Reading "Little house on the Prairie"

All around them there was nothing but grassy prairie spreading to the edge of the sky.Quite near them, to the north, the creek bottoms lay below the prairie. Some darker green tree-tops showed, and beyond them (1)bits of the rim of earthen bluffs held up the prairie's grasses. Far away to the east, (2)a broken line of different greens lay on the prairie, and Pa said that was the river.

"That's the Verdigris River," he said, pointing it out to Ma.

(1) Does the sentence mean that the earthen bluffs were holding the prairie's grasses?

Or, does it mean that the earthen bluffs were stopping the prairie's advance or march or going forward?

(2) Does the "a broken line" mean that the line it self was broken by different greens?

Or does it mean that the prairie was broken by the line and the line was the river?