# Traps (geology)

Schematic representation of the parameters that define the position of a "geological area" (planar) in space:
blue = horizontal plane as a compass rose, green = area perpendicular to the horizontal plane along the north-south axis, red = area to be defined, z = streak line, Z = direction of sweep, σ = angle of sweep, F =  direction of fall (in the horizontal plane), φ =  angle of fall .
Stroking and falling are always perpendicular to each other. Illustration on an inclined layer surface

In geology, falling or collapsing is one of the two parameters that define the orientation of a given geological surface (planar) in space. The other parameter is the brushing .

## definition

When falling, a distinction is made between the angle and direction of fall . The fall angle is the angle between the horizontal plane and the steepest line ( fall line ) that can be made on a planar. The angle of inclination thus defines the inclination of the planar. The direction in which the line of fall, projected in the horizontal plane, points is the direction of fall. It indicates the direction in which the planar tilts and is always perpendicular to the stroke. Together with the direction of strike, the angle of fall clearly defines the spatial orientation of any planar.

A specification like "035 / 20SE" means that the angle of fall of the geological surface is 20 degrees and that the direction of fall is southeast. In this example, "035" is the value of the stroke direction, which indirectly also indicates the exact direction of fall (with direction of fall SE: 35 degrees + 90 degrees = 125 degrees, with direction of fall NW: 35 degrees - 90 degrees → 360 degrees - 55 degrees = 305 Degree). The form of the above is the traditional geological notation . A newer form is the so-called Clar notation (after Eberhard Clar ). It contains the numerical values ​​of the direction and angle of fall and would be given as 125/20 in the above case.

A surface that is exactly horizontal (at the bottom ) has a fall angle of 0 degrees and thus no fall and consequently no stroke direction. An exactly vertical ( saiger standing) surface has a fall angle of 90 degrees and a clear stroke direction, but not a clear fall direction (stroke direction ± 90 degrees).

## Measurement

A suitable instrument for measuring strike and fall is the structural or geological compass . The angle of fall is read off on a scale that is located on the hinge between the compass body and the compass cover. Historically, falling u. a. determined with a stratameter .

## Apparent and true falling

Apparent and real falling: The geological area concerned is shown in green. α is the true angle of fall, because the profile line a runs 90 ° to the stroke. α 'is an apparent fall angle, because the angle β is smaller than 90 °.

In a geological profile section, a distinction is made between the apparent falling observed in the profile plane and the true or real falling, if the profile plane is not exactly perpendicular to the strike or parallel to the fall line.

The true angle of incidence, apparent angle of incidence and profile angle (horizontal angle between the stroke and the profile line) are in a trigonometric relationship - if two of the angles are given, the third can be calculated. Let β be the profile angle, α ′ the apparent falling and α the true falling, then:

 ${\ displaystyle \ tan \ alpha '= {\ frac {g} {a'}}}$ (1)
 ${\ displaystyle \ tan \ alpha = {\ frac {g} {a}}}$ (2)

and

 ${\ displaystyle \ sin \ beta = {\ frac {a} {a '}}}$ (3)
Nomogram according to HS Palmer for apparent falling

From (1) and (2) it follows:

 ${\ displaystyle \ tan \ alpha '= {\ frac {a} {a'}} \ tan \ alpha}$ (4)

Substituting (3) into (4) gives

 ${\ displaystyle \ tan \ alpha '= \ sin \ beta \ cdot \ tan \ alpha}$

In addition to calculating with a pocket calculator or computer, the value you are looking for can be determined using various non-electronic arithmetic aids : By drawing construction, construction in Schmidt's network or by reading from nomograms developed for this purpose .