3

THE BRUNTON COMPASS

But first...two methods of stating direction

The bearing (or quadrant) method-is no doubt acarry-over from the ancient compass rose with itsfour quadrants. A bearing is an angle measuredeastward or westward from eithernorth or south, whichever is closer.The method employs a circledivided into four quadrants:northeast (NE), northwest(NW), southeast (SE), andsouthwest (SW) (Fig. 1A).Each of the four quadrantsis divided into 90°, beginningwith 0° at the north and southpoles and ending with 90° ateast and west. So, bearing isalways less than 90° measuredeastward or westward from eitherthe north pole or the south pole.

A bearing direction can be specified by stating (first)the pole-north or south-from which the angle ismeasured; (second) the magnitude of the anglemeasured; and (third) the direction-east or west­toward which the angle is measured. Four ex­amples are shown in figure 1B.

By the way... if you compare the face of a compasswith figure 1A, the compass might appear to bemis-labeled (Le., west in place of east, and east inplace of west). A compass is labeled in this mannerso that when you rotate it progressively westward(for example) the compass needle "reads" progres­sively westward (rather than progressively east­ward).

4

The azimuth method-of stating direction employsa circle divided into 360°, beginning with 0° at thenorth pole and increasing clockwise to 3600 at thenorth pole (Le., 00 and 3600 are coincident) (Fig. 2).An azimuth circle is graduated in a manner analo­gous to that of the face of a clock. Only instead ofbeing a clockwise sweep of 60 minutes, an azimuthcircle is a clockwise sweep of 360 degrees.

The four directions illustrated in figure 2 are thesame as those illustrated in figure 1, which servesto contrast one method with the other.

0° & 360 0

(north)

900

I (east)/

I /---------+-.---------- >

1800

(south)

Fig. 2. The azimuth method of stating direction.

The Brunton compass is available with either thequadrant or the azimuth circle. The quadrant circleis more traditional, but the azimuth circle is lesssUbject to recording-error; and, azimuth data aremore easily processed with a computer. TheBrunton in figure 3 has an azimuth circle.

0'1

tD:0c::Z-loZ

»z»d:s:-<

(QOJ-l(I) .... ::rQ.§(I)

o-OJ~.g 2en 5" 3­. 0

..... :J00 0COoj3_. -0CIl Dl_CIl::rCll(I)~

(f)::I1=Et!J(jj'wCIl-

»1.... -03 Dl'<ro;>i;":J:J ­_. (I)

<Dc.00­-'<::l>CJ(I).

a:~

-----~--~:~:::::---.....=Y 0~\=-==7 .~

Large sight Peep sight

Semi-circular scale (In l'increments) for reading vertical

angles

Circular scale (In l' increments) forreading direction

Needlelift pin

Long level

on mirror

';} s.,n ",0< / 7)/~ S;'h"~~-~~\ .. M,:,'3;f1 '.

DeclinationMagnetic needle; . adjustmentd north-seeking. screwwhite end uth-seekingblack en so

Vernier arm forvertical angles

(attached lever viewedon oppOSite side)

Fig. 3. The Brunton compass. A quarter-circle scale (hidden in this view) shows percent-grade. Ascale on the vernier arm provides for reading fractions of one degree when mounted on a tripod.

10

MEASURING DIRECTION TO AN OBJECT

Case I-When the elevation of an object is notmore than a few tens of degrees at>ove-normore than approximately 15° below-theelevation of the viewer, a Brunton should beheld as in figure 8.

Fig. B. How to holda Brunton whenmeasuring thedirection to anobject whoseelevation is notmore than a fewtens of degreesabove-nor morethan approximtely15° below-theelevation of theviewer.

Step 1-With your elbows hugging your sides forsupport, hold the Brunton approximately level inone hand (as indicated by the round level). Withyour free hand, swing the large sight and the mirrorabout their hinges to positions that allow you toview the reflection of the object either through theslot in the large sight or along its tip. (A little trial­and-error is required here.) Then bring the roundlevel to center. You will no doubt have to pivot your

. body and Brunton a bit to maintain your view of theobject along the axis of the large sight. You mustalign your eye, the axis of the large sight, the centerline of the mirror, and the object. You may moveyour head in order to achieve this alignment, solong as the round level remains centered.

Step 2-Read the bearing or azimuth indicated bythe white (north-seeking) end of the needle.

11

Case II-When the elevation of an object is morethan approximately 15° below that of the viewer,a Brunton should be held as in figure 9.

