Eighth Edition
by Harold L. Levin

Chapter 5 - page 13

The Sedimentary Archives

Depicting the Past

There are various ways in which the distribution of rocks can be depicted. These include:

• Geologic Columns Columnar sections show the vertical succession of rock units at a given location. They are used in correlation and in the construction of cross-sections.

Columnar section.
• Stratigraphic cross-sections tie together several geologic columns from different locations. The purpose is to show how rock units change in thickness, lithology, and fossil content across a given area.
• Structural cross-sections show the timing of tilting, folding, and faulting of rock units. The datum is a level line parallel to sea level. The tops and bottoms of the units are plotted according to their true elevations. Folds and faults are depicted clearly.
• Geologic Maps show the distribution of various layers and types of rocks in an area. Geologic maps are prepared by geologists who locate the positions of contacts between formations in the field, and plot them on a map. Map symbols indicate structural features (folds, faults, etc.) and formation names.


Steps in the preparation of a geologic map.


A = Base map.
B = Outcrops and contacts are plotted. Structural orientation is shown using strike and dip symbols.
C = Completed geologic map.
D = Cross-section along line A - A' on map C.
E = Block diagram illustrating strike and dip.
• Paleogeographic Maps are interpretive maps which depict the geography of an area at some time in the past, for example, maps showing the distribution of land and sea in the past.
• Isopach Maps show the thickness of formations or other units in an area.
• Lithofacies Maps show the distribution of lithofacies that existed at a given time over an area, or show the percentage of some lithologic component (such as clay), or show the ratio of one rock type to another within the unit.


Diagram illustrating the preparation of a lithofacies map from a subsurface time-rock unit.