Ah, after examining the image more closely in MS Paint, I discovered that some of the pixels only
look black, but are actually a very dark shade of blue.
This makes more sense now, so it's actually
not true that the original terrrain acts like this:
X
X
X
even though that's what it looks like in the screenshot.
I'll now figure out exactly which pixels are truly black (ie. empty space) and rework the level from there.