Will the mountains melt and the valleys cleave?
Radak (3), Malbim: All this is a Mashal for Churban ha'Aretz and the awesome afflictions. And so it says "va'Tig'ash va'Tir'ashu ha'Aretz u'Mosedei Harim Yirgazu" (Tehilim 18:8), "Harim Nazlu Mipnei Hashem" (Shoftim 5:5). Malbim - the mountains will melt from awe of Hashem and His grandeur, "ka'Donag" (like wax). The valleys will cleave "ka'Mayim Mugarim b'Morad." The solids will be transformed to liquids. This is a Mashal for Churban ha'Aretz ? it will return to be water, like at creation ? Tohu.
What is "Donag"?
Rashi: It is wax.
What is the meaning of "Mugarim b'Morad"?
Rashi citing Targum Yonasan: They will flow on an incline 1 . Mugarim is an expression of flowing water, like "Eini Nigerah me'Ein Hafugos" (Eichah 3:49).
Radak: It goes quickly. All this is a Mashal.
