Archaeological excavation is conducted in controlled levels or spits. Since in the process of excavating a site you destroy it, you must be very cautious as you dig, particularly in the very first opening of a site, since there's no way to know for sure what you'll find.
Thus, at the beginning of an excavation, you may start out digging in arbitrary levels--5 or 10 centimeters (2-5 inches) thick, until the patterns of intact structures or features are identified. Then, the archaeologist must adapt her processes to match the pattern of the exposed features.
Jargon Watch
These archaeologists are using picks and shovels to clear out the room interiors and expose an intact wall of the Mayan residence called Toucan House. The white powdery soil represents the remnants of a plaster layer which covered the interior and exterior floor and walls.
