Geology in Crossection

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Geology in Crossection

Plan (Map) View vs. Crossection View
Some Guidelines to determine relative ages in rock crossections
Strike Dip and Apparent Dip
Stress in Rocks

Plan (Map) View vs. Crossection View

map view vs. section view

Some Guidelines to determine relative ages in rock crossections..

four laws to determine relative age in rocks

1. Law of Superposition:
 in any undisturbed sequence of rocks deposited in layers, the youngest layer is on top and the oldest on bottom, each layer being younger than the one beneath it and older than the one above it.

law of superposition

2.Law of Original Horizontality
layers of sediment were originally deposited horizontally under the action of gravity.

Law of Original Horizontality

3.Law of Lateral Continuity

originally extended in all directions until they thinned to zero or terminated against the edges of their original of deposition basin

Law of Lateral Continuity

4.Law of Cross-cutting Relationships
the geologic feature which cuts another is the younger of the two features

Law of cross cutting relationships

5. Baked Zone
An intrusive rock with a baked zone is younger than the rock it intrudes

Baked zone

6. Principle of Inclusions
fragments of  rock in another rock are older than the rock they are embedded in

principle of inclusions

7. Key Bed
for example below: the global Cretaceous-Paleogene boundary  - raised iridium levels suggest and asteroid impact 66.043 million years ago

K T boundary

8 Faunal Succession
index fossils and fossil successions can be attributed to particular ages of the rocks

faunal succession

Illustration of rules

illustration of guidelines

Strike, Dip and Apparent Dip

Dip is the acute angle the rock surface makes with a horizontal plane
Strike is the compass direction of the line formed by the intersection of the rock surface with a horizontal plane
Strike and Dip are always perpendicular to each other on a map

strike and dip explained   

How the rocks appear inclined depends depends on where you take your crossection

tilt 10 degrees    tilt 45 degrees    tilt vertical

Apparent Dip  - is the inclination of the geologic beds as seen from any vertical crossection not perpendicular to the strike
Apparent gives a false impression of the true thickness and extent of rock units and so must be converted to true dip

true dip   apparent dip
apparent dip of coal bed 1of 2   apparent dip of a coal bed 2 of 2
apparent dip in a valley

Three Kinds of Stress in Rock

stress in rock


normal fault  REVERSE FAULT  STRIKE-SLIOP FAULT  thrust fault


anticline  syncline

fold geometry and terminology

plunging syncline  plunging anticline   age of rocks


Joint Pattern form Compresional Forces
  joints  compression

Joint Pattern form Tension Forces

joints tension

Joint Pattern Around an Intrusion 

joints over intrusion
Joints around a Fold
joints folding
Joints from Cooling Contraction
joints cooling
Indicators of movement along joints

Joint infilling

gouge and breccia infilling