Some of the fingerlike drainage of erosion cuts into plowed lands during a rainy spell. This suggests what might be done with a spray, a mud bank, and a tank, to see how the finger valleys form. This erosion of the runoff of water was imitated in the Harvard laboratory.
A beautiful river pattern on a slope, like the trickle of raindrops on a windshield, was made by tipping up a rectangular glass plate covered with very liquid clay. A portion clung to the glass, and exquisite fernlike streams formed on the upper half of the plate, with a bank of distributaries of V-shape on the lower slope.
This glass plate was used for a surface of stamp mill slimes, of thicker beds, and was eroded with an atomizer and water by means of a barbershop air compressor. The slimes are very fine pounded sands with angular fragments. To get a stream pattern, this is necessary, so as to have fine grit to cut down the rivulets between the coarse grit remnants. This resembles the requirements for ripples.
The spray was kept going for hours. Meanwhile the river pattern at the steep sides of the sloping plate ate into the bank of sediment, robbing the streams of the main slope, because the side streams were oblique cascades. They dug deep, took off the water, and left the main slope streams without their headwater drainage. The pattern of the main slope became the headwater branches of the side streams, the streams which in plan drained over the edge of the uplifted plate right and left. This was somewhat like stream robbery.
For example, the Lewis River at the south end of Yellowstone Park once drained Yellowstone Lake, including the Lamar River, which is now the headwaters of Yellowstone River. The Yellowstone plateau formerly drained south into the Snake River and the Pacific Ocean. The Yellowstone River headwaters suddenly tapped the system, thanks to geyser erosion and acid corrosion, and the Yellowstone Canyon cut down rapidly, reversing to the north the outlet of Yellowstone Lake. Thereafter the lake flowed into the Mississippi and the Gulf of Mexico. At some critical time about the glacial period the continental divide made a leap of thirty miles from the present head of the Canyon to the neighborhood of Lewis Lake, or from one end of Yellowstone Lake to the other. This is stream robbery.
Spray and runoff and rainfall and wash did not alone cut down the Yellowstone Canyon. The essentials were the rotting of rock and the pull of gravitation on the fragments. The Yellowstone rotted away on the north side, but it was hard granite and mountain-built quartzites on the south, toward the Tetons. Hot spring rotting, geyser erosion, acid waters, and sulfur decomposed the north country. The underground water head followed the easiest channels, and the canyon was the result. The canyon line encircles Mount Washburn, the old volcano, and conceivably is over an old crack concentric to the dome.
Water is a transporter, and cracking opens ways to the rotting agents. Only in rivulets and floods does water actually corrade, or grind, the bottoms of streams. In our spray and fern patterns there is analogy to rainfall springs on flat strata, but nine-tenths of the elements of erosion are left out: jointing, weathering, ice, faulting, gravitation, rotting down, quaking, solution, sliding, and last, spring water.
Erosion by sliding continues by wind action in desert mountains, and on volcanic cones under bombardment, and by rocks snapping under chill and sunshine on the moon. Creep of loose stuff is the greatest eroder on earth. Rainfall cloudbursts certainly help, especially where soil is not held together by a mat of roots.
The process of erosion is supposedly slow, as all geological processes are slow, if we neglect the possibility of such submarine landslips or supramarine upheavals as occurred at Yakutat in 1899. But even New England has floods, hurricanes, landslides, forest fires, and cloudbursts which are exclamation points on an otherwise sleepy history. And in the past it has had ice sheets, and subsidences beneath the sea.
In other words, the making of valleys and stream patterns for the map is accented occasionally, and the occasions may come in climatal waves unknown to us. The stream patterns in the Bad Lands, Tennessee, Pennsylvania, the Grand Canyon, and New England make very different maps. The rotting of the rock, limestone caverns, rainfall, faults, and sloping underground strata bearing spring water all influence these maps. What is erosion and what index is written on the land to say the Grand Canyon and tributaries are being carved downward faster than the Mystic River in Boston?