Page 296 - Laboratory Manual in Physical Geology

When the strike is expressed in degrees east or west of true a horizontal plane, is the dip angle . Dip is always expressed
north or true south, it is called a quadrant bearing . However, in terms of its dip angle and dip direction. The dip angle is
it is more common to express strike as a three-digit azimuth always expressed in degrees of angle from 0 (horizontal) to 90
bearing in degrees between 000 and 360. In azimuth form, (vertical). The dip direction can be expressed as a three-digit
north is 000° (or 360°), east is 090°, south is 180°, and west azimuth direction or as a quadrant direction.
is 270°. Because the azimuth data represents directions with
a number, instead of letters and numbers, it is easier to enter Strike-and-Dip Symbols
it into spreadsheets for numerical analysis. Strike is usually A strike-and-dip symbol consists of a long line showing
expressed relative to north ( FIGURE 10.4 ). the orientation of strike, plus a short line for the direction

of dip (see FIGURES 10.3 , 10.4 , 10.5 ). Note that the dip

What Is Dip? direction is always perpendicular to strike and points
Dip is the angle between a horizontal plane and an inclined downdip —the direction that drops of water would flow
(tilted) surface, measured perpendicular to strike. The surface or a ball would roll. Accompanying numerals indicate the

may be a formation contact, bedding plane contact, fault, dip angle in degrees. See FIGURE 10.4 for examples of how

or fracture. As you can see in FIGURE 10.3 , a thin stream of to read and express strike and dip in quadrant or azimuth

water poured onto an inclined surface always runs downhill form. Also note that special symbols are used for horizontal
along the dip direction , which is always perpendicular to the strata (rock layers) and vertical strata ( FIGURE 10.3, 10.5 ).
line of strike. The inclination of the water line, compared to
High angle fault; U for up
Strike and dip of strata Strike and dip of a U and D for down to indicate
63 63 normal fault D relative movement

Vertical strata Axis of an anticline Reverse fault: teeth are in
the side of the hanging wall
(upper block); number
25 indicates dip of the fault
Axis of an plane
overturned anticline Axis of a syncline
Contact or other line solid
where known, dashed
Axis of an
overturned syncline where approximated, and
dotted where concealed
Axis of a
plunging anticline
Strike and dip of Synclinal bend of a
20 overturned strata Axis of a monocline: short arrow is on
plunging syncline steeper beds, long arrow is on
more nearly horizontal beds.
Horizontal strata 34
Anticlinal bend of a
or Trend and plunge monocline: short arrow is on
of a line
steeper beds, long arrow is on
more nearly horizontal beds.
Lateral, or strike-slip
Strike and dip fault; half arrows
42 of foliation indicate relative Unconformity
movement

D Devonian J Jurassic N Neogene

S Silurian T R Triassic P G Paleogene

O Ordovician P Permian Q Quaternary

C Cambrian PP Pennsylvanian T Tertiary

pC Precambrian M Mississippian K Cretaceous

FIGURE 10.5 Structural geology symbols and abbreviations used on geologic maps.

266 ■ L ABOR ATORY 10