you could use a black/white collission image for example or in the case of suggletruck you might get away with only storing the height of each "column" (with the height at 0 or -1 for "gaps") then you can use the height to generate the actual displayed terrain.

You could also use polygons for the terrain (and store the level as a list of polygons)

you could use a black/white collission image for example.