I have seen it handled slightly differently to account for acceleration. 1d6 for first 10', 2d6 for second 10', 3d6 for third, etc. so it would do 21d6 (1d6+2d6+3d6+4d6+5d6+6d6) damage for the 60' fall which seems a bit more realistic if it actually hits. Of course, in real life, depending on the air resistance, you will eventually hit a maximum velocity, so damage is usually capped at 60' in games I have played in just for convenience; i.e. falling 2000' is just as damaging as falling 120' since you are at the same speed when you hit the ground. And if you want to get really analytical, hitting stone is probably a lot more painful than hitting say a marshland or bog at the bottom of the fall.