Geologic Maps and Cross Sections

Definition: A map which shows the distribution, nature and the age relationships of rock units and the occurrence of structural features.

Geological Maps and cross-sections Outcrop pattern Topography and attitude of the geological structures control the shape of the outcrop. Outcrop pattern = attitude of beds + topography Horizontal beds will follow the contour lines. A vertical bed will appear as straight line on the map irrespective of the topography. All dipping beds will cut across the contour lines and have zig-zag appearance on the map.

The Rule of Vs One of the most important generalities is the "Rule of Vs", which states that where dipping surfaces are incised by stream valleys or canyons, their map view will make a "V" in the direction of dip. If the surfaces dip vertically, then they do not make a "V"; if they are horizontal, then they will follow the contours: the "V" will point uphill. The only exception to this rule comes when the dip of the bed is less than the gradient of the valley. Then, similar to horizontal surfaces, the surface will make a "V" that points upstream.

Oblique View. Beds dip progressively less as you go to the left Oblique View. Beds dip progressively less as you go to the left. Block on right shows vertical beds. Looking straight downwards (Map View). The vertical bed shows no "V", the two dipping beds show "Vs" in the direction of dip, the horizontal bed shows horizontal stripes on either side of the valley. Note that the "V" becomes increasingly pronounced as the dip decreases.

The rule of Vs, also shows other effects of variable topography on the outcrop pattern. Where the terrain is steep, and so at a high angle to the dipping bed, the outcrop width is relatively narrow. However, where the terrain is gentle, and so nearly parallel to the dipping bed, the outcrop width is much wider.

A B C D E A:  The formation is horizontal, therefore the formation contacts, i.e., the lines on the map where the top and bottom surfaces of the formation intersect the topography, are parallel to the contour lines. B:  The formation is vertical, therefore the formation contacts transect the map as straight lines. C:  The V points up the valley, therefore the formation dips up-stream direction. The formation dips steeply, much more steeply than the stream gradient, so the “V” pattern is shallow or blunt. D:  The V points up the valley, therefore the formation dips up-stream direction. The formation dips gently, much more gently than the stream gradient, so the “V” pattern is exaggerated.  E:  The V points down the valley at an angle which is steeper than the stream gradient. The formation dips moderately towards down-stream direction.

Geological Map Cross-section

6 7 8 9 9 8 7 6 6 7 8 8 7 B A

1000 1000 . 500 500 6 7 8 9

1000 1000 . 500 500 6 7 8 9

1000 1000 . 500 500 6 7 8 9

1000 1000 . 500 500

1000 1000 500 500 A B

1000 1000 500 500 A B

1000 1000 . 500 500 A B

1000 1000 500 500 A B

Stratigraphic succession Lithology Thickness (feet) 100 + partly eroded Basalt 100 Rhyolite 100 Sandstone 200 Conglomerate 450 + base not seen Granite

Stratigraphic succession Date: Cross-section Stratigraphic succession Scale