Field Trip 10: Rainbow Basin Natural Area

Introduction

This chapter is written for a 3 day, 2-night field trip; over which time students learn how to make a geologic map. This area could also be studied during a daytrip or even briefly visited en route to or from Pisgah/Amboy craters, but this chapter is written with assumption that groups will participate for 3 days, which is the minimum amount of time needed for an introductory group to map a portion of the Rainbow Basin Natural Area (Rainbow Basin).

Photograph of the syncline at Rainbow Basin. 

Accommodation options include camping at Owl Canyon Campground, which is within a part of the study area, or using hotels in nearby Barstow. Groups will NOT be able to access the Rainbow Basin by charter bus and high clearance vehicles are recommended; but 4-wheel drive is not necessary.

Rainbow Basin is situated in the Mud Hills within the Mojave Desert, about 20 minutes north of Barstow and has, for very good reasons, long been a draw for earth scientists. Vegetation is scant, resulting in nearly 100% exposure of the sedimentary rock strata, making for easy observation and study. These rocks are vividly colored and have been deformed into a nearly perfect syncline, with limbs of the syncline being offset by movement along faults. Paleontologists also value this area, because the excellent mammalian fossils collected from the strata have provided valuable information about animals and the ecology from 12-16 million years ago. This is all contained within a geographically small area, making it ideal for a weekend-long project.

Additional materials

• Map board (large clipboard, drawing board, something with a clear cover and without metal would be best)
• Hiking shoes/boots with good soles
• Field book
• Protractor
• Ruler
• Mechanical pencils w/0.5 mm lead and good erasers
• Drafting pens (00 and 2 weight or 0.3 mm and 0.6 mm)
• Liquid paper pen/white out
• Box of colored pencils
• Brunton compass – provided by instructor
• Mud Hills USGS 7.5 minute topographic map – provided by instructor

Safety Considerations

• Rattlesnakes – watch where you step and where you place your hands
• Cliffs and Climbing – climbing steep slopes is not necessary to complete the assignment and pose a unnecessary safety hazard; follow trails and if you reach an area that is uncomfortably steep, don’t push it
• Water – all students need to have a means of carrying at least 2 quarts of water (more in warm weather) on their person and should be encouraged to hydrate before and after field work
• Weather – It can be dangerously warm during late spring, summer, and the early fall, so visits should be made during the late fall, winter or early spring months. On the contrary, it can also be quite cool in the mornings and evenings, so be sure to consider the weather forecast when packing for a visit.
• Wide brimmed hats will help reduce the possibility of heat exhaustion and sunburn

Geology

The principal strata exposed at Rainbow Basin are part of the Miocene Barstow Formation, which has been dated at 12-16 million years old, based on the excellent assemblage of mammal fossils that have been recovered from the strata. The upper and middle sections of the Barstow Formation are a fine-grained and colorful mix of tan, brown to red-brown, and green-brown mudstone, siltstone, fine sandstone, and tuff that accumulated as ancient lakebed deposits. The lower section is distinctly coarser grained, consisting of sandstone and conglomerate from ancient stream and alluvial fan deposits. The tuff deposits within the upper and middle members of the Barstow Formation stand out in relief and color and are useful as “marker beds”.   Three of the most conspicuous tuff deposits, Puma Tuff, Thin Tuff, and Skyline Tuff, will be mapped and used to subdivide the Barstow Formation into 4 members: Tertiary upper Barstow 1, Tertiary upper Barstow 2, Tertiary upper Barstow 3, and Tertiary middle Barstow, named Tbu1, Tbu2, Tbu3, and Tbm, respectively. The lower member will not be mapped. These rocks have been deformed into a syncline around an east-west trending fold axis and have been offset along several faults.

