Hi, I've been reciting "I have a dream." speech by Martin Lutheer King. I can't see the meaning of the phrase;"every valley shall be exalted." The context is; I have a dream that one day every valley shall be exalted, every hill and mountain shall be made low, the rough places will be made plains, and the crooked places will be made straight, and the glory of the Lord shall be revealed, and all flesh shall see it together. If it is a mountain that shall be exalted, I think it would be natural, but why valleys? Is there any occurrence that people exalt valleys in the Bible? Thanks in advance.