Once the feature has been dug out, you have to draw a cross section of it. A cross section is a drawing that looks as if you've cut the feature in half. It tells the Archaeologist what shape it was, and possibly, how it was created and destroyed . A cross section is also drawn to scale. A unit is usually dug until you find subsoil .
Subsoil is different in each area, but it is usually a hard clay or sand .
Subsoil is sterile, which means there are no artifacts in it. Sometimes, if you are lucky, you might find some non-European features in the subsoil. Once you find subsoil in your unit and you are sure it is sterile, you can draw a profile of the unit. A profile is also drawn to scale. Once the profiles have been drawn and photographed, you can backfill the unit.