Excavation documentation aims at recording the materiality of the archaeological site uncovered and partially destroyed by the excavation process, with an emphasis on spatial information. The knowledge about the past is built on the recording of physical materiality and its spatiality. In archaeological knowledge production, temporal information is inferred from material and spatial relations. The main objective of this paper is to model the spatio-temporal reasoning process applied on excavation documentation of stratigraphic units and interfaces with the CIDOC CRM family of models, thereby enabling the mapping of Harris matrices to CIDOC CRM, while clarifying the distinction between process documentation and reasoning. We apply a methodology of knowledge representation on the Harris laws. The result is a formal modelling of the Harris matrix laws in the CIDOC CRM family of models, with competency questions (CQs) and associated queries.