07-20-2012, 07:48 PM
The ravines are placed completely randomly. They are 64 - 128 blocks long, their bottom is at Y 5 to 37 and their top at Y 32 to 64. On average there is one ravine in a 128x128 blocks area.
This is what they look like in Minutor:
This is what they look like in Minutor: