East of Dorchester the river runs over sands, clays and gravels which overlie the chalk; as the valley gradient is gentle the Frome has deposited much sediment here and thus created a broad floodplain. Historically this contained marshes and gave the name to the Durotriges, water dwellers, the Brittonic Celtic tribe who inhabited Dorset. The river forms a wide, shallow ria at its estuary, Poole Harbour.
East of Dorchester the river runs over sands, clays and gravels which overlie the chalk; as the valley gradient is gentle the Frome has deposited much sediment here and thus created a broad floodplain. Historically thi?s contained marshes and gave the name to the Durotriges, water dwellers, the Brittonic Celtic tribe who inhabited Dorset. The river forms a wide, shallow ria at its estuary, Poole Harbour.