Deuteronomy 21:4

אל נחל איתן UNTO A VALLEY WHICH IS איתן — which is hard; i.e. one that has never been tilled (Sifrei Devarim 207:2; Sotah 45b). וערפו AND THEY SHALL STRIKE OFF [THE HEIFER’S] NECK — i.e. one breaks its neck with a hatchet. The Holy One, blessed be He, says, as it were, Let a heifer which is only one year old and which therefore has brought forth no fruits (no offspring) have its neck broken at a spot (the untilled valley) which has not brought forth fruits, to expiate for the murder of him whom they did not permit further to beget children (Sotah 46a).

וְהוֹרִ֡דוּ זִקְנֵי֩ הָעִ֨יר הַהִ֤וא אֶת־הָֽעֶגְלָה֙ אֶל־נַ֣חַל אֵיתָ֔ן אֲשֶׁ֛ר לֹא־יֵעָבֵ֥ד בּ֖וֹ וְלֹ֣א יִזָּרֵ֑עַ וְעָֽרְפוּ־שָׁ֥ם אֶת־הָעֶגְלָ֖ה בַּנָּֽחַל׃

and the elders of that city shall bring down the heifer to a rough ravine, which is neither ploughed nor sown, and shall break the heifer’s neck there in the ravine: