John Gill 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)

And I took your sin, the calf which you had made
Which was the object of their sin, which lay in making and worshipping it; see (Isaiah 31:7)

and burnt it with fire, and stamped it ;
with his feet after it was burnt, to bring it into small pieces:

and ground it very small ;
or, as the Targum of Jonathan,

``ground it in a mortar well;'' the burnt and broken pieces:

even until it was as small as dust ;
being ground to powder, as in (Exodus 32:20)

and I cast the dust thereof into the brook that descended out of the
mount ;
and made the children of Israel to drink of it, as in the previously mentioned place; (See Gill on Exodus 32:2); all this was done before the prayer for Aaron and the people.