What are Geologic Cross Sections?

 A third standard geologic diagram is a cross section, which views the earth as if it were cut open and seen from the side. Geologic cross sections are interpretative, since the relations can generally not be observed directly. Only in areas of deep canyons or high mountains can natural cross sections be observed. In most cases cross sections require inference about subsurface structure. Sometimes drill holes or geophysical exploration provide data from which cross sections can be constructed. 

 Look at the cross-sections on page 5 and page 15a of Rocks, Rails and Trails as examples of this type of geologic diagram.