"I would have seized and attempted to arrest the pendulum. I might as well have attempted to arrest an avalanche" means the pendulum was too ____?From The Pit and the Pendulum by Edgar Allen Poe
The most likely answer here is "heavy" or "big."
What he's saying is that he tried to stop the pendulum from swinging. But trying to stop it was like trying to stop an avalanche. This clearly implies that he was completely unable to stop the pendulum. The most likely reason why you would be unable to stop a pendulum from swinging would be that it was too big or heavy so that when you tried to stop it it just kept swinging.
I suppose you could also say that it was too slippery or smooth, but I think big or heavy is more likely.