Sylvia,
You must be producing super dogs out there.  Our 2+ year old male GS seemed to have scaled a 6' flat wood fence this weekend to chase a roaming cat. The cat escaped down a storm sewer and was found hours later unharmed.  Our GS "Berkeley" was fine as well.   I never would have thought he could get over the fence but there were claw marks all the way up the fence near one comer and that is probably where he climbed over.  There was no other possible exit other than over the top since the gates were still locked and there no areas to crawl under.  The cat has taunted him before by walking along the top of the fence and one time before was almost caught.  Have you heard of GS climbing over a 6 foot fence like this before?  He is 80+ lbs of dog and I'm very surprised by this.
In retrospect I will tell you another interesting story.  I took him to a farm and he was free to run following me on my ATV.  I thought I would wear him down into to nothing and get him good and tired by having him chase me on the 4 wheeler.  He ran all day long following me chasing fish eating birds off the ponds all over 2000 acres and still had plenty energy at the end of the day.  There went my hopes of ever wearing him down by walking him around the block a few times since he probably ran 15 or more miles that day.
The other free roaming (and fit) farm dogs gave up running after maybe a few hundred yards of 20+ mile per hour running.   They just did not have the genetics and lungs to come close to keeping up for any longer distance. These are some serious dogs with remarkable running abilities.  I bet most other working breeds could not match their long distance running or speed.
No doubt this is an industrial strength dog that is very intelligent, always on alert (but not nervous), will test you, maybe wants to be in charge and is not for a novice dog owner that wants a mild quite little pet.

Best regards,
Anthony