Image 1 of 3
Image 2 of 3
Image 3 of 3
Grand Mesa
Basalt That Filled a Valley
Grand Mesa, the largest flat-topped mountain in the world, rises to 11,000 feet in elevation above the Colorado River near Grand Junction. Its cool climate, forested meadows, and alpine lakes are a world away from the dry adobe desert at its base. If you want to escape the summer heat, drive across the mesa on CO 65.
The mesa is an excellent example of inverted topography, which means that what is at its top was once the low spot in the topography. At the top of the mesa is basalt that erupted from vents in the Grand Mesa volcanic field to the east around 10 million years ago. Like all liquids, fluid basalt will find the lowest points in the landscape, pooling in and flowing down riverbeds. More than twenty lava flows stacked up, filling in irregularities and cooling into a very flat surface.
Because the ancient riverbed was filled and dammed with volcanic materials, the river changed course and continued to erode the surrounding landscape, removing vast quantities of Mesozoic and Cenozoic sedimentary rock. The more resistant basalt remained intact, resisting erosion and shielding the softer sedimentary layers beneath it. Now, after 10 million years of uplift and erosion, Grand Mesa stands high in the landscape. Gravels and cobbles deposited by the former river are preserved beneath the lowermost basalt flows, now visible at about 10,000 feet elevation! Features in these river deposits tell us that the river flowed west toward the Colorado Plateau.
Erosion continues to eat away at the sides of the mesa as softer rock beneath the lava cap is eroded. The steep slopes of Grand Mesa are in constant battle with gravity, and landslides are common.
Donahue, Magdalena S, and Marli B Miller. “Grand Mesa.” Colorado Rocks!, Mountain Press Publishing Company, Missoula, Montana, 2021, pp. 43–43.