The following link presents some of the evidence used to conclude that what formed the unusual scablands terrain up in the American Northwest was a series of megafloods. You will notice that the geological evidence includes not only erratics, but erratics plus massive ripples, vertical scoured cliffs, shorelines, etc. All evidence of fast-moving water.
See:
Exploring the Scablands
Those pieces of evidence are what lead geologists to conclude a series of megafloods carved the scablands as opposed to glacial processes, which result in vastly different geological formations.
See:
Landforms of Glaciation
Therefore, while both megafloods and glaciation can leave erratics, the ultimate cause of those erratics is extrapolated entirely from the surrounding geology.
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