Fig. 9. How to hold aBrunton whenmeasuring the directionto an object whoseelevation is more thanapproximately 150 belowthat of the viewer.

Step 1-Hold the Brunton as in case I, but in areversed orientation. Position the mirror and largesight so that you can sight over the tip of the largesight, through the window in the mirror, to the objectbeyond. (Again, a little trial-and-error is requiredhere.) Level the Brunton with the round level, whilekeeping the object in view-moving your head andbody as necessary.

Step 2-Read the bearing or azimuth indicated bythe black (south-seeking) end of the needle.

Holding the Brunton as in figure 9 is troublesomefor two reasons: (1) Inasmuch as your head is notdirectly above the Brunton, you must read its facewith oblique vision, which hides that part of thegraduated circle nearest you. (2) Also-dependingon the position of the needle-the large sight mighthide the needle's tip from view, requiring you to pullthe large sight back toward you to a point whereyou can see the needle's tip, which can result inslight misalignment of the Brunton while reading it.

12

Case III-When the viewer's waist-high line-of­sight to an object is blocked by some obstacle,a Brunton should be held as in figure 10.

When the elevation of an object is as in case I, butyour waist-high line of sight (Fig. 8) is blocked bysome obstacle, you can hold the Brunton at eye·height and use it as a "prismatic compass" (Fig.10).

Fig. 10. How to hold a Brunton when measuring thedirection to an object in a case where your waist-high line­of-sight to an object is blocked by some obstacle.

Step 1-View the object either (a) along a line thatcrosses both the tip of the large sight and the tip ofthe small sight; Of, (b) along a line that passesthrough both the slot in the large sight and thewindow in the mirror.

Step 2-WhiJe holding the Brunton level, asindicated by the reflection of the round level in themirror, read the bearing or azimuth. (This is a littleawkward-like cutting your own hair in a mirror.) Asin case II, the direction to the object is indicated bythe black (south-seeking) end of the needle.

15

SOLVING FOR MAP DISTANCE REPRESENTEDBY SLOPE ANGLE AND SLOPE DISTANCE

Refer to figure 14. After determining slope angle(y)-as in figure 12-tape or pace to determineslope distance (S), then solve for the horizontaldistance (H) by...

H=Scosy

Fig. 14. Measuring slope distance (S) with a tape. Slopeangle (y), elevation difference (E), and horizontaldifference (H) are also labeled.

SOLVING FOR DIFFERENCE IN ELEVATIONREPRESENTED BY SLOPE ANGLE AND SLOPEDISTANCE

Refer to figure 14 once again. Solve for thedifference in elevation (E) by...

E = S siny

16

SOLVING FOR DIFFERENCE IN ELEVATIONUSING SUCCESSIVE EVE-HEIGHTMEASUREMENTS

Set the long level of the Brunton at zero index andhold it as if measuring a vertical (Le., slope) angle(see again figure 12). Or, more conveniently, use atube-like instrument called a hand~level that ismade for the purpose. Spot a point up-slope that isthe same elevation as your eye, move to that spotand repeat the procedure until you arrive at yourdestination (Fig. 15).

The difference in elevation between point A andpoint B in figure 15 is 4 x eye~height plus theestimated distance between eye-height #4 andpoint B.

A;XintB~

;~r ------ Eye-height #4

f:\I))) /

- - - - - _J.t-'/Eye-height #3

Eye-height #2

Poil1tA

Fig. 15. Determining difference in elevation by using aBrunton as a hand~/evel.

17

STRIKE AND DIP-DEFINITIONS

Strike-is the map direction of the line formed bythe intersection of (a) an inclined plane and (b) animaginary horizontal surface. The top of the blockdiagram in figure 16 represents an imaginaryhorizontal surface, and the shaded layer representsan inclined limb of a fold bounded by planes. Theirline of intersection is strike, which, in this case, isoriented north-south. Strike has traditionally beenexpressed in terms of the acute angle between theline of intersection and north, for example... N. 300

E. (instead of S. 300 W. nor E. 600 N.).

Fig. 16. lfIustrating strike and dip of an inclined plane(e.g., that of a limb of a fold, or that of a fault). Strike isdue north; dip magnitude is 45°; dip direction is east.

Dip-has two attributes: magnitude and direction.Dip magnitude is the (vertical) angle of inclination ofthe inclined plane. Dip direction is simply thedirection toward which the plane is inclined down­ward (Le., its 'down-hill' direction). Because dipmagnitude is the maximum angle formed betweenthe inclined plane and the imaginary horizontalsurface, dip direction is necessarily perpendicular tostrike (Fig. 16), so only the general direction of dipneed be given (e.g., NW, SE, etc.).

