Where does Momaday shift the sentence length and structure? Why?

Explanation:

When Momaday describes where his grandmother grew up and her life there, he uses a series of long, descriptive sentences, which has the effect of showcasing the long history of the place and the vast landscapes of the area:

Great green and yellow grasshoppers are everywhere in the tall grass, popping up like corn to sting the flesh, and tortoises crawl about on the red earth, going nowhere in the plenty of time. Loneliness is an aspect of the land. All things in the plain are isolate; there is no confusion of objects in the eye, but one hill or one tree or one man.

Then, when he tells the story he learned from his grandmother, his sentences become shorter, as if he is mirroring her storytelling:

Eight children were there at play, seven sisters and their brother. Suddenly the boy was struck dumb; he trembled and began to run upon his hands and feet. His fingers became claws, and his body was covered with fur. Directly there was a bear where the boy had been. The sisters were terrified; they ran, and the bear after them. They came to the stump of a great tree, and the tree spoke to them. It bade them climb upon it, and as they did so it began to rise into the air. The bear came to kill them, but they were just beyond its reach. It reared against the tree and scored the bark all around with its claws. The seven sisters were borne into the sky, and they became the stars of the Big Dipper.

Hope this helped you!

"the sandy walls seemed to collapse in on themselves"

and

"the bloodshot waters seemed to have no bottom"