Trenches and Gullies
Often where you have high general sand levels, the only productive ground will be at the bottom of any trenches or gullies. This can be only a six-step “swath”sometimes running along an entire beach. As well, the actual shape of these trenches is important in that where you have a trench with flat ground at the bottom (termed a “floor”) this can mean more productive ground to hunt. At the shore-ward side of a trench–a flat slope will often move away from the base or “marl” whereas when you see a low, curved contour–this can often mean ground that is low enough to keep more of the base within detector range. A trench that is abrupt–dropping off sharply, is also a good sign. This means that sand is being moved around with considerable force–rather then just the gradual, gentle contours that result from silt being washed around.
Where you see these formations in the water, try to spot something on shore to landmark your location with. This will not only help you to return and monitor the section but also to see if there has been movement–forward, back into deeper water, or laterally. As with the example above, having the patience to return again and a again to monitor a section that shows contour changes can result in some great finds.
From: “Water Hunting: Secrets of the Pros, Volume II”