When an excavation is done it reveals TWO dimensions about the past.
The horizontal (sideways) excavation reveals the site
as it was at a time in the past.
For example, the horizontal dimension shows the
house floors with furnishings like benches and
things like hearths and rubbish pits.
The vertical (up and down) excavation shows the sequence of change over time at the site. From this, archaeological detectives can figure out the relationships between one time period to another.  For example, if I excavate down in a trench and find housefloors I may be able to tell if people had lived at that site all the time and never left for hundreds of years or if people had come and gone over time, leaving the area unlived in for periods of time.
I know, I know, this is a bit tricky to understand but be patient,
I know that you'll get it!
We can excavate both horizontally and vertically on a
site but we use different techniques for each one.
In this Top Secret Trick I'll explain the open area system for
horizontal excavation - be on the look out for the other one
in another trick section !!
Open Area Excavation - for Horizontal Excavation
This involves uncovering the site layer by layer by layer.
 This is important in sites where the horizontal dimension is more important than the vertical one, for example, where structural (building) remains are rare.  When archaeologists do this, they make sure that the vertical dimension is recorded on paper, but it is never seen or photographed because the soil is taken away layer by layer.
An easy way to picture an open area excavation is to imagine
that you have a big cake. The top layer is icing, then
underneath that is chocolate cake, underneath that is more
fudge icing and then underneath that is more cake.
If you eat your way from top to bottom, you are doing an
open area excavation on your cake!
This is how all good archaeologists eat cake !!!  (No, I am just joking)
O.K. So, did you get the basic idea???
If yes, give yourself a big pat on the back !!!