In The Way To Rainy Mountain, the author compares the prairie in the summer to an 'anvil's edge.'
Traditionally, anvils were metal blocks used as forging tools. Today, many blacksmiths still use anvils to form horseshoes and other types of tools/weapons. The metal (to form the horseshoe) has to be heated so that it is malleable enough to shape. Have you ever heard of the saying 'strike while the iron is hot?' When the iron is cool, it becomes brittle and difficult to shape. Among other things, an anvil's edge can be used to make clips on horseshoes (some people believe that clips make the horseshoe fit and stay better on the horses' hoofs). Of course, the horseshoe will have to be heated so that it is malleable enough to make the forming of clips possible. Watch this short video here to see how it's done:
The idea here is that the horseshoe is being pummeled by a hammer on the anvil's edge. It's a pretty hot area temperature-wise, and you probably don't want to touch it with your bare fingers. In the story, Momaday equates the prairie to an anvil's edge on a summer's day. He's telling us how hot it gets at Rainy Mountain in the summer. The next sentence gives us an idea of how hot:
The grass turns brittle and brown, and it cracks beneath your feet.
He provides further imagery in case we are still wondering how hot it gets in the prairie in the summer:
At a distance in July or August the steaming foliage seems almost to writhe in fire.
So, the author is trying to tell us that it gets so hot in the prairie in summer that one might as well be on an anvil's edge.