Based on AwfulKanwful - inspired by the new, faster, Flash player for Scratch. Use UP/DWN to change speed. Awful keeps going forever *infinite scrolling", if you want to make more terrain, add costumes to the TerrainOne, Two and Three sprites, just make sure they begin and end at the same level so the terrain matches up - easy to do if you just copy the costume and edit the middle part...
