Earthquakepocalypse- Great Falls

So today was my friend Kimberly's birthday.  We spent the day at Great Falls.  So there we are sitting on a rock, overlooking the falls when the earth starts shaking.  It just so happened to coincide with a big gust of wind, so at first we thought the wind was making the rock move (yeah, we're geniuses I tell you).  Afterwards I started moving my butt back and forth trying to make the rock move and it didn't, so I was thinking... there's no way wind did that.  A few minutes later my mom called to check on me after the "earthquake".  Only then did we put two and two together and realized what had happened.   Where were you when the earth shook?