The Brick Marsh is a marsh from pre-tractionist London. It was formed when the River Thames burst its banks and flooded London's Southern Boroughs sometime during the Scriven's rein. Auric Godshawk had a house in the swamp. A tunnel leading from the house to the Barbican allowed the Scriven to move between both places in secret. In Scriven times, the marsh was crossed by a long causeway. By the time of Fever Crumb, it is passible only though the use of boats, or Godshawk's tunnel.
Dr. Crumb was briefly employed there as a research assistant and began an affair with Wavey Godshawk. During the Scriven Uprising, the remaining Scriven went to Godshawk's house in the Brick Marsh. Several days later, Bagman Creech and a posse of Skinners arrived to finish off the Scriven. The Scriven shot all of them except Bagman, who escaped by submerging himself in the marsh and hiding beneath his hat. Later that night he crept into the house and slaughtered the Scriven, but spared the lives of Wavy Godshawk and baby Fever.
Bagman Creech returns to the marsh with Charley Shallow in order to find out if Fever Crumb is a Scriven or not. He attempts to kill Fever and is shot by Kit Solent. Later, Charley Shallow chases Fever through the marshes, intending to finish what Creech started, and shoots her with a magnetic gun that disables Godshawk's memory device inside her head.
It is likely that the marsh was drained by The Movement during the construction of Traction London.