For the majority of the time the ocean floor would have been very quiet with only a gentle rain of mud particles and plankton. However, approximately every hundred or so years this stillness would be interrupted by an avalanche of sediment. Known as turbidity currents, these avalanches originate on the continental shelf and are often triggered by earthquakes. They flow down submarine canyons at speeds of around 20 metres per second. As they slow down, the sand and mud they were carrying is deposited to form a new layer on the sea bed. Over many thousands of years, these flows build up layer upon layer of sediment into a giant pancake-shaped body known as a submarine fan. Preserved within the Manx Group are several submarine fans shown with yellow, orange and pale green colours on the central map.
Diagram redrafted from Moore, D G 1969. Reflection profiling studies of the California continental borderland: Structure and quaternary turbite Basins. Geological Society of America Special Papers, 107, 1-142