Charles Ellicott Commentary Deuteronomy 9:21

"And I took your sin, the calf which ye had made, and burnt it with fire, and stamped it, grinding it very small, until it was as fine as dust: and I cast the dust thereof into the brook that descended out of the mount." — Deuteronomy 9:21 (ASV)

I took your sin ... and I cast the dust thereof into the brook. —The stream from the rock in Horeb not only gave Israel drink, but bore away their "sin" upon its waters. And that Rock was Christ. This identification of the sin with the material object is in harmony with the Law in Leviticus, where "sin" and "sin-offering"—"trespass" and "trespass offering"—are respectively denoted by a single word.