As rivers flow over the surface of Earth, they cut
canyons in some places and deposit sediments in other
places. Geologists assume that rivers flowing in the
past behaved in the same ways as rivers that flow today.
This is an example of one of the most basic principles
of geology. It is called the principle of uniformitarianism.
The principal of uniformitarianism states that the
processes affecting Earth today are the same ones that
affected it in the past. For another example, at an
active volcano we can observe lava cooling to form layers
of basalt. Therefore, any time we see layers of basalt,
we can assume that they formed from cooling lava.