Why can't I clear rocks on DragonVale?

Since this question has languished for so long and has no less than five duplicates attached to it, I decided to throw myself into the fire and reinstall DragonVale. By signing out of GameCenter I was able to restart my park and quickly play through to this part in the tutorial.

The screen I was presented with differs from the screenshot in this question and from what I remember the first time I played the DragonVale tutorial, so it seems likely that Backflip Studios has updated the game since this question was posted.

Regardless, in the current version of DragonVale, at this point in the tutorial, you are asked to clear one specific rock, which is marked with a big green arrow. You are not asked (and cannot) clear any rock in the park. Selecting an incorrect rock results in the spade icon being disabled.

Here is a screenshot of me selecting an incorrect rock:

Here is a screenshot of me selecting the correct rock and then being able to remove it and continue on with the tutorial: