quicksand
noun
a bed of soft or loose sand saturated with water and having considerable depth, yielding under weight and therefore tending to suck down any object resting on its surface.
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If the river crossing was quicksandy the cattle were taken out and driven back and forth across the stream until the quicksand was packed sufficiently to be considered safe.
From The Indians' Last Fight Or The Dull Knife Raid by Dennis Collins