This word is still in common, everyday use in Scotland; although it is strangely difficult to find in conventional English dictionaries. This is a great pity for it is a lovely word, and definitely not obsolete. People will still speak about "going up on the knowes" (e.g for a picnic, letting their children or dogs play about.)
It occurs a lot in Scottish place-names. It is also found in traditional Scots poetry (e.g. Robert Burns "Ca' the yowes tae the knowes" i.e. "bring the ewes to the hills".)
It usually refers to natural hills, but the Salt Knowe in Orkney is thought to be man-made, with an ancient chambered cairn inside.
I can present photographic evidence of both natural hills called Knowes, and the Salt Knowe in Orkney.http://www.orkneyjar.com/history/brodgar/salt...http://www.orkneyjar.com/archaeology/knoweosk...http://www.panoramio.com/photo/5953728http://www.geograph.org.uk/photo/638707
Notice how in the last example, from the North of England, the word "knowe" is side-by-side with the more commonly-used, more Germanic word "hill".
I hope readers will find the links interesting.