Experiments in connection with Daly's hypothesis on the formation of submarine canyons

Shepard has made us familiar with the shape and various properties of submarine gorges and in a number of publications he has shown the great importance to earth-science of these wonderful features of the sea floor. Various hypotheses have been brought forward to explain the formation of submarine canyons. The only hypothesis to which there are no grave objections, is that suggested by Daly. During the low sea levels of the ice age mud was stirred up on the shallowing shelves and the dense water ensuing, flowed down the continental slopes, gradually eroding the gorges. This theory can be substantiated by experiments. These proved that a suspension will flow down a slope without being much diluted by mixing, that such a flow will concentrate in slight depressions in the flat and follow a rut down the slope, that the current will begin to form ripple marks in sand at a velocity of 10—20 cm/sec and that the speed varies with the root of the density and probably also roughly with the root of the depth. Rough estimations, based on the data of one of the Georges Bank canyons, show that the velocity in nature should be somewhat less than that of larger rivers, but encreasing as it proceeds and catches up more sediment. At the lower end the suspension has gained 6 times the (effective) density and two to three times the velocity it started with. At this stage the amount of sediment has caught up the average of most larger rivers and reached that of the most muddy major rivers. Some 10.000 flows must have occurred, or one every 5 years, each eroding as much sediment as the Mississippi carries to sea in two or three weeks. The current lasted many hours and its discharge was several times the average of the Mississippi. The relatively large percentage of materials carried by the currents is a consequence of its having slumped down the sides of the gorge before the current picked it up. However rough these figures may he, they show that no preposterous assumptions have to be made to arrive at reasonable relations between velocities, amounts of water and sediment and size of the gorges for Georges Bank. Where the shelf is narrow, as at some of the Californian canyons, there appears to have been hardly enough sediment to set up the necessary currents. The chief remaining uncertainty lies in the necessity of assuming small strength of the rocks forming the continental slopes. But all evidence available except some of Shepard's dredging results, points to the same conclusion. Older, smothered gorges may have been cleaned out by this mechanism. In any case, as Stetson points out, the mechanism must have played an important part in the sedimentation on and beyond the continental slope. Groningen, March 1937. An 8-mm film illustrating the experiments, of about 20 meters length, can be obtained at the price of 8 guilders from the Rijksmuseum van Geologie at Leyden.