If you want to make it yourself then I would get some food coloring and a few pastry bags with different tips. You could use malt balls for rocks or boulders and you can make shrubs and grass with the pastry bag tips. Its really very easy. Just don't squeeze the bag too hard a slower pace is better. You can even make a steam with a pastry brush and blue food coloring. You could also use crushed oreos as dirt if you wanted.
My son is having a 'Land Before Time' theme for his 4th Birthday - have a look at ebay - Dinosaurs Fingeroos Cake Kit Toppers - item no 350330845758. It is not 'really' land before time - however my son loves it and has decided this is the cake for him!! (and it sounds EASY!!)
You should go to the store and get a Disney cake made without the figurines. You could try one that's got dinosaurs in it? Then you could put Land Before Time dinos on there...