Bedrock often appears in outcroppings following a diagonal orientation rather than parallel to the earth's surface. I think seeing the remarkable geological examples of this at Gros Morne National Park Newfoundland in 2008 might have triggered my imagination for trying to build on the slant.
Later that same year I visited a garden designer in California and built a small wall using stones that were in the discard bin of a stone supply company in San Francisco. This decorative border wall incorporated all kinds of different flag stones and slate along a diagonal. The local California grocery store sold a wine with a name which necessitated me buying a bottle just to use it in a photo with the wall in the background.
The low retaining wall I built in Berkeley for Mariposa Gardening in California was a prototype for a project I was asked to build the next year in Danville California in 2009.
Later that same visit we built a slanted wall and Gothic arch at the Flower and Garden Show March 18th to 22nd in San Mateo California.