When ice covered the continental shelf it left deposits behind which contained a mix of sand, gravel and mud. In many areas large icebergs were broken off as the glaciers receded. These large icebergs scoured and ploughed up the seabed, making long deep scratches or furrows in the seafloor. The ploughing of the icebergs caused large boulders to be pushed up to the seabed and concentrated along the edge of the furrow. The furrows are in such deep water that they have remained unchanged for thousands of years. The large boulders in deep water are a stable site for attachment and growth of sponges or other filter feeding organisms. These are the initial sites of growth of the sponge reefs where a hard substrate is available in a very stable area of low sedimentation and relatively fast - (about 1.6 kilometres per hour) currents. These currents ensure an adequate supply of nutrients is delivered to the reef sites and that suspended sediment remains in suspension and does not blanket the reef surface.