What are depositional environments, and why can changes be seen vertically in a sequence of sedimentary rocks?

Assuming that the rocks in a given area have not been
tilted or otherwise moved around by geological action, you can see changes in the
environment vertically in the sedimentary rocks.  This is because of the fact that older
layers of rock will be covered over with newer layers.  When an excavation or a river or
something cuts through a deposit, the layers can be seen in vertical order, oldest at
the bottom, newest at the top.

The changes in depositional
environment can be seen in the types of rock in each layer.  The rocks will reveal
aspects of the environment in which they were laid down.  They will show what kinds of
sediment were laid down, they will show some things about the types of plant life that
were around at the time.  They can also show what laid the various strata
down.

So the changes in environment can be seen because
sediment will be put down in layers that will differ as the environment
changes.