I have a hypothesis that, like minstrelsy and vaudeville, the threat of quicksand was a holdover from earlier, pre-film times. Specifically, I would bet that it traces back to the western emigration dating from the 1848 Gold Rush through the 1860s. Everyone who came West in those days took wagons, usually drawn by oxen, on one of a handful of major trails running along the great rivers of the Plains, and in those wide, undammed braided streambeds quicksand must have been a constant threat.
The great emigration left a deep stamp upon the national narrative, not least on those who made it to the promised land. They and their descendants brought waves of residents over the rails to Los Angeles and San Francisco, and when the first movie studios arose in California around 1920, the writers repurposed the living memories and sturdy memes from that not-so-long-ago time—and I think one of those easy licks was quicksand.
More:
About quicksand
Hard Road West, a geologist's account of the Emigration
Sherlock Holmes and the Wild West
The gold rushes
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One year when we had lots of rain I was trail riding with a friend. We were only a few dozen yards off my property when my old quarter horse Smoke, a true veteran, started to fuss in an area that was a slightly damp depression. I turned to tell my friend and the bottom dropped out underneath of her and her much larger horse Havoc. Neither my friend nor her horse panicked, and in about eight tries he managed to jump to ground that would hold him. When we checked, the stirrup leathers were wet half way up. We returned by another route, and there was the slightest dimple where they had gone in.
It is the sort of experience that stays with you.
Addendum: We live in Colorado, in the rather geologically interesting area just north of the Air Force Academy.
When I was a boy there was an old quarry myself and friends enjoyed exploring and playing in ( without permission of course ).
One year we went down after a week worth of moderate to heavy rain and as we walked along the incline and the bottom of the pit, one of my friends suddenly cried out in alarm.
He’d sunk in up to his hips and was flailing his arms about.
We spoke as one, ” QUICKSAND ! ”
Another had sunk too, but only up to his shins.
He quickly retraced his steps onto solid ground, losing both shoes and a sock doing so.
My other friend by this time was crying as he’d sunk up to his waist. He was straining side to side and pushing down in vain with his hands on the bog surface.
We advised him to stop moving.
He was panicking too much and by the time the words had sunk in, he’d slipped in up to his chest with both arms below the surface as well.
At least, he’d stopping sinking.
The three of us formed a human chain as we’d seen in movies and exorted him to pull his arms out so we could reach him.
His had some difficulty due to the suction, but eventually pulled one and then the other arm free.
We tried unsucessfully to pull him out but found all we could do was turn him around so he was facing us.
The pain and strain was too much for him and he yelled at us to stop pulling as he felt like he was being torn apart.
It was decided two of us would run for help while one stayed behind for ” moral support. ”
Luckily help was only a few minutes away and after a rescue team arrived with pumping eqipment, he was pulled free after being in the quicksand for about an hour.
He was treated for hypothermia ( a turtleneck, sweater and jeans does you no good when soaked in quicksand up to your chest when it’s 30 F degrees, apparently ).
We all recieved the lecturing of a life time and grounded for a month.
As for the quarry, steps were made so it was more difficult for quicksand to form after heavy rain and warning signs were placed around the fence line.
In the sand dunes of Wyoming and other places that get alot of snow in the valleys the snow can build up and then get covered by sand. When the snow melts it can create an air pocket. If you were to try and walk accross this air pocket you may not live to tell about it. The sand goes into a liquifaction state and you sink like a rock in water and you may sink 20 feet in less than a second.