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? Ralph Stone tackled the Mystic River, and marked ledges and set stakes opposite the flood plain meanders. The idea was that ledges split by winter freezes, and that the meanders of a stream build on one side and cut on the other. Maps were made repeatedly, and the ledge cracks were measured in millimeters. Some movement was found, but a college year was not enough time. If we could combine as a motion picture, photographs from the air taken once a year for many years, doubtless the film would show that the stream meander pattern is migrating toward the sea like a wiggling snake. Stone next made a model three inches thick in a tank of water, by sedimenting sixty-one very thin layers of marble dust, coal dust, clay, red lead, and sand. He tipped it up as an island and sprayed it in periods which lasted one to ninety-two hours, up to a total of 719 hours. A forking stream and its delta were formed in the lagoon of the tank. The stream cut a canyon with waterfalls, treelike branches, esplanades, and a flood plain. There were three principal hard white multiple strata layers in the model, separated by sand. The white layers made waterfalls and were eaten back to form the canyons. When the cross section of the delta was sliced with a knife, it showed three white layers foreset at thirty degrees under the tank pool and separated by more sandy strata. The bottommost of these was the sediment of the top thick marble dust layer of the model as first eroded by the spray, and the top frontal layer of the leaf-shaped delta was the product of the erosion of the canyon bottom on the lowest of the white layers. This must happen in nature where one formation in reverse order is derived by river erosion undermining a stratified older pile of sediments.