It has been proved by experiment that the rapidity at the bottom
of a stream is every where less than in any other part of it, and
is greatest at the surface. Also, that in the middle of the stream
the particles at the top move swifter than those at the sides. This
slowness of the lowest and side currents is produced by friction; and
when the rapidity is sufficiently great, the soil composing the sides
and bottom gives way. If the water flows at the rate of three inches
per second, it will tear up fine clay; six inches per second, fine
sand; twelve inches per second, fine gravel; and three feet per second,
stones the size of an egg.--_Sir Charles Lyell._