This method of digging is called stratigraphic excavation. Strata is a word for different levels. If the square is not kept level, it would be easy to discard or cut through what could be otherwise obvious evidence. Digging a hole is the opposite of stratigraphic excavation. The second thing that you should know about how archaeologists dig is that they collect important evidence along the way and keep it all organized by square and locus.
Evidence includes soil samples, floral and faunal samples and the collection of all objects.
Pottery is very important. You have learned that pottery changes over time. Changes in pottery, like coins can also be used to set a latest possible date for a particular level of an excavation. All of this evidence has to be carefully collected and organized. Some people spend each afternoon, after long hours in the sun, working with pottery.