The shirasu is spread out across the garden to represent a water surface, as you can see in today's photo, taken inside the Hirayama Katsumi garden.
EXIF info:
Nikon Df
Lens: 14mm F/2.8D
Focal Length: 14mm
Aperture: F/6.3
Shutter Speed: 1/1000s
ISO Sensitivity: ISO 800