expanse of sand, pebbly beach
磧 centers on a broad, open area of sand or pebbles, especially along a shore or riverbed. The two given meanings are natural variations of the same landscape feature.
磧 combines 石 (stone) with 責, which likely contributes the sound. The character refers to a stony or sandy expanse, fitting the stone radical.
The left side 石 (stone) suggests a rocky or pebbly place, and the right side 責 looks like a pile of shells and coins. Imagine a beach covered with pebbles and scattered treasures: an expanse of sand and stones.
For セキ, picture a seagull pecking at pebbles on the beach: seagull -> セキ, and the bird is searching among the stones.