Fishlake National Forest

Thousand Lake Mountain

Despite its name, there are few lakes on Thousand Lake Mountain. Local legend has it that the names of Thousand Lake and Boulder Mountains were switched after someone told government topographers their correct names. Other locals then told a "fish story" to the topographers in order to protect favorite fishing spots on what is now Boulder Mountain, the one with many lakes.

Thousand Lake Mountain rises on the east side of the Forest like a giant layer cake. The top is capped with volcanic rocks that flowed out onto a preexisting surface. Fissure vents are visible on the sides of the Flat Top. These vents occur where the molten lava was pushed up from the depths to flow out over the surface. Because the lavas were quite fluid, they formed a nearly flat surface similar to the one that is the Flat Top today.

Mesozoic sedimentary rocks ranging from the colorful Moenkopi Formation, found on the south side of the mountain, to the drab grey Mancos Shale, found to the north, lie beneath the cap of basalts and other volcanic rocks. For the most part, however, these sedimentary rocks are buried beneath landslide debris that cascade down the mountain side like frosting dripping off a giant layer cake.

The landslides give Thousand Lake mountain its distinctive form. They also are responsible for the few lakes that are present such as Deep Creek, Neffs, Meeks, and Morrell. The landslides result from two factors 1) high relief and 2) the sequence of rocks present.

The high relief is caused by the uplift of Thousand Lake Mountain along a prominent, north-south trending fault that runs along the west side of the mountain. Movement along this fault has made Thousand Lake Mountain stand higher that the valley to the west. Bending of the rocks on the east side of the mountain has made it higher than the valleys to the east.

The sequence of rocks consists of a cap of resistant basalts and fractured volcanics. The fractures permit water to seep down through the cap to the shales and sandstones beneath. When the water reaches the underlying shales it is forced to move laterally toward the edge of the mountain. It also converts the shales into slippery clay. The combination of reduced friction, along the shale layer, and water pressure pushing toward the edge of the mountain can cause blocks of the resistant cap to slide. As they slide downward they rotate backward toward the mountain, often forming a closed depressions where water can accumulate. During periods of wetter climate, such as the time of glaciers over 10,000 years ago, this landsliding was fairly active. In today's relatively dry climate the landsliding is relatively inactive.

Fremont River Ranger District
138 South Main Street
P.O. Box 129
Loa, UT 84747
(435) 836-2800