Great Sand Dunes National Monument

Background• Located between the San Juan and Sangre de Cristo

Mountains in Southwestern Colorado’s San Luis Valley

• Over 12,000 to 130,000 years old (exact time they formed is unknown)

• Approximately 42,000 acres

• Composed of black magnetite, pink feldspar, red and tan sandstone, green epidote, white quartz, and many other minerals.

Medano CreekFluvial Process

• Seasonal stream that runs through the national monument

• Created from mountain run off and seasonal rain, this determines the volume of flow.

• Moisture from the creek keeps dunes stabilized so they don’t move

• “Conveyor belt for sand”- flow carries sand downstream and when the creek dries up the wind blows the sand back to the dunes

Drying up Light Flow

Heavy Flow

Different flows of Medano Creek

Wind (aeolian) TransportationArid Land

• Dunes are created by a combination of wind patterns. Northeast wind blows strongly across the valley, weaker winds blow southwest through a gap in the mountains

• Particles are moved by saltation and traction or in other words bounced and rolled

• The surface layer moves slowly downward as a result of saltation, this is called creep

• When the sand is deposited (known as aeolian deposition) it creates a sand dune

Aeolian Transportation Diagram

Different Types of DunesStar Dune

Seifs

Barchan

Transverse Dunes

Where all the Sand Came FromGlacial Modification of Terrain

• During the Pleistocene Epoch the San Luis Mountains were covered in glaciers

• Relatively warm summers during this time and the melting glacier seasonally flooded the valley

• Glacial plucking occurred as the glaciers moved down the mountain which is when the glacier picks up rocks and sand

• Meltwater carried tons of sand that the glaciers had scoured from the mountains and deposited it in an alluvial fan

• Winter winds carried the sand across the valley

• The Sangre de Cristo Mountains blocked the wind and the sand piled up forming the dunes.

Glacier of a Mountain Alluvial FanCaused from Meltwater

Glacial Plucking

Wind Blowing and Moving Sand

Sand Dunes are Formed