Ever wonder where building materials come from to support London’s railway projects? Much of it is from ships like these that work 24/7 around the UK coastline dredging sand and gravel from the sea bed. (I worked for one of these companies before getting into rail projects). https://twitter.com/dredgingtoday/status/1301831452955422720

Contrary to what some would have you believe, sand and gravel dredging is highly regulated with extraction limited to licence areas and ships constantly tracked via GPS to ensure they don't stray. The licences are owned and monitored by The Crown Estate.

Different licence areas yield different cargos. Material from the Norfolk coast tended to be smaller stone and fine sand whereas the more recently licensed areas in the English Channel provided coarse sand and larger stone. These are often blended together whilst being processed.

Blending ensures that the material coming out of the plant is consistent and can make the required grades to meet concrete specifications. During the processing a lot of the contaminants are also removed - like shells (although some is permissible)