18

RECORDING STRIKE WITH THE RIGHT-HAND RULE

A growing practice is to express strike simply as anazimuth value. But, inasmuch as a line (in this case,a strike line) has two directions, a choice must bemade. By convention, American geologists recordthe azimuth along which one looks while positionedso that the dip direction is to hislher right-hence,the 'right..hand rule.' The person in figure 19 ispositioned so that he can apply the right..hand rule.Dip direction is to his right; and, he has the Bruntonoriented with the large sight away from him so thathe can read the strike azimuth directly with thewhite (north-seeking) end of the needle. Only themagnitude of dip need be recorded, inasmuch asthe direction of dip is indicated by the strike value(Le., dip direction has to be to the right of strike).

THE OTHER RIGHT-HAND RULE

The right-hand rule described above might betterbe called the 'right-arm rule'-to distinguish it fromthe other right..hand rule used by British geologists.To apply the other right-hand rule (Fig. 17): Positionyourself so that the thumb of your right hand pointsdown-dip while the heal of your hand is flat on thebed's surface. Your index finger is extended topoint in the direction of the recorded strike. Result:An implied dip direction opposite that derived fromthe 'right-arm rule' above.

.n..Fig. 17. The other right-hand rule. iJ-

19

THE AZIMUTH METHOD OF DESCRIBING THEORIENTATION OF AN INCLINED PLANE

The idea of strike and dip must have been con­ceived in the dark of the moon. A much morepractical method for describing the orientation of aninclined plane is to simply state the azimuth andmagnitude of its dip. (Strike is inferred as 90° fromthe azimuth of dip-in case you need to know it.)For example, an inclined plane whose orientationmight be described as...

Strike: N 45° E, dip to the southeast at 30° (Fig. 18)

...can be more simply described as 135° at 30°.

Fig. 18. Strike N 45° E, dip to the southeast at 30° can bemore simply described as dip (azimuth) 135° at(magnitude) 30°.

The azimuth method is more direct, and azimuth ismore easily digitally processed (in computers) thanis strike. The stereographic-net solution of theoriginal orientation of structurally-tilted cross-beds(page 57) uses this method to good advantage.

20

MEASURING STRIKE-CONTACT METHOD

Few beds are smooth enough to allow for making'contact' strike and dip measurements. However, abed's irregularities can be 'smoothed' by placing amap~board on the bed's surface (Fig. 19). Then,when the bottom side~edge of a Brunton is heldagainst the inclined map-board and the roundbubble is centered, the axis of the Brunton isparallel to strike, so you can read its bearing orazimuth.

Fig. 19. The contact method of measuring strike.

21

MEASURING DIP MAGNITUDE-CONTACT METHOD

The contact method of measuring dip magnitudecan be achieved by using a Brunton as a clinometer(Fig. 20). Turn the Brunton on edge against theinclined surface and swing it to the position ofmaximum vertical angle indicated by the long level.A bit of trial-and-error is required to find themaximum angle.

Fig. 20. The contact method of measuring dip magnitude.

22

USING TWO OUTCROPS TO MEASURE STRIKEAND DIP

A most accurate method-Two outcrops of thesame layer on opposite sides of a valley present themost accurate method for measuring strike and dip(Fig. 21). This method might not seem as tangibleas the contact methodf but it has two advantages:(a) the two-outcrop method avoids effects of bedroughness; and (b) the extent ot bedding used inthe two-outcrop method is larger-scale than thatused in the contact method.

Measuring strike-With your head at some bedboundary, and with the Brunton's long level set atzero, sight (as you did in figures 12 and 15) to apoint on the same marker across the valley whilekeeping the long level at zero (Fig. 21A). The mapdirection ot this horizontal line-at-sight is strike. Nowhold the Brunton in the direction-measuring mode(Fig. 21 B) and read the point's bearing or azimuth.Be sure that there is not a fault separating the twooutcrops!

Fig. 21. The two-outcrop method for measuring strike.

23

Measuring dip magnitude-Hold the Brunton atarm's length and bring its edge into alignment withthe stratigraphic marker sighted in figure 21 (Fig.22). While holding it in this position, adjust the longlevel to its level position (Le., center its bubble).Read the dip magnitude.

Fig. 22. Two-outcrop method for measuring dip magnitude.

