In a smooth grassy valley, a location called the Keypoint can be found where the lower and flatter portion of a primary valley floor suddenly steepens.
The Keyline of this primary valley is revealed by pegging a contour line through the keypoint, within the valley shape.
All the points on the line are at the same elevation as the keypoint.
Contour plowing parallel to the Keyline, both above and below will automatically become "off-contour" but the developing pattern will tend to drift rainwater runoff away from the valley centre and incidentally, prevent erosion.
Keyline pattern cultivation on ridge shapes is done parallel to any suitable contour but only working on the upper side of the contour guide line.
This automatically develops a pattern of off-contour cultivation in which all the rip marks left in the soil will slope down towards the centre of the ridge shape.
Xisca Nicolas wrote:Contours on maps are never parallel.
The tighter the steeper.
So ok, the plowing is off-contour.
Now, in what sense is "parallel" on the ground?
How can you do this, do you need to imagine a "view from above", as in a map?
Is it more difficult to do it than seeing it on the map?