The great sand dunes in the sky
While chasing snow on a recent holiday in Colorado I came across one of the most radical landscapes I've seen.
It wasn't an abrupt mountain face but a Mars-like landscape of sand dunes in a remote and desolate corner of the otherwise snow-bound state.
The dunes are over 200m high and sit 2,650m above sea level, higher than Australia's largest 'peak' and have an appearance not dissimilar to Stockton Bight.
The dune field spreads out for miles, a geological wonder tucked into the base of the Sangre de Cristo Mountain Range immediately to the east which contains a handful of Colorado's '14ers'. That being mountain peaks surpassing 14,000 ft (4,267m), which in Colorado number 53.
The first thing I thought was, How did the sand dunes get here? They're over 1,000km from the nearest ocean and far above sea level.
Recent studies have identified the source of the sand and its age by looking at the surrounding geography and geology.
The sand dunes sit in the north-east corner of the San Luis Basin, a diamond-shaped basin that's bound to the north by the junction of the Sangre de Cristo Mountain Range and San Juan Mountains, while to the south the Sangre de Cristo Range meets the Tusus Mountains.
During glacial times the Rio Grande River, which runs down from the San Juan Mountains into the western side of the basin, delivered large volumes of meltwater and sediment, flooding it and forming Lake Alamosa which consumed the northern half of the basin.
A large alluvial fan also formed at the base of the Rio Grande and when Lake Alamosa drained rapidly some 440,000 years ago due to its volume and pressure purging volcanic deposits at its southern end, this provided the sediment for which the sand dunes could form.
Predominant west to south-west winds blew the sand east over hundreds of thousands of years until it was captured in the corner of the Sangre de Cristo Mountain Range. This and the swing in local winds from west to east during storm periods results in the formation of reverse sand dunes which remain fairly stationary and grow in height instead of moving across the landscape.
Analysis of diverted rivers flowing down from the Sangre de Cristo Range identified that the earliest the sand dunes could possibly have formed was 130,000 years ago, but sometime after the lake drained 440,000 years ago.
Further analysis of the sand dune profile identifies that only 11% of the material is from the local Sangre de Cristo Mountains immediately to the east, with the majority of the sediment originating from the San Juan Mountains some 80km to the west.
All this geology talk may only interest some, but for anyone travelling to Colorado I highly recommend heading to and camping in the Great Sand Dunes national park to enjoy the juxtaposition between the dunes and towering snow-covered mountains.