Geology of the Sangre de Cristos

The Sangre de Cristo range is one of Colorado's few fault-block ranges, geologically similar to the Tetons or Sierra Nevada. The main uplift of the Sangres happened about 15 million years ago, so these are some of Colorado's youngest mountains. Interestingly, until about a half million years ago the northern part of the San Luis Valley was filled with a giant lake, until it drained catastrophically cutting a channel to the Rio Grande. It's possible that the Great Sand Dunes were initially formed from sand from the dried up lakebed of the ancient Lake Alamosa.