In the Canyonlands of Utah and Arizona, Canyon Walls Often

In the canyonlands of Utah and Arizona, canyon walls often support "hanging gardens" in which plants cling to sandstone cliffs and these hanging gardens are typically along shale beds in the sandstone. Why is this happening in a desert setting?

A) The shales trap groundwater along the shale horizons that helps aid plant growth.
B) The shales are more fractured than the sandstones so the plants have an easier time starting in the shale.
C) The shales form better soils than the sand, so the plants grow there.
D) The shales hold the water better than the sandstone, so plants preferentially grow in them.