What a fabulous President's Day, a ski tour atop the "largest flat topped mountain in the world". The Grand Mesa is a large mesa found in western Colorado. I am not sure who decided that it was so special, but I read it on the Internet...so it must be true.
The top of the mesa is covered by lava flows dated at 10,000,000 years ago. These lava flows make quite a nice cap rock and have slowed erosion down. The top of the mesa sits a little over 11,000 feet, and in the summer the volcanic evidence is abundant. In the winter, not so much. On either side of the mesa, the Colorado River to the north and the Gunnison River to the south, deeper valleys have been incised down to 5,000 feet in elevation.
Because we can date the lava flows and we can measure the depth of the valleys next to the Grand Mesa, it makes for a very convenient yard stick. I teach geology classes along both the Colorado and Gunnison Rivers. By doing some simple math, (6,000 feet in 10,000,000 years) we get an idea how long it takes for neighborhood valleys to erode. We then measure nearby tributaries and get an idea how long that tributary has been down cutting. I love getting simple math into classrooms!
The Grand Mesa is important for more than just being a yard stick. It is a formidable barrier to moisture as it makes its way east. The south side of the mesa is part of the Gunnison River drainage while the north side of the mesa is part of the Colorado River drainage. Much of the water needed throughout the arid west will be dropped here as snow each winter.
The Grand Mesa seen from a distance. It makes an obvious landmark for mile around.
A summer time shot from the valley bottom.
Winter! Skiing next to a basaltic ridge. 
The state flower, The blue Columbine growing among vesicular basalt boulders.
