A formation is a set of layers that have a specific lithological composition and spread and extend over a relatively wide area. By examining sediments and deposits, geologists find out the sedimentary environment. The formation boundaries with the lower and upper layers is clearly defined. The top and bottom of a formation is defined, but its thickness has no definite limit. The junction of each formation with its upper and lower formations is called formation boundary. This border can be in different forms. Specific and sudden boundaries, gradual, discontinuity, between fingers, etc. make all kinds of boundaries of formations. In any case, the boundaries of the formations are clear and distinguishable from their upper and lower formations, whatever their state. When two adjacent formations cannot be separated due to their similarities, they can be named together. In this paper, determining the boundary of a geological formation in a sedimentary basin will be discussed.