The northern and southern West Elk Mountains have contrasting geologic histories and surface features. In the north, the prominent peaks are laccoliths, formed when magma intruded into Mancos Shale about 30 million years go. Since then, the overlying Mesozoic sedimentary rock, including the relatively soft Mancos Shale, has eroded away, exposing the laccoliths. Laccoliths in the West Elk Mountains include Marcellina Mountain, Mount Gunnison, East Beckwith Mountain, the Anthracite Range, Mount Axtell, Carbon Peak, and Whetstone Mountain.[1][2]
In contrast, volcanic rocks dominate the southern portion of the range. Shortly after the laccolith intrusions in the north, volcanic activity began to the south. A large stratovolcano and other vents ejected material that accumulated over what is now the southern West Elk Mountains. Most of these volcanic rocks are included in the West Elk Breccia Formation, a heterogeneous collection of volcanic materials including extensive mudflow deposits. West Elk Breccia is in places over 3,500 feet (1,100 m) thick.[1][2][3]
On top of the West Elk Breccia, volcanic ash was deposited through repeated eruptions in the San Juan volcanic field to the south. Most of the ash was deposited 26 to 27 million years ago. The resulting rock, tuff, is relatively soft, but the ash landing toward the southern edge of the West Elk volcanic field was hot enough to fuse into harder welded tuff. These welded tuffs are more resistant to weathering than the underlying breccia and today they cap multiple south-sloping mesas in the southern West Elk Mountains.[1][2][3]
Erosion has cut valleys and defined the mesas and peaks we see today. The highest point in the West Elk Mountains is West Elk Peak, which is located near the center of the large volcano that once dominated this landscape. Stratigraphic profiles of these rock layers can be seen at the southern edge of the West Elk Mountains where the Gunnison River has eroded through the volcanic strata. A good example can be seen at the Dillion Pinnacles in Curecanti National Recreation Area. The resistant welded tuff that caps Dillon Mesa is on top, overlying the West Elk Breccia, which has eroded into the pinnacles. Exposed under the breccia are the older, underlying Mesozoic sedimentary rocks including the Mancos, Dakota, and Morrison Formations.[1]