These ancient walls are made of distinctive reddish pink local sandstone. Not fired brick. It's made the walls durable against enemies, but sadly not against time.
This sandstone dissolves into fine pink sand at the slightest provocation. Since it leaves the mortar more or less intact, you get what looks like a strange honeycombing effect as the mortar juts outward and the sandstone just recedes.
Erosion of city wall
Recorder's Steps, Chester
Below the Recorder's Steps