Stratigraphy is that branch of geology that deals with the study of stratified or layered rocks. Stratigraphy occupies a central position in geology, linking branches of sciences as a single broad spectrum. Most stratified rocks are sedimentary rocks, closely linked with time. The role of time in geology is defined partly in terms of stratigraphic features. While time can be measured in years or hours, it can also be represented in a material sense by features of the earth whose age relationships may be determined through application of stratigraphic principles. These gouche and metallic ink paintings illustrate basic principles of Stratigraphy.