What you have quoted are the last five sentences of the Introduction’s first paragraph. Momaday sets the trend at once by using specific descriptive language so that readers can imagine or picture the place for themselves. Here’s how this paragraph begins.
A single knoll rises out of the plain in Oklahoma, north and west of the Wichita Range. For my people, the Kiowa, it is an old landmark, and they gave it the name Rainy Mountain. The hardest weather in the world is there. Winter brings blizzards, hot tornadic winds arise in the spring, and in summer the prairie is an anvil’s edge. The grass turns brittle and brown, and it cracks beneath your feet. There are green belts along the rivers and creeks, linear groves of hickory and pecan, willow and witch hazel. At a distance in July or August the steaming foliage seems almost to writhe in fire.
Here—in addition to the kinds of verbiage analyzed by the other posted answer—the author includes even more descriptive language. He could have merely said that the prairie gets hot in the summertime. But no: it becomes “an anvil’s edge” (a metaphor) where all of the leaves and greenery seem “almost to writhe in fire” (a simile). The winds are not merely gusty; they’re “tornadic” (adjective). The moisture that the rivers offer results in the growth of “linear groves” of trees that outline any waterway on the horizon (descriptive phrase). He’s been here, he knows what it looks like, and he conveys the look and feel of the landscape as best as he can to us.
The bottom line is that, in this entire paragraph and throughout the book, Momaday shows his respect for the land by paying close attention to it. Someone who didn’t care could have passed through the same landscape and could have seen nothing, or seen very little. Instead, Momaday tells us what this part of Oklahoma looks like in each season, right down to the grass and the grasshoppers. He conveys the visuals, sounds, and feelings of the scene through his word choices. This geography is important to him, as it was to his Kiowa ancestors. The best way that we can honor and respect anything, including a place or a person, is to give it our full attention. If we truly see it, we can describe it so that others can, too.
Further Reading
Momaday's adjectives and descriptions in the passage you cite simultaneously create images of specific, concrete details and a vast, open, and isolated landscape. The adjectives are mixed with verbs and nouns, of course (to create concrete details), so I'll italicize the adjectives.
The "Great green and yellow grasshoppers" (visual imagery) pop up like corn to "sting the flesh" (tactile imagery), and they are "everywhere [adverb]." Turtles crawl on "red earth." These concrete details bring verisimilitude, or realism, to the scene being described and create the localized, immediate view of the scene.
The view moves outward, then, to the vast landscape. When one scans the big picture, so to speak, "there is no confusion of objects in the eye," as only "one hill or one tree or one man" is seen. The plain is a lonely place.
The concrete details of the local view combine with the images of the overall view to establish the setting that leads to the writer's conclusion, which demonstrates his profound respect for the plain, where, one might think, creation was begun.