Delta deposits extend upstream, within the mould of the cavern within ice of the glacial period. Thus the map shows a snake-like ridge of gravel, ending in a maple-leaf flat, with lobate frontal slopes. These slopes were much steeper where the dump of the stream on the delta fell over the beach line at the lagoon or lake level in which the delta was built. This was like the delta shown in Stone’s erosion model. Stone prospected the idea of torrential deltas in a tank, while E. W. Dorsey and I started a tank imitation of the glacial sand delta. In the glacier, the ice tunnel had been supplied with water by melting through the ice crevasses, just like tunnels seen in Switzerland, floored with sand ground up by the ice. There was thus a torrent pouring along inside an arched tunnel, the mouth of which emerged on a delta in a pool, with water surface either at the tunnel level or above it against the rounded front of the ice mass. In imitation of a rounded bank of ice with a pool of water in front and with a subglacial meandering cave fed with sands and a torrent, an apparatus was built and supplied by a hose. A sheet of lead was bent in the form of the glacier surface, with an arched opening, and set in our tank. This fitted over a tunnel of sheet iron, soldered so as to meander in plan, and fitted at its upper end with pipe and hose connection. A sheet-iron funnel rose from the upper end of this artificial cavern, wherewith to supply different colored sands to the model subglacial river, represented by the hose jet and iron tunnel. The iron tunnel ended flush with the leaden arch. The object of the experiments was, first, to set the leaden glacier in a pool of water in the laboratory tank. Next, to jet water through the tunnel, supply sediment in successive colors through the funnel, and let that accumulate on the bottom of the tunnel and in a delta in front of the artificial leaden glacier. The deltas and their sliced cross sections in different experiments represented the noted difference of kinds of sand supply or difference in water level of the pool. In one case the water level was below the ceiling of the tunnel where it emerged from the arch entrance. In another, it was above the cavern mouth, so that water of the cavern stream, debouching from the submerged cavern mouth in the lagoon, spurted up with its mud and made a half crater against the glacier front. These experiments illuminate the gravel-quarry sections of Massachusetts. In those cuts in eskers (serpent ridges) and sand plains (glacial delta fans) were seen topset beds, or flood wash, or foreset beds at forty-five degrees which are the sublagoon frontal wash, and occasionally backset beds where cavern wash gushed upward.