Several years ago I was wondering just how Lake Tahoe was formed. I discovered that geologists believe the Sierra Nevada on the west and the Carson Range on the east were once massive blocks of crystalline rock. These areas were uplifted on both sides of the present day Tahoe Basin. A large down-dropped block created the Basin in between these ranges which were joined in a large U-shape at the south end and open at the northern end. The open north end allowed snowmelt to flow out of the basin toward the east and down to the Great Basin in the area covered by Reno, Nevada today. About 2 million years ago, when large volcanic eruptions were occurring in the area, a large volcano called Mount Pluto poured lava and volcanic mudflows into the northern outlet from the valley blocking the previous outlet and allowing a large lake to form behind it. This lake which was a predecessor to the present Lake Tahoe had a water level about 600 feet higher than the present lake. During the last Ice Age, less than a million years ago, glaciers shaped the surrounding mountains and scoured the landscape, this time depositing a pile of rocks scoured by the moving glaciers as an end moraine blocked the northern end even further. The result was that the outlet of the valley was changed to where the Truckee River outlet now exists at Tahoe City.