Using a single outcrop-The technique illustratedin figures 21 and 22 can be applied to a singleoutcrop provided that there is enough beddingsurface exposed to allow you to look along thatsurface. That is, if you can position yourself so thatyour line-of-sight is both (a) within the plane of thebed, and (b) horizontal, then your line of sight isstrike. Dip can be measured as in figure 22.

Before closing, a 'trick of the trade'-The contactmethod of measuring strike won't work in cases ofvery low dip because the raised ring protecting thelong-level lever prevents aligning the edge of theBrunton with the inclined surface. In such cases,measuring strike is still possible: Set the long levelat zero and place the base of the Brunton flat onthe inclined surface. Now rotate the Brunton asneeded to bring the bubble of the long level tocenter. Can you see that the axis of the Brunton isnow parallel to strike? Read the bearing or azimuth.

24

MEASURING TREND AND PLUNGE OF ALINEATIONDefinitions-The trend of a lineation is the mapdirection (bearing or azimuth) in which it pointsdownward. Its plunge is the vertical angle betweenit and a horizontal plane (Le., its 'dip magnitude'). Itis commonly difficult to apply contact methods tomeasuring the orientation of a lineation. Where thisis the case, reasonably accurate measurementscan be obtained as follows (Fig. 23 and steps 1-3):

Step 1-Position a pencil on the outcrop so that itis parallel to a lineation. (A bit of tape might beneeded to hold the pencil in place.)

Fig. 23. Measuring the trend of a pencil positioned parallelto a lineation.

25

Step 2-Move to a position where you can lookdirectly down the pencil. Hold the Brunton as infigure 23 (see again figure 9) and read the bearing(or azimuth) of the pencil's trend.

Step 3-While in the same position, hold theBrunton as in measuring a slope angle (see againfigure 12A) and, while sighting down the length ofthe pencil, read its plunge.

MEASURING THE TRACE OF AN OUTCROP

A vein commonly occurs as a path of rubble with nohint of tabular shape (Fig. 24), so dip magnitudeand direction are enigmatic. And, if the ground isinclined, strike is difficult to assess as well. Still, itstrace (Le., the direction of its outcrop) can provideimportant information in mineral exploration andstructural studies. Of course, if you can trace thevein into an area of relief, you can there solve for itsstrike and dip.

/"

. .-r-p/- /~ p~-..--/

~./ =/<::)

Fig. 24. Sighting on your field partner is easier thansighting on a path of rubble. Average your two readings.

26

MEASURING AN INCLINED STRATIGRAPHICSECTION WITH A JACOB'S STAFF ANDBRUNTONA Jacob's staff and a Brunton compass can beused to measure the stratigraphic thickness of adipping stratigraphic section (Fig. 25).

Equipment-A Jacob's staff can be fashioned froma five-foot length of one-by-two lumber. The endthat supports the Brunton must be sawed at a 90°angle with the length of the board so that the axisof the Brunton will be perpendicular to the staff.

Note: Your traverse must be in the direction of dip.

Step 1-5et the dip magnitude on the Brunton (Fig.26), place the Brunton on the end of the staff, andlean it toward the section through an angle requiredto bring the long level to horizontal. The pointsighted on the ground is five feet higher, stratigra­phically, than the point at which you are standing.

Step 2-Go to the point sighted on the ground andrepeat the procedure.

Step 3-Always measure in Jacob's-staff incre­ments. Stratigraphic markers (e.g., lithic changes,unconformities, etc.) can be interpoJated (byestimated footages) into the measured section.

Lateral off-setting-ean be done freely in order togain a topographic position that is more favorablefor continuing your traverse. Simply follow astratigraphic marker to its occurrence elsewhere,and resume measuring. It doesn't matter if youcross a fault in your off-set; but, if dip changesacross a fault, you must re-set the dip magnitude.

27

.~ •...

1'\_:.... _1(,.'~\

\'\ " '.-l._~

~,

Fig. 25. Measuring the thickness of a dipping stratigraphicsection with a 5-foot Jacob's staff and Brunton.

(d) lilt Jacob's staffto bring long level to

horizontal

--- ((/9J~\~JC~~)~4JJJj~ ~//~ ~'-~ (b) Fold peep\ ~ / "~" sight into

( ~~ _~~~ position for'------s;; (a) Set dip onQ~ sighting throughclinometer \ '.\ \ sighting window

(c) Swing mirror scale. \ '---\\\ (in mirror) tointo position so that \"\~ '" ground beyond.compass face can ~'be seen while sighting ~ ,from peep sightthrough sighting window.

Fig. 26. Preparation of a Brunton on a Jacob's staff formeasuring the thickness ofa dipping stratigraphic section.

