Unpopular theory time: Sand Shower is the name of the desert zone and Dust Hill is the name of the rock world. Eroding rocks are dusty. Don't at me. In all seriousness though. It makes sense that the mountain in the past map labeled Rock Zone could have eroded away (or been destroyed by Robotnik) and the result have been a desert. Also the original demo for the Oil Ocean Zone music gives a much more Arabic feel to it. My guess is that was the original music planned for Sand Shower (which was probably meant to be an Arabic style desert like in Sonic 4 E2). But Brenda Ross made an American style desert (since she's American) and the Japanese team didn't like it so they canned the whole stage. Thus the music was modified to feel a little less Arabic and used in Oil Ocean Zone.