"Song" in the "Jiyun" (集韵) is written with the ancient character "䛦". Detailed notes can be found in the section for four strokes. The "Zhengtong" (正字通) believes that "Gu" (谷) has the meaning of a deep trap, hence the character is formed from "Gu" (谷). The form written in the "Yupian" (玉篇) is incorrect.