from the northern Great Barrier Reef, Australia

90 km across the continental shelf. This is the deepest shelf valley yet found in the Great Barrier Reef and is well below the maximum depth of fluvial incision that could have occurred under a 120 m, eustatic sea level low-stand, as what occurred on incision model may explain the observation of other workers, that sediment is exported from the Great Barrier Reef shelf to the Marine Geology 220 (2005) 181–204 www.elsevier.com/locate/margeothis margin during the last ice age. These valleys appear to have formed by a combination of reef growth and tidal current scour, probably in relation to a sea level at around 30–50 m below its present position. Tidally incised depressions in the valley floor exhibit closed bathymetric contours at both ends. Valley floor sediments are mainly calcareous muddy, gravelly sand on the middle shelf, giving way to well-sorted, gravely sand containing a large relict fraction on the outer shelf. The valley extends between broad platform reefs and framework coral growth, which accumulated through the late Quaternary, coincides with tidal current scour to produce steep-sided (locally vertical) valley walls. The deepest segments of the valley were probably the sites of lakes during the last ice age, when Torres Strait formed an emergent land-bridge between Australia and Papua New Guinea. Numerical modeling predicts that the strongest tidal currents occur over the deepest, outer-shelf segment of the valley when sea level is about 40–50 m below its present position. These results are consistent with a Pleistocene age and relict origin of the valley. Based on these observations, we propose a new conceptual model for the formation of tidally incised shelf valleys. Tida