Below the Barstow formation is the early Miocene, brown to red conglomerate and breccia of the Pickhandle Formation. The bedding contact between the two units represents an unconformity: an erosional surface separating a younger rock unit above from an older rock unit below, with a significant volume of rock (and geologic time) between the two units missing due to erosion. The top of the Barstow Formation at Rainbow Basin also represents an unconformity. Quaternary sand and gravel has been deposited upon the tilted and eroded beds of the Barstow Formation, providing a nice example of an angular unconformity. These pinkish gravels cap the cliff tops around Rainbow Basin and are the product of geologically recent alluvial fan deposits, hence this rock unit is called fanglomerate. Younger, but geologically similar fanglomerate deposits are found in-between stream channels within the mapping area, as buff-colored sand to boulder-sized gravels, which will be mapped as Qf, for Quaternary fanglomerate. The active stream channels contain unconsolidated deposits of sand and gravel (alluvium), which will be mapped as Qa.

Taken as a whole, the rock units at Rainbow Basin record a sequence of geologic events: abrupt, episodic mountain building, represented by the breccia of the Pickhandle Formation, then erosion of the mountains and deposition of the sediment in alluvial fans and stream beds. These deposits are the conglomerate and sandstone of the lower section of the Barstow Formation. Finally, these deposits were submerged by a lake, upon which fine-grained sediments and volcanic ash accumulated, consolidating to become the siltstone, mudstone, and tuff of the middle and upper Barstow Formation (Sharp and Glazner, 1993). Sporadic volcanic eruptions produced the interbedded tuff layers.  

Learning Objectives

Through participation in this field trip, students should be able to:

1. Identify unconformities, i.e., angular unconformity, disconformity, nonconformity
2. Locate on a topographic
3. Measure and record the attitudes of strata, faults, and folds
4. Locate and map contacts between rock units 
5. Accurately locate and map faults and the fold axis of the Barstow syncline
6. Describe the lithology of rock units
7. Summarize the geologic history of the field area
8. Create a geologic map that clearly conveys the structure and geology of the field area
9. Create a geologic cross-section that clearly conveys the structure and geology of the field area

Key Vocabulary

• Attitude – the position of a horizontal surface, like a bed of sedimentary rock, relative to horizontal
• Azimuth – direction a compass needle is pointing based on the 360^o of the compass rose, e.g. 030, 170
• Bedding contact – the planer contact between two types or ages of rock
• Compass bearing – direction a compass needle is pointing as measured in degrees east or west, relative to north or south, e.g. N 30 E, S 10 E
• Dip – the amount of tilt of planer surface, like a bedding plane, measured in degrees from horizontal
• Fault – a fracture in Earth’s crust along which movement has occurred
• Fold axis – the hypothetical line around which the strata of fold was bent
• Plunge – the amount of tilt or dip of the fold axis, measured in degrees from horizontal
• Stream terrace – topographically flat surface elevated above the active stream channel representing the past position of the stream bed or flood plain for the stream
• Strike – the trend of planer surface, like a bedding plane, measured as a compass direction
• Syncline – a fold where rock layers have been compressed downward, into a U-shape

Pre Field Trip Questions/Activities

1. Draw a syncline and label the fold axis and the limbs.
2. What is an unconformity? Draw an example of each of the three types of unconformities.
3. Using the compass wheel below, indicate in each box the direction the compass needles are pointing as azimuth and compass bearing.

Compass rose.

4. Using the cross-section cartoon below, measure the dip of the bedding plane. The diagonal lines represent tilted strata, with the top diagonal line representing the ground surface.  

Cross-section drawing of a slope.  Ground surface represented by blue line. 

