Laccoliths are isolated rock formations, mountains or mountain ranges that result from the injection of magma into overlying layers of sedimentary rock. Cooling into igneous rock, the laccolith is uncovered as the crust in which it lies is uplifted by tectonic forces and the softer sediments encasing it erode away.
Some of the most famous laccoliths in the United States are found in the Four Corners region of the Colorado Plateau. The La Sal Range, just southeast of Moab, the Abajo Mountains, west of Monticello, the Henry Mountains, west of Canyonlands National Park and Navajo Peak, ENE of Page, Arizona (just north of the Utah-Arizona border), are prominent features of the landscape in southeastern Utah. In extreme southwestern Colorado, Sleeping Ute Mountain (southwest of Cortez; photo below) and the La Plata Range (northwest of Durango) are well known laccoliths.
The intrusions in Utah occurred during the Oligocene and Miocene Periods (20-30 million years ago) while those in southwest Colorado were emplaced during the Laramide Orogeny (the first uplift of the Rocky Mountain Chain), some 65-70 MYA. Of course, over the coming eons, these scenic mountains will also succumb to the relentless forces of wind, water and ice. Better go see them before it's too late!
