To do a post on valleys and not one on mountains does not seem right. The Hebrew word har is used six times in Joel. A word for hill is used once in 3: 18, but depending on the translation you may read “hill” even though it is the word Har. (Got to love translators and their work.)
Three times in Joel Har is used with the word Zion or holy (2: 1, 32; 3: 17). The other three times the NIV adds an “s” making them plural, so to me, that makes them all the other mountains in Israel. Mountains in Israel and other places are often sacred places. Strong’s Concordance states they are often associated with deities and serve as symbols of strength. Israel did have problems with using mountains as worships sites for other gods throughout its history. If you read this thought (what we think as strong or wrong gods) into the verse when you read 2: 2, 5, and 3: 18 it transforms the verses into statements of God’s greatness and blessings over and above the gods that would war against Him and his people.
In 3:18 it is the mountains and hills that are giving the wine and milk. These are the “areas” above the valleys where I would normally think of when it comes to food production. I will let you draw your own conclusions here but it should give you another look into the greatness of our God.
Reference – Strong’s/Vine’s Concordance