var sh,ah,dirt_level,stone_level,adl,size;
size = sprite_get_width(spr_grass);
sh = room_height - 512;
ah = sh;
for(xx = 0; xx < room_width; xx += size)
{
dirt_level = ah + size * irandom_range(5,7);
stone_level = room_height;
object = instance_create(xx,ah,ground)...