a rocky mountain
岨 centers on a mountain that is rocky or stony, describing a rugged peak or slope composed of rock.
岨 combines 山 (mountain) with 且, which likely contributes the sound and may have originally depicted a piled or layered shape, suggesting a rocky, uneven mountain.
A mountain 山 with a rough, layered shape 且 on the right: picture a rocky peak with jagged layers of stone.
For ソ, imagine a rocky mountain so steep that you need to sew (ソ) your climbing gear together to ascend safely.