They arnt. The tile set is missing alot of tiles sadly. Those werent even available until the last tile set was released. There is another tile type missing. That is neeed for making desert to cliff tiles. They have it for grass to cliff but no sand to cliff.
here is an exampe It really limites my dried up river since it can only go at \ this angle. So the dried up river can never twist and turn this way. /
Wait a second no ignore the screen shot on this post. That isnt the missing tile. I can't remember which tile it is thats missing maybe its the bottom one. Hmm guess I'll try and add more to figure it out because I cant remember. x.x