My experiments with volcanoes

by Thomas Augustus Jaggar

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.

So our cross sections, cut with a knife in the delta, and the winding cake extending upstream in the cavern showed topset, foreset, and backset strata after draining the tank and lifting out the apparatus. From the embryo delta the flood-plain beds overlap the earlier frontal, or foreset, beds. The frontal beds are always under the lagoon. The flood-plain beds (topset) were made by a meandering river course under the air. Always this plain is built at beach level as a wash fan shaped like a leaf, with the cavern stream bottom as the stem of the leaf.

New England has been covered with mountainous ice, miles high. Subglacial streams and subglacial clear ice caverns are abundantly found at the lower ends of all glaciers in the world. They merely represent the melting snow and ice in pulses of sunshine, snow at the source, ice in the course, crevasses and gravitation making water seep through. This water shapes a channel for itself and erodes a sewer system of scouring along the bottom of the subglacial valley. This grinds and melts the bottom ice into arched caverns; and the sediment builds up on the stream bottoms, eventually carving the roofs of the caverns into high arches or arcades. The subglacial caverns are self constructed drainage pipes.

The glacial stream is really a river flood cutting its valley. The ice river grinds and scrapes, and the water under the ice pipes and drains the melting. The ice carries chisels of broken rock. The enormous weight, in gliding plane layers of ice, flows in accordance with the crystal laws of snowflakes and ice crystals. The moraines, or debris fields, at the sides and on top and underneath the eroding ice jumble yield mud and sand and boulders. The torrent underneath removes the rubbish.

The delta in front follows laws of sedimentation. If there is no lake in front, the delta is a flat wash fan, or valley flood plain. All these things become clear to the student who makes a baby glacier out of tinware, sand, a tank, a hose, and a faucet.

I have spoken of cataclysms, or what early geologists called catastrophe, happening occasionally in the world of erosion and subterranean geysers. Such were the Yakutat crash and the Bandaisan explosion. But each glacier-period field, like an ice mountain over Europe and America, constituted a cataclysm lasting 500,000 years, and this happened four times even in the centuries of early man. The Mediterranean and the Great Lakes are offspring of such cataclysms. But Lyell carried the doctrine of uniformity to extremes; he thought that what man sees is what always happens. I do not believe Lyell ever realized that earth or sun might conceivably explode in a month of our time. Again, this is not likely.