Lines 38-45 Summary

Get out the microscope, because we’re going through this poem line-by-line.

Rock-ribbed and ancient as the sun,—the vales

Now the speaker starts a description of the whole earth, of the geography of our globe. Remember, "Thanatopsis" started out as a nature poem, and now we’re headed back to those themes.
He begins by talking about the hills. He refers to them as "rock-ribbed and ancient as the sun." We sort of love that image – it makes the hills sound like giant, old sleeping animals.

Stretching in pensive quietness between;

Next he describes the quiet, thoughtful ("pensive") valleys that stretch out between the hills. Again, the idea that a valley could be thoughtful makes this whole imaginary landscape feel kind of alive.

The venerable woods—rivers that move
In majesty, and the complaining brooks

Just a few more stops on this tour of the world’s landscape:
We take a quick peek at the "venerable" (that means something old and deserving of respect) forests. We picture these woods as being like Fangorn in Lord of the Rings – ancient, and full of wise Ents.
We also see the majestic rivers, and their little cousins, "the complaining brooks." (A brook is a little stream, and the speaker calls them complaining to create an image of the constant, burbling sound they make.)

That make the meadows green; and, poured round all,
Old Ocean’s gray and melancholy waste,—

After that, the speaker takes us to some pretty meadows, which the brooks have watered and turned green.
Finally, we arrive at the "gray and melancholy waste" of the "Old Ocean," which surrounds everything else.
A couple things to notice about that last image. First, Bryant spends a lot of times telling us how old (or "ancient" or "hoary" or "venerable") everything in the world is. We think that adds to the peaceful, serious tone of this poem. Second, we’re back to some grim imagery – does "melancholy waste" sound nice to you? – an echo of the sad moments in the first stanza.

Are but the solemn decorations all
Of the great tomb of man. The golden sun,

All of these places in Nature – hills, valleys, forests, streams, the ocean – are compared to "decorations" on a "tomb." Every last hill and valley and river is just a way to spruce up the giant grave that all humans will share.
Like a lot of moments in this poem, it’s not super-happy. Still, there is a kind of quiet beauty to the idea of "solemn decorations."