Rockfall does 6 Crushing damage to everyone in the affected squares. Weak Block blocks things that do 4 damage or less, which therefore includes spell effects that do no damage at all (like Memory Loss). A goblin's Weak Block stopped the damage from a Rockfall, presumably because Rockfall states the damage within the text of the card, not in the little box in the corner. This strikes me as odd, and I guess there must be some complicated reason (based on effect triggers) for why. But then, Acid Spray does an actual 3 damage as stated in the corner of the card AND causes a terrain effect. Why not Rockfall? For that matter, Rockfall may count as having a "text bug": is doesn't use the standard keyword "Burst," and instead spells out the full definition of Burst in plain text. I suspect it is an older card and could be brought more in line with Acid Spray, using a keyword (this should be Burst, while Acid Spray uses "Cone") and putting the damage in the corner.
