bring the heifer to a valley with running water that has not been plowed or sown, and break its neck there by the stream.
Care had been taken by some preceding laws for the vigorous and effectual persecution of a wilful murderer (Deu 19:11 etc.), the putting of whom to death was the putting away of the guilt of blood from the land; but if this could not be done, the murderer not being discovered, they must not think that the land was in no danger of contracting any...
Commenting on Deuteronomy 21:1-9
The elders of that city shall bring down the heifer unto a rough valley,.... Cities being generally built on hills, and so had adjacent valleys, to which there was a descent; but here a rough valley, or the rougher part of it, was selected for this purpose.
And the elders of that city shall bring down the heifer unto a rough valley, which is neither eared nor sown, and shall strike off the heifer's neck there in the valley: (b) That the blood shed of the innocent beasts in a solitary place, might make them abhor the fact.