En Route Talking Points: I-605 north, I-10 east, I-15 north, I-40 east

• I-605 north
  • The 605 takes us across part of the Los Angeles Basin. This incredibly deep basin formed from roughly 16 million to about 1 million years ago as tectonic forces slowly peeled the Transverse Ranges away from the Peninsular Ranges, rifting open a series of basins. The sea flooded these pits as they slowly opened, meaning what is today the metropolis of Los Angeles and Orange counties were once at the bottom of the Pacific Ocean. All the while, sediment being eroded from the continental highlands filled these depressions with silt, sand, and gravel, up to 6 miles in thickness.
  • San Gabriel River
    • The San Gabriel River Freeway (formal name for I-605)
      • Follows a path of least resistance from river erosion
      • River erosion creates natural pathways
      • Pathways become footpaths
      • Footpaths become horse trails
      • Horse trails become thoroughfares
      • Thoroughfares become highways
    • River transports sediment from the San Gabriel mountains to its base level, the Pacific Ocean
    • Whittier Narrows water gap. The San Gabriel River and Rio Hondo River, just to the west, have created the Whittier Narrows by eroding faster downward than the hills have been uplifted, resulting in carving a topographic saddle, a “water gap”, 2 miles wide and 800 feet deep, bisecting the Puente Hills into 2 parts: the Montebello Hills to the west and Whittier Hills to the east. For a more detailed description read the excellent discussion in Geology Underfoot in Southern California, “Vignette 9 – A Boon to Communication, The Whittier Narrows”
    • Similarly, Santa Ana River erosion has created water gaps through hills that are now occupied by the 91 and 57 freeways
  • Whittier and Puente Hills (east of the 605) are being actively uplifted along the Whittier fault, which runs along the base of these hills; this fault is active as evidenced by the recent Whittier (1987, M 5.9), Chino Hills (2008, M 5.5), and La Habra (2014, M 5.1) earthquakes.
  • Montebello (Repetto) Hills (west of the 605)
    • Western extension of Puente Hills
    • Like Puente Hills, the Montebello Hills are made up of steeply tilted, south-dipping sedimentary rock that is typical of sediment in the LA Basin: mudstone, siltstone, sandstone, and conglomerate
  • On a clear day point out the San Gabriel Mountains making up the northern skyline and its highest peak, the 10,064 feet tall Mt San Antonio, a.k.a. Mt Baldy.
• Eastbound on I-10
  • “The 10” takes advantage of the San Gorgonio Pass, 1 of 3 topographic passes through the mountains surrounding the greater Los Angeles area, the other two being the Tejon and Cajon passes, utilized by I-5 and I-15, respectively.
  • Just east of Azuza Ave. the hills (San Jose Hills) buttressing the north side of I-10 are partly composed of lava flows know as the “Glendora volcanics”
    • ~16 million years old
    • Volcanic eruptions were triggered by the rifting that formed the Los Angeles Basin
    • Exposed as olive brown rock, as opposed to buff-orange rock exposed in preceding hills
• Northbound on the I-15
  • The 15 freeway takes advantage of the Cajon Pass, one of three topographic passes through the mountains surrounding the greater Los Angeles area, the other two being the Tejon pass, utilized by I-5 and the San Gorgonio Pass, over which the I-10 stretches to Arizona and beyond.  
  • San Gabriel and San Bernardino Mountains
    • Represent the eastern Transverse Ranges
      • A unique mountain range in that they are one of the only east-west oriented mountain ranges in the western hemisphere, going against-the-grain of the other mountain ranges in California, which all trend roughly north-south
    • Stream erosion along the San Andreas fault Zone has created the Cajon Pass, which divides the Transverse into 2 geographically distinct mountain ranges, the “San Gabriels” and the “San Bernardinos”
    • Compressional tectonic forces have uplifted the mountains, over the past 5 million years at a rate as fast as 70 feet per 1000 years (Sylvester and Gans, 2016)
      • San Gabriels could be growing as fast as the Himalayan Mountains (Prothero, 2011)
      • Rapid uplift evidenced by very deep, steep-sided canyons and triangulated ridges
    • Contains metamorphic rock as old as 1.7 billion years, as well as Proterozoic plutonic rocks; these were intruded by magma of diorite to granite composition that was generated by subduction of the Farallon Plate during the Mesozoic time
  • The I-15 crosses over Lytle and Cajon creeks, at 5 and 7.5 miles north of I-210 interchange. These two major tributaries of the Santa Ana River contain abundant cobble and boulder-sized clasts of bedrock from the adjacent mountains.
  • The San Andreas fault and the plate boundary
    • About 2.5 miles after passing Kenwood Ave, just after I-15 curves due north, you cross over the San Andreas fault, the plate boundary between the Pacific and North American plates 
• I-15 to the I-40 interchange
  • The 15 climbs up the Cajon Pass, then descends onto the southern margin of the Mojave Desert
• After crossing the truss bridge (see directions below) you may wish to stop at the Mother Road and Railroad Museum occupying the old Harvey House.  
• Just after passing the Railroad Museum First Street traverses the Mojave River
  • Normally dry
  • Streamflow is as groundwater through porous channel deposits

