A strip (usu. 10-25 centimeters) of unexcavated earth left in place between excavated units, pits, or trenches for the purpose of revealing the stratigraphy of an excavation for as long as possible. The balk provides a constant reference to the original pre-excavation level of the site, and also carries all sections along or across the site. In an excavation carried out according to the grid method, 25% of the site may consist of balks. Balks may also serve to facilitate access to different areas of the excavation.