Japanese Karesansui flat surface

Quick answer：

Karesansui, often known as 'dry landscape' or 'dry garden,' is a traditional Japanese art form that uses carefully placed rocks, sand, and often moss to represent a natural landscape. A 'flat surface' in this context likely refers to the flat, gravel-covered ground that forms the base of a karesansui garden, providing a stable foundation for the placement of rocks and other elements.