Directions to Rainbow Basin

As you near the I-40 interchange, prepare to exit Barstow Road (SR-247), turn left and proceed north for 0.8 mile. Turn left (west) when Barstow Road ends at Main Street. Proceed 0.2 mile to First Avenue and turn right (north), following signs for CA-58: Bakersfield, Fort Irwin, Las Vegas. Continue for 0.9 mile, passing over the truss bridge spanning the train tracks and Mojave River, then make a left on Fort Irvin Road, which is just after Buzzard Peak. Buzzard Peak is a resistent plug of Miocene dacite (Sylvester and Gans, 2016) that forms a prominent red-brown hill topped with a white water tower. After about 6 miles look for the road signs indicating a sharp right curve and a road intersecting Fort Irwin Road, and a small sign for Rainbow Basin. Make a left on Fossil Bed Road and proceed for 3 miles until you reach the entrance to Rainbow Basin. Turn right onto Rainbow Basin Road and continue about 2 miles into Rainbow Basin to “Rainbow Basin Basecamp”, passing the road for Owl Canyon Campground along the way.

Rainbow Field Trip Stops

As stated in the introduction, the stops in this chapter are NOT accessible by charter bus. Instead, commercial vans or high clearance 2-wheel drive cars are recommended.

Day 1, Stop 1 – Rainbow Basin Observation Area

Rainbow Basin observation area 

Vehicles will enter Rainbow Basin and park in the designated parking area, referred to as “Rainbow Basin Basecamp” on some maps and signified by a binoculars road sign. From here, exit vehicles and walk east about 20 yards to the “Rainbow Basin observation area”, which lies between the two low knobs, where there is a wide, flat area with the semi-circular remains of a building foundation. Here, you have a good look at the structure of the Barstow Formation and a sense of the area to be mapped, starting tomorrow. Looking to the left, or north, strata is tilting or dipping to south, whereas strata off to the right, is dipping to the north. These strata dipping in opposite directions make up the limbs of syncline and the viewing area upon which you are standing is more or less the fold axis around which the rock was bent. Folds like this one are the result of compressive stresses squeezing the crust, folding it like a rug being pushed into a wall (see short explanatory video: video description of making a fold Links to an external site.). In the Mud Hills the compressive stresses are caused by a bend in the Calico fault; a right-lateral strike slip fault that bends in the same way the San Andreas fault does in southern California. Shear stresses act on either side of a straight strike-slip fault, but when a fault bends, compressive stresses are localized on either side of the bend (see illustration below). This is similar to what happens when a fault splits into separate branches (like the Indio Hills discussed in Chapter 1).

Map view drawing of a right-lateral strike-slip fault, where the bend causes a zone of compression. 

From this observation area, several faults may also be observed. One is just north of the fold axis, where it offsets strata in this limb of the syncline and another should be visible by turning around and looking at the west wall of the basin. With more time and geologic mapping several other faults will become evident in the study area.

Looking back to the eastern wall and towards the top of the slope one can see that the tilted strata has been truncated and capped by a bed of sediment that is nearly horizontal. This is an example of an unconformity, where rock has been eroded and sediment has been deposited on the erosional surface.

Return to the vehicles and head to the Owl Canyon Campground.

Day 1, Stop 2 – Owl Canyon Hike and warm-up exercises

Continue along Rainbow Basin Road until you come back to Fossil Bed Road. Turn left (east), then left again on to the Rainbow Basin access road. Turn right on the road to Owl Canyon Campground and park at your campsite, or at the end of the road at the Owl Canyon Hiking Trail. Park and prepare for 1.5-2 hour hike. Ask students to pair-up.

