Deciphering the outcrop expression of beds and topography
This post is part of the How To… series
Geology maps and topography maps go hand in hand. Try drafting a map of surface geology without reference to the local topography and you will end up in a heap of trouble. The rules inherent in topography maps are pretty basic: contours represent horizontality, contours always close (except on the edge of a map), contours never cross although they may become very close on steep terrain or cliffs, closely spaced contours represent steepness, and the geometry of contours changes in predictable ways between valleys and ridges. The latter is called the Rule of Vs, where:
- contours V up-gradient ( upstream) in valleys, and
- down gradient on adjacent ridges.
Geologists have borrowed the Rule of Vs to describe the geometry of mapped strata. The Rule also serves to predict the surface location of strata in areas of poor exposure. The Rule applies to the simplest case where bedding is a flat plane, dipping at any angle. Keep in mind that the topographical expression of bedding represents erosional remnants of rock layers that once extended above the present surface; a lot of rock has been removed to produce the topography you see today.
To get a sense of this in three dimensions, take the example where bedding dips upstream (up gradient). Imagine these beds extending above the present surface. In your mind’s eye, whittle away at the strata until your imaginary surface coincides with the present surface. The mapped expression of the beds will show them V-ing upstream in the valley and down gradient on the adjacent ridges.
Uplift and erosion of a flat but dipping bed, and the resulting outcrop expression
Map expressions for six different bedding orientations are shown in a short animation, in 3D block views and their corresponding map views. I have taken the liberty of modifying the examples shown in Donal M. Ragan’s excellent text Structural Geology: An introduction to geometrical techniques. Pause each frame if you need to spend more time looking at it. The animation shows, in sequence, beds that dip: horizontally, upstream, vertically, downstream dip greater than stream gradient, downstream dip equal to stream gradient, and downstream dip less than stream gradient. The second part of the animation is presented at a faster frame rate to give a more ‘animated’ sense of changing map expressions.
D.M. Ragan, 1968. Structural Geology: An introduction to geometrical techniques. John Wiley & Sons Inc. 166 p. This book is into its 4th Edition.
Some other useful posts in this series:
Solving the three-point problem
Plotting a structural contour map