1877) in Corry (1988).
Colorado has many laccoliths and is the first place in the world where they were recognized and named. Corry (1988) lists more than 80 by name and location. Many of these might be questioned as to how good an example they might be. But, the point is — Colorado has a lot of laccoliths!
A large field of mid-Tertiary intrusions form a geographic triangle in Colorado where many laccoliths intruded.
The laccolith triangle extends from Carbondale on the north to Crawford on the southwest to Crested Butte on the southeast. This area is one of the best places in Colorado for viewing laccoliths.
Annotated view of the notable intrusions of the “Laccolith Triangle” – Click on the image to enlarge.
This area is also noted for its anthracite coal — the highest grade of coal. The laccoliths are the principal reason for the creation of the anthracite grade of coal because the heat from the intrusions baked the bituminous coal into anthracite coal.
The heat from one of the laccolith intrusions converted the dingy-looking Leadville Limestone into pure, white Yule Marble, Colorado’s State Rock. Click to learn about the metamorphism of Leadville Limestone into the beautiful Yule Marble.

Some of the most spectacular scenery in Colorado is found in mountains created by laccolith formation. Click on the images or links below for larger views of some of Colorado’s many laccoliths:

Gothic Mountain is a place where the flat floor of the laccolith is exposed. A sill below the laccolith is also prominent, intruding parallel to layers of Cretaceous shales in the Mancos formation.
The resistant Marcellina Mountain is a nice example of the domed shape of a laccolith. In plan view, it is nearly symmetrical.
This iconic image looking down the Roaring Fork Valley from Glenwood Springs is Mount Sopris, a laccolith.
Aerial photo of the snow-capped Mount Sopris laccolith.
The Mount Sopris laccolith bowed up the sedimentary rocks into which it intruded. This image shows steeply dipping, white and red strata on the west side of Mount Sopris. Note that the slope of the upper grayish rock of the uniform igneous intrusive is parallel to the bowed up sedimentary strata.
The red and yellow strata were bowed up by the Mount Sopris laccolith. Here they are dipping to the south. The shrub-covered slopes in the foreground are composed of the igneous rocks of the laccolith.
The Sleeping Ute Mountain profile as seen from Utah illustrates how it got its name.
The Sleeping Ute laccolithic complex is isolated in a sea of Cretaceous sedimentary rocks just east of Cortez.
The West Toe of Sleeping Ute Mountain is interpreted to be a bysmalith — a special type of laccolith that has penetrated its roof.
West Beckwith Mountain on the right is quite symmetrical whereas East Beckwith Mountain in the left background is more elongated.
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West Beckwith (left) and Mt. Gunnison (right) are prominent laccoliths that can be viewed from the Kebler Pass Road between Paonia and Crested Butte.
The Raggeds are the southwestern edge of a large laccolith south of Redstone.
The Tater Heap laccolith has baked shales bowed up along its margins.
This closeup of the porphyritic rock of the laccolith near Crawford is typical of the igneous rocks of the “Triangle of Laccoliths”.