Long before the days of domed stadiums with retractable roofs, baseball was a game played exclusively in the great outdoors. It was, and still is, a game directly associated with seasons. (Just think about books like TheBoys of Summer, or the fact that the World Series is also known as the "Fall Classic.")
So, it should come as no surprise that Casey contains some nature imagery. What is surprising is the nature imagery's far-reaching implications. It seems those events in Mudville may just affect us all.
Line 1: The town's name, Mudville, might not qualify as typically natural imagery, but it definitely gives us a rural, outdoorsy feeling. Mud is pretty nature-y, right? When we imagine folks from Mudville, we are more likely to picture farmers and ranchers than bankers and boutique owners. This place is more Mayberry, less Manhattan. From the poem's first line, we are placed in a natural setting. We imagine the ball field surrounded by farms and fields, not high-rise buildings. If the team was from Gotham or Metropolis we'd start the poem with an entirely different landscape in mind. "Mudville" sets the scene and the tone.
Lines 18-19: When things start looking up for the Mudville faithful, they let their team hear it. They let out a cheer that echoes from the valleys to the mountains and across the plains. It's as if their collective cheers are so mighty that they can be heard across the whole country. By including these various landscapes, Thayer includes, in a sense, everybody in the scene that's unfolding in Mudville. It changes the scope of things. Suddenly the Mudville fans seem to represent all of us, and our desire to see the hero come through and save the day. Go Mudville!