In Ezekiel 26:11, this signifies the love of learning or intellectual things. (Arcana Coelestia 3727)

In Revelation 6:2, this signifies the love of understanding the Word. (Apocalypse Revealed 298)

In Zechariah 12:4, this signifies that the intellectual should be filled with falsities. (Arcana Coelestia 2383[2])

In general, 'a horse' signifies knowledge or understanding of the Word. In an opposite sense it signifies the understanding of the Word falsified by reasonings, and likewise destroyed from self-derived intelligence. 'A dead horse' signifies no understanding of truth from the Word.

Mountains in the Bible represent people's highest points, where we are closest to the Lord -- our love of the Lord and the state of caring for one another. By extension, then, it makes sense that valleys represent our lowest points, the ones most distant from the Lord: Our external, bodily lives in the day-to-day world. Valleys can also represent the external level of other things, depending on context. For instance, the Valley of Shinar, where the Tower of Babel was built, represents the external state of worship people had at the time. And in Genesis 26:19, when Isaac's servants dig a well in a valley, it represents seeking true ideas in the external, literal sense of the Bible.