The Harris Matrix - formulated by Dr. Edward C. Harris in 1973 - is the established way of representing the archaeological stratigraphy of an excavation. The Harris Matrix is a sequential diagram defining relations between stratigraphic units. It is an important method to document the stratification that is destroyed by the excavation process and hence a vital tool for analysis. In this paper a new application is introduced, called Harris Matrix Composer (HMC) that allows creating and editing Harris Matrices with an intuitive graphical interface. Dr. Harris was involved in the evaluation of early prototypes to guarantee compliance with his theory. User tests were undertaken to address usability problems. The HMC supports valid Harris Matrix creation and indicates invalid units and relations. The theory has been extended to allow for temporal relations as well. Furthermore units can be grouped into structural entities called phases and into periods, assigning them to a historical epoch.