Let’s talk about… dunes.
No, like sand dunes.
No, like the Great Sand Dunes.
Of Colorado.
No, like the National Park.
Great Sand Dunes National Park.
Chalamet fans, I love ya.
I love ya boy, but we talk about parks on this channel.
You can spread your Chalamet love in the comments.
I’ll allow it.
For now though, let’s talk about Colorado’s Great Sand Dunes National Park (and Preserve).
This park protects the tallest sand dunes in North America and one of the most complex
dune systems in the world.
The tallest dune here, Star Dune, rises 750 ft, nearly 230 meters, from its base to its
crest.
The dunes were originally protected as a National Monument in 1932, but were redesignated as
a National Park and Preserve in 2004.
The Park is also incredibly diverse, protecting more than just the dunes themselves.
You’ll find alpine and tundra habitats here, along with forests, grasslands, wetlands,
and more.
Which is just as well, because the dunes here don’t grow to their considerable height
all on their own.
They rely on a complex system of factors and processes.
All parts in the system are connected and they all work together to make these dunes
as high as they are.
So, the fact that Great Sand Dunes National Park protects this entire system is a really
good thing - because that means the dunes themselves are protected also.
Alright, let’s dive into it.
You got your basic elemental processes here at Great Sand Dunes: Earth, Wind, Water, and
Fire.
Wait, no.
No, there’s no fire involved here.
Unless you count the volcanoes?
But I’m not really counting the volcanoes because they’re not super important to the
story.
Anyway, we’ll just stick with Earth, Wind, and Water for now.
But there are volcanoes.
If you’re interested in that sort of thing.
No, but we’ll start off with…
Earth and the formation of two important mountain ranges.
To the West of the park, you’ve got the San Juan Mountains, which are primarily volcanic
in origin (see I told you there were volcanoes).
To the East, and encompassing parts of the park itself, you’ve got the Sangre de Cristo
Mountains.
After these mountains uplifted, a broad, flat valley remained between them - the San Luis
Valley.
This is the setting for our story.
The dunefield and most of its encompassing parts lie within this valley.
Now, all of this happened a long time ago and the San Luis Valley doesn’t look today
like it did back then.
No, in fact the Valley was home to a large lake.
Lake Alamosa.
At its largest, Lake Alamosa covered a surface area of 1,500 square miles, or 4,000 square
kilometers.
That’s bigger than the US State of Rhode Island.
So, you’ve got Lake Alamosa sitting in the San Luis Valley, bounded on the West by the
San Juan Mountains and bounded on the East by the Sangre de Cristo Mountains.
Both mountain ranges are depositing large amounts of water and sediment into the lake.
This sediment is important.
We’ll come back to it.
But, Lake Alamosa started getting a little too big for its britches.
The rocks at its southern boundary eventually give way, and they’re the things, you know,
like keeping it in a “lake shape.”
So, Lake Alamosa starts to drain and eventually empties, leaving behind just a couple of smaller
lakes here and there, but nothing like the large body of water it once was.
But that sediment is still flowing in.
The San Juans and Sangre de Christos are still depositing sediment into the Valley floor.
Now eventually, even those tiny lakes dry up, leaving behind all that sediment.
This is the origin of what we today call the Sand Sheet.
Now, this is a good time to talk about the 4 main areas of the Dune System here at Great
Sand Dunes National Park.
You’ve got the Mountain Watershed.
This is part of the Sangre de Cristo Mountains that receives all that alpine precipitation
and drains it down toward the second area: the Dunefield.
The dunefield is, you guessed it, where the dunes are.
It’s 30 square miles, or 78 square kilometers, and contains 1.5 cubic miles, or 6.5 billion
cubic meters of sand.
That is enough sand to fill 2.5 million Olympic sized swimming pools.
But that’s not even the best part.
Because the next component of the dune system, the one we’ve already talked about, knocks
that figure out of the water.
The Sand Sheet contains 90% of the sand in the entire system.
That’s right, the dunefield itself contains only 10% of the sand in this system.
I’ll let you work out for yourself how many swimming pools you can fill.
Now, the sand sheet may have a lot of sand, but it’s not characterized by the big, tall
dunes like in the actual dunefield.
The sand sheet is more of a vegetated grassland, but it’s where the dunes themselves actually
start off, before they migrate in to play with the big kids.
The Sand Sheet surrounds the dunfield on 3 sides, constantly supplying it with the sand
it needs to replenish itself.
And lastly, beyond the Sand Sheet, you’ve got an area known as the Sabkha.
The sabkha is characterized by a crusty layer of sand that is formed when surface water
evaporates away and leaves behind a salt which cements the sand grains together.
You won’t find a lot of dunes here.
But, let’s get back to the ones you will find and how they’re formed.
So, the remnants of Lake Alamosa have dried up, leaving behind a nice, large pile of sand.
The Sand Sheet.
This is where our dunes start to form.
Because of the orientation of the San Luis Valley, the predominant winds in this area
blow in from the Southwest toward the Sangre de Cristo Mountains.
And of course, as they do that, they blow the sand in that direction.
Now, sand grains are tailor made for this.
They're the perfect size.
Too big and they wouldn’t blow at all, too small and they’d just be lifted away.
No, instead, sand grains are just light enough to be picked up by the wind, but just heavy
enough to not be carried away in the air.
They kind of just bounce and roll along the surface until they come to a stopping point.
Which, in this case, is the Sangre de Cristo Mountains.
These mountains were nice enough to develop a nice little pocket for all the sand grains
to collect in as they blow in from the Southwest.
So, again, Southwesterly winds blow all this sand from the sand sheet toward the Sangre
de Cristo Mountains, where it starts to accumulate.
Eventually, the sand just builds up on itself, forming dunes.
But wait, there’s more!
Yeah, that’s not all that’s happening.
At the same time as these winds are blowing in all this sand from the Southwest, storm
winds from the mountains themselves blow down from the Northeast - the exact opposite direction.
These winds then, blow the sand right back in the direction it came from.
But the Southwesterly winds are still blowing, of course, and the effect is that the sand
grains migrate vertically.
That’s right, these so-called bimodal winds are what push the dunes upward, contributing
to their great height.
These sort of environments, where wind blown sediment is the dominant process, are known
as aeolian environments.
But wait, there’s more!
Yeah, sorry.
We’re still not done.
We’ve talked about Earth and we’ve talked about Wind.
Now, let’s talk about Water.
See, draining off of the Sangre de Cristos are two seasonal streams: Sand Creek and Medano
Creek.
Sand Creek runs along the dunefield to the Northwest, while Medano Creek straddles it
along the East.
These creeks are responsible for a process called sand recycling.
Essentially, these creeks, running along the boundaries of the dunefield, pick up sand
near the mountains and deposit it further back in the dunefield, where the southwesterly
winds can once again act upon it and blow it toward the mountains.
These streams, quite literally, replenish the dunefield and allow the dunes to maintain
their massive height.
Scientists speculate that if these creeks were to disappear, the dunes would spread
themselves over a larger area, but not be as tall.
For now though, they are that tall.
Maybe not the tallest in the world, but the tallest in North America and certainly some
of the most complex.
All of the factors at play here - the wind, the creeks, the ready supply of sand - means
you can find all sorts of dunes here.
You got reversing dunes, star dunes, parabolic dunes, barchan (bar-kin) dunes, transverse
dunes, nebkha dunes.
That’s… that’s about it.
But dune types and Forest Gump references aside, this is an incredibly rich and diverse
geological environment.
And it’s protected here for us to see it and enjoy it.
Not just the dunes themselves, but the whole system.
The whole system that makes the dunes possible.