62

CONFRONTING AN OUTCROP-points to ponder

1.. The value of reconnaissance-Before takingpencil and notebook in hand and putting your brainin low-gear, invest some time in cruising aroundyour study area. Is there a field geologist whohasn't spent valuable time at a particular location(describing, sketching, interpreting, etc.), only tolater learn that just around the bend there is anexposure more enlightening and illustrative?(Curses!)

2. The utility of field sketches-IA picture is wortha thousand words.' (No doubt first exclaimed by afield geologist') You don't have to be a ThomasMoran to enhance your field descriptions withsketches. Not only does a sketch capture thegeometry of a field relationship more thoroughlythan do words, but a sketch also serves as a basefor recording locations of field measurements androck samples. Lastly, and universally, sketching

63

requires that you observe the object more thor­oughly, thereby prompting discovery. In this lastway, sketching is superior to using an instantcamera.

Sample XR-12Blah, blah

blah, blah

/~.1meterl ._~~_..J ~-_________

Looking eastward at locality 43

3. The importance of contact relationships-Theend purpose of most field studies is to decipher thegeologic history of an area, andgeologic history is punctuated bycontact relationships. Observe,sketch, and record the geometryof contacts and the character ofrocks on either side. The varietyof possible features istoo numerous to listhere, but examplesinclude:

In sedimentary rocks-Contacts: abrupt, gradual,truncating (both macro- and micro-truncation).Underlying intervals: weathered, mineralized,penetrated. Overlying intervals: sole-marked,graded, conglomeratic.

In igneous rocks-Alteration effects (e.g., chilled,baked, micro-intruded). Inclusions. Injections.

64

MAPPING-PACE·AND·BRUNTON METHOD

Definition-Pace-and-Brunton mapping is a 'quick­and-dirty' method of mapping features within arelatively small area (Le., a size comparable to thatof a football field or less). The map is made by'stepping-off' a series of distances, each of which isin a direction (either bearing or azimuth) measuredwith a compass, such as the Brunton.

Procedure-First,you will need to know the lengthof your pace, which can be determined by countingthe number of paces required to cover a knowndistance (e.g., a distance of 100 feet staked outwith a tape). Pace is conventionally considered tobe two steps, so count every other step (i.e., everyright step or every left step) along the traverse.

Example-A pace-and-Brunton map is simply aplot of the positions of selected points on theground, either natural or man-made, constructedfrom bearings or azimuths and paced-distancesfrom point to point (Fig. 51). Data (employing theazimuth method) plotted in figure 51 are as follow:

83

SOLVING FOR TRUE DIP WHEN A CROSS­SECTION IS NOT PERPENDICULAR TO STRIKE

Trigonometrically-Where a line of cross-sectiondiverges from strike at some angle (0°-90°), truedip can be computed trigonometrically:

.... 'S.-.... ---_.--

...... 0

70 80 9010 20 30o

tan apparent dip =(tan true dip) (sin divergent angle from strike)

Graphically-Figure 62 illustrates W.S. TangierSmith's graphic solution of true dip whenapparent dip and the divergentangle (from strike) of thecross-section areknown. 1()

Fig. 62. Graphic solution of true dip. In this exampleapparent dip is 22°. A line from this value is drawn to theapex of the diagram. The cross-section is 30° from strike.The intersection of the 30°-line and the line to the apex isprojected downward to a true dip of 40°. (From EconomicGeology, 1925, v. 20, p. 182, fig. 28).

84

LAND SURVEY SYSTEM

Latitude and longitude-Figure 63 shows theglobal distribution of latitude and longitude.

A

Parallels(lines of latitude)

B

Meridians(lines of longitude)

Fig. 63. Global distribution of latitude and longitude.

92

TRIGONOMETRIC SOLUTIONS OFSTRATIGRAPHIC THICKNESS

T:::: stratigraphic thfckness

5 :::: slope distance (determined by pacing or tapIng)y = slope ang Iex:::: dip angle

Cases where slope and dip are in oppositedirections.

Fig. 69. Slope plus dip < 90°.

T = 5 sin (x + y)

\ ~ --\ -------/'r------ //I (()

~ /I\

- ,\ - - - -,_\~ - - - - - - - - - - - - - - -

Fig. 70. Slope plus dip> 90°.

T = S cos (x + y - 90)

Cases where slope and dip are in the samedirection.

Fig. 71. Slope < dip.

T =S sin (x y)

Fig. 72. Slope> dip.

T = S sin (y x)

93