Here's another Page Publications digest-size comic I snagged recently - Nightmare Suspense Library #2:
I've checked all my usual sources and I can't find a reference to the original appearance of the story "When Mountains Moved", but I'm going to take a punt and say it originally appeared in Nightmare Suspense Picture Library #10 from MV Features. I say this because the pages include a job number "n10" as a prefix to the page number, and i'm assuming "n" refers "Nightmare" and "10" to the issue number. As per the link, there are two issues missing from the GCD gallery - #'s 10 and 13 - and my guess is that #10 included this story.
Furthermore I'm guessing that Keith Chatto's cover is based on the original cover, as per other instances of this pattern.
Indeed, I wouldn't be surprised to find "When Mountains Moved" has been reprinted elsewhere by Page.
We shall see in due course.