This is a really descriptive passage, but it's also figurative. After all, when was the last time you met someone with sand for hair? (Outside of an Etch-a-Sketch.)
mountains her breasts two Niles her tears. (5-6)
Wow – we're dealing with some seriously epic proportions here. This woman isn't just well-endowed; she's got mountains for breasts. And the Nile River, in case you haven't heard, is pretty long. Picture that as a teardrop, then imagine the size of the body that could produce such a teardrop. See what we're saying?
Black through the years. (8)
Strangely enough, this description isn't just one of appearance – it's of appearance over time. Angelou wants to make sure we know that the first stanza is a form of history, just like the other two.
now she is striding although she has lain. (25-26)
The end of the poem creates an image that hearkens back to the strongly personified images of the first stanza. Africa isn't just rising – she's a woman who's striding into the future. The parallel between the first and the last lines makes this progression something like a return to the magnificence of the long-ago past as well. Talk about empowerment!