Measurements and Techniques
The most obvious thing to do when trying to decipher the structural history of a formation is to describe it. One way of doing this is to measure the
dip and
strike. The dip is the amount a bed of rock is tipped from the horizontal. The strike is the direction which is ninety degrees from the dip, i.e. along the horizontal line on the bed. The strike can be in two directions, hence the dip could be in one of two directions also. There is a convention for the strike to be the in the direction you are facing if the rocks are dipping to your right. Some geologists prefer to measure the dip direction, rather than strike, as it is slightly simpler. However, all maps use dip and strike, not dip direction.
This is complicated slightly by
apparent dip. This is due to the fact that you are not always looking edge on (perpendicular) to the bed you are measuring. If you are looking at a bed at a slight angle, then you see the apparent dip. The
true dip will be greater than the apparent dip, as it is the maximum amount of dip, so the apparent dip can appear to be anything from 0° to the maximum (true) dip
Figure 1: Dip and strike of a bedding planeIn this diagram, the dip is 30°, with a strike north/south (0°/180°), the dip direction is 270°. On a geological map, symbols are used for the dip and strike. The strike is represented by a bar, and the dip by a mark on the strike bar on the
downdip direction with the dip written alongside, as shown on the map below left
A geological cross section can be drawn from the map showing the subsurface structure. Obviously, only features which can be seen on the surface can be represented. The cross-section below right is drawn using the values in the map alongside
Figure 2a: A geological mapFoldsFolding of rocks is caused by the compression of rocks. This occurs slowly, over a long period of time. If this happened quickly, the rocks would break, and
fault. This is due to the mechanical properties of rocks, namely it's plastic nature. If a rock is stretched slowly, then it will behave in a ductile fashion. If stretched quickly, the rock behaves in a brittle fashion. This behavior can be mimicked by using Blu-Tac. A typical fold is shown below, outlining the terms used in describing folds
Figure 3: Nomenclature used when describing foldsHinge: Where curvature of the fold is at a maximum Crest & Trough: Where fold surface reaches a minimum and maximum respectively Limb: Beds between two hinges Antiform & Synform: Convex upwards or convex downward folds respectively Anticline & Syncline: Older or younger beds at the core respectively. Can be used in conjunction with antiform and synform, i.e. an antiformal syncline Folds are classified by shape and the chronological order of rocks in them. The shape of a fold is described by the angle between the limbs, which are given the terms:
gentle (120-180°), open (70-120°), close (30-70°), tight (5-30°) or isoclinal(0-5°)The chronological order of the rocks in a fold are described by syncline and anticline, as described above
See if you can identify some features on the picture below
Figure 4: Lulworth cove, DorsetFaultsFaults are caused by short-term stress on rocks. They occur discontinuously along
fault planes, and are the cause of most earthquakes. Faults are classified in terms of the type of force causing the fault which determines the direction of movement. There are many terms used in describing faults. These are shown in the diagram below
Figure 5: Fault nomenclatureThe strike is the horizontal distance moved. The throw is the vertical distance and the heave is the distance moved perpendicular to the fault.
The types of fault, with arrows showing motion, are shown below
DiagramNameMode of Formation...Normal FaultExtension
(tension)Reverse Fault (or Thrust)CompressionStrike SlipShear
Faults rarely fall exactly into these categories, as they have some of the other types of motion as well, particularly some Strike Slip and rotational motion. Faults can also be
reactivated, meaning a normal fault can be "reused" at a later time in a compressional regime. This produces complex
drag folds along the side of a fault. Drag folds occur as the rock is bent due to the movement of the fault
