Geologic Timeline
 During
the Paleozoic Era, over 500 million years ago, shallow seas and river
systems deposited sedimentary materials throughout the region. The Sangre
de Cristo mountain range east of the dunes is composed of Paleozoic rocks
as well as metamorphic rocks from the much older Precambrian period.
 Eons
later, pressures from within the Earth pushed the landscape high above
sea level, limiting sedimentary deposition and leaving a gap in the region's
Mesozoic record.
Around 40 million years ago, an arid landscape dominated where rivers
and intermittent lakes emerged and subsided upon the land. Evidence of
this period are found in the silts, clays, sands, and gravels of the early-Tertiary
Blanco Basin Formation.
Recurring lava flows poured onto the landscape about 34 million years
ago from volcanic vents in the San Juan mountains, creating a subsurface
layer that geologists call the Conejos Formation.
 After
the San Juan Mountain lava flows subsided, volcanic activity continued
and for four million years ash flows blanketed the earlier lavas of the
Conejos Formation. These ash-flow tuffs are visible on the west side of
the Sand Luis Valley.
Rapid uplift of the Sangre de
Cristo Mountains began about 19 million years ago. Rifting associated
with the uplift widened the valley and caused its floor to drop significantly.
(This process continues today.) Water-born and eolian sediments were deposited
in the subsiding San Luis Valley, creating the Santa Fe Formation.
Around five million years ago, a new series of volcanic activity rocked
the region, at times damming rivers that flowed out of the southern half
of the San Luis basin. The Alamosa Formation formed during this time,
made of lake, river, marsh sediments and wind-deposited layers of clay,
sands and silts.  Locally
known as the "blue clay layer," the Alamosa Formation is a confining
layer between aquifers found under the valley floor.
The Pleistocene began 1.8 million years ago as climate changed globally.
Glaciers grew in mountain valleys, some pouring ice and rock far into
the San Luis Valley. Many scientists consider the Pleistocene to be the
period in which dune formation began in the San Luis Valley.
 Cooler
temperatures also caused the Earth's oceans to contract, creating, land
bridges that permitted a worldwide diaspora of animals between North America
and Asia.
Only about 12,000 years ago, a warming climate melted many glaciers worldwide
and signaled the end of the Pleistocene. Large quantities of silt, gravel
and sand were carried by rivers and streams into the San Luis Valley.
Although the dunes existed long before the Pleistocene ended, glacial
outwash materials provided new sources of sand for winds to rework into
even greater sand dunes.
 Today,
the rivers and creeks continue to transport sediment into playa lake systems
which are sources of sediment for dune-forming winds of the San Luis
Valley.
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