Addresses learning objectives:
1. Identify unconformities and name the specific type, i.e. angular unconformity, disconformity, nonconformity
4. Measure and record bedding attitudes, strike and dip, and the attitude of faults and fold axis
5. Locate and map bedding contacts
6. Describe lithology of rock units
7. Summarize geologic history of field

At trailhead sign, inform group of learning objectives.

Activity 1: Direct students to use compasses to help them position their bodies so they are facing north. Ask: What direction is the surrounding strata tilting/dipping? – North. How much? – 110/36 N

Activity 2: Have students work with their partner to measure the bedding attitude of strata at trailhead. Good places to take strike and dip measures are between the tan-colored fenced structure and campsite to the right (#25). If students are inexperienced, expect that many will need lots of guidance. Have them repeat until they can get consistent answers. Walk amongst groups of students to help them measure bedding attitudes.

Activity 3: Assemble students at trailhead sign and ask them to point out the stream channel and evidence of mass wasting. – Stream channel is just below you and mass wasting is happening across the stream channel.

Walk down trail and stop where it encounters the stream terrace, just above the dry wash that is the active streambed. Here you can point out the angular unconformity between recent gravel deposits and the Barstow Formation, as well as the lithology of the Barstow Formation, which consists of alternating layers of mudstone, siltstone, sandstone, and some volcanic ash. This fine-grained stratigraphy represents ancient lake and stream deposits.  

Activity 4: Ask: Are the beds oriented the same as they were where we parked? – Yes. Are we on the southern or northern side of the fold axis? – South. Ask: As we walk towards the fold axis, should the strata be getting younger or older? – Younger  

Activity 5: Direct students to observe the lithology of the clasts in the streambed. – Lots of granitic cobbles and boulders, and boulders of a pink conglomerate. Ask: Where are these boulders coming from? – Upstream; lower Barstow Formation and Pickhandle

As you proceed upstream, as well as upsection within the Barstow Formation, direct students to observe the lithology.

About 500 feet from the trailhead, after the 2^nd zigzag in the channel, the unconformity is nicely exposed on the eastern wall. Walking up onto the stream terrace on the western side of the channel offers a nice view. Look left to right.

Angular unconformity. 

Activity 6: Ask: What direction are the beds tilting? – North. Are you south or north of the fold axis? – South

Activity 7: Measure the bedding attitude by taking strike and dip measurements of beds. If students are inexperienced, expect that many will need lots of guidance. Have them repeat until they can get consistent answers.

Activity 8: Describe lithology of Barstow Formation. Descriptions should include: (1) name and member, i.e. upper Barstow Formation; (2) fresh and weathered color; (3) texture, including grain size, angularity, and sorting; (4) composition of grains, i.e. granitic, basaltic, gneissic; (5) thickness of bedding; (6) sedimentary structures, i.e. ripple marks, cross bedding, etc.; and, (7) and evidence of fossils/trace fossils.  

Activity 9: Draw and name the unconformity. – This is a great example of an angular unconformity.    

As you walk upstream, watch for light-colored tuff bed, which stands out in relief, compared to the mudstone strata above and below it.

After another 100 feet or so note the oversteepened terrace walls.

Activity 10: Ask: What will eventually happen to the terrace walls? Why? – Collapse due to oversteepening. Look upstream to observe several blocks that have fallen into the channel, as a result of oversteepening.  

Continuing upstream you’ll encounter another tuff bed then a little further an approximately 8-inch high “step” in the streambed. Here, bedding is essentially flat lying, because you are very close to the fold axis of the syncline. After proceeding another 100 feet or so, look to the east wall of the channel where it makes a sharp right turn. Here, you can get a sense of the fold axis, although discerning the attitude of bedding may be challenging due to faulting. If you keep a close watch, you may find some smaller faults further upstream.

Normal fault.  Car keys for scale.  

About another 100 feet farther north, observe the strata is now dipping towards the south, meaning… You have crossed the fold axis.

Activity 11: Measure the bedding attitude on the north side of the syncline.

At the fork in the stream channel, stay to the right where you can observe the dip of strata and an angular unconformity.

Angular unconformity.

After another couple hundred feet rock fall has resulted in large blocks of the Barstow Formation blocking the channel. Here you may wish to discuss rock fall and mass wasting.

About 150 feet past the large rock fall is a tunnel that extends 20 yards into the cliff. You’ll need a flashlight if you wish to explore. From here and over the next several hundred yards, the Barstow Formation is coarser grained, comprised mostly of sand and pebbles. This is the sandstone and conglomerate of the lower member of the Barstow Formation.

Activity 12: Describe the lithology of the lower Barstow Formation.  

As the walls of the wash begin to close in, observe that the Barstow Formation becomes coarser grained. Eventually, the stream channel becomes a dry waterfall and impossible to navigate without climbing. You’ve reached the bedding contact between the Barstow Formation and the underlying Pickhandle Formation.

Activity 13: Describe the lithology of the lower Pickhandle Formation. Students should note that the grains are very coarse and angular. If possible, measure the dip of the Pickhandle Formation, which should be about 20 degrees steeper than the Barstow Formation.

If you are feeling adventurous and confident in your group’s climbing skills, continue through the narrow section until the slot canyon opens up to a slope of granite peppered with Joshua Trees. This is the end of the hike and provides a good place to review what’s been observed and discussed, take questions, and enjoy a little rest before heading back for the night.

Day 2 – Rainbow Basin Mapping Assignment

Remind students to pack everything needed for the day: water, food, sunscreen, hat, jacket if needed, and all field equipment. Prepare maps by drawing a north arrow and/or a grid of lines spaced at 2 inches, trending north-south and east-west.

Drive back into Rainbow Basin and park in same spot as yesterday. With all gear needed for the day, walk back to the “observation area”. Organize group into 2 student-mapping teams. Resist requests for larger groups, as larger groups tend to be more prone to socializing. Remind students of the safety considerations: snakes, climbing, and dehydration. With inexperienced groups, you might suggest that they stick to pathways trodden by past students and to stay away from overly steep slopes and cliffs.

Through participation in this field trip students should be able to:

1. Identify unconformities and name the specific type, i.e. angular unconformity, disconformity, nonconformity
2. Accurately locate position on a topographic map
3. Accurately measure and record bedding attitudes
4. Measure and record bedding attitudes, strike and dip, and the attitude of faults and fold axis
5. Accurately locate and map contacts between different rock units by drawing lines on the map delineating the contact between different rock units on Earth’s surface
6. Accurately locate and map faults and the fold axis of the Barstow syncline
7. Describe lithology of rock units
8. Summarize geologic history of field area
9. Create a geologic map that clearly conveys the structure and geology of the field area
10. Create a geologic cross-section that clearly conveys the structure and geology of the field area

Mapping Assignment Directions

1. Students: orient your map with north, so that you and the top of the map are facing north.
2. Instructors: clarify the limits of the area to be mapped. It’s recommended that the east-west limits include The Sphinx and Curvier, while the north-south limits are Cope and 3323 (see associated Canvas file "Rainbow Basin Base Map").
3. Students: working with your partner, identify on your map the following topographic features (you don’t need to mark them on your map, simply point each topographic feature to your partner on your map):
   1. Cope, the highest, gravel-capped peak rising above ridge of light-colored rock on the skyline to north
   2. 3418: High peak to northeast formed by steeply dipping resistant bedding
   3. 3195: The Sphinx: the sharp, reddish point rising from basin floor 500 feet to northeast
   4. 3323: To southeast on rim of basin
   5. Double Peak, 3258 and 3244 on skyline to southwest
   6. Try to locate where you are and the parking area, which is not designated on the map. Hint: use the small closed loop in the (heavy) 3150 foot contour near the observation point.
4. Instructors: walk your students down into the wash, about 400 feet to the east. There, define the difference between loose alluvium and consolidated terrace gravels. Students will map these separately. Demonstrate where mapping contacts with dotted, dashed, and solid lines. Walk back up to the observation area.
5. Students: these are the primary mapping objectives:
   • Map the contacts between different rock units, Tbu1, Tbu2, Tbu3, and Tbm, as delineated by the 3 tuff units, Puma Tuff, Thin Tuff, and Skyline Tuff
   • Map in the trace of faults and the axis of the syncline
   • Begin mapping with the resistant yellow-brown tuff bed that forms the prominent south-facing slope, approximately 250 north of the northwest corner of the parking area. This is the Puma Tuff. The Puma Tuff delineates the contact between stratigraphically highest and youngest members of the Barstow Formation, Tbu1 and Tbu2 (the next member down).  Walk with your partner to your first location, locate your position on the map, and take a bedding attitude (strike and dip).   Record the strike and dip symbol and dip measurement in pencil on your map. Remember to use a straight edge to draw the strike and dip symbol and to write, small and neat. Puma Tuff outcrops at arrow tips. 

• • Measure and record bedding attitudes along the contact between Tbu1 and Tbu2. Do this at least once every 300 feet or so, on either side of the fold axis. Once you’ve confidently located the contact at a few locations, begin to “connect the dots” with a pencil line. Take measurements more often when close to the fold axis and faults. Map the Puma Tuff on both limbs of the syncline as far east as 3195. Hold off mapping the unit where it is exposed on the low hill immediately southwest of the parking area (near the road) until you’ve accurately mapped it elsewhere. Map the tuff beds down section from the Puma Tuff on both flanks of the syncline.

IMPORTANT mapping assignment considerations:

• Record all attitudes, comments, rock descriptions, etc. in your field notebook while you are there. Forgetting something then having to come back will waste time and effort.
• Abbreviate, e.g. outcrop = oc, fold axis = fa, etc. BUT, make a glossary of your abbreviations because you will forget what some of them mean, even the ones that seem completely obvious at the time.
• Zoom out! When up close on the outcrop, it’s easy to be overwhelmed by the detail and to lose sight of the big picture. If you’re stumped, take some steps back and consider the bigger picture. Sit down and soak it in. Perhaps take a drink of water, have a snack, sketch it out in your notebook, and generally allow your eyes and brain to absorb what you are seeing. What have initially seemed a chaotic mess, will resolve itself into distinct structures given time and an open mind… Ommmmmm.
• Before marching on to the next outcrop, consider the upcoming geology from a distance. Make a sketch. Estimate where the trace of a bedding contact or fault is. Sketch on the map what you see in the distance.
• Be prepared to erase and correct where you’ve located contacts and/or bedding attitudes – it is often the case that moving on to a new location gives a fresh perspective and with it, better confidence about locations.
• Get in the habit of making sketches of complicated areas in your notebook.

Additional instructor notes for mapping assignment:

• As students map, they should carefully observe and describe the lithology of the Puma Tuff and the other marker beds. This will help in recognizing them throughout the study area, especially in areas where faulting makes identification more challenging.  
• If students are succeeding with the mapping, encourage them to find some attitudes directly on the axis of the syncline in order to determine the plunge.
• During the first few hours, try to visit each group at least once to ensure that they are able to complete the objectives. Encourage them to keep moving. Inexperienced mappers can easily get bogged down.  
• Remind students to map the stream alluvium and terrace gravels. It will be convenient to color the stream alluvium yellow and the terrace gravel orange on their map as they go.
• Measure and plot attitudes of fault surfaces where possible. Faults are not as well exposed as marker beds; their position will be established in part by accurately plotting locations where marker beds are truncated.
• As they work their way through the assignment, keep in mind: accuracy is more important that quantity, but don’t get bogged down. Consider the size of the area to be mapped and try to keep on pace to cover this entire area. Neatness counts.

6. Students: post-fieldwork assignment. Once back in camp or at the hotel, complete the following:
   • Ink all contacts (#00 pen or 0.3 mm) and faults (#2 pen or 0.6mm), and color the map.
   • Erase stray marks.
   • Color each unit with color that seems appropriate.
   • Consider places that you’ll need to revisit tomorrow. The map to the right is an example of how your map should start to look.  
   • Construct and arrange your map legend so it describes to the viewer what is depicted on your map. Set up your legend using the example below as a guide. Be sure to color the explanation boxes to match the map.

Example of a geologic map. 

7. Before turning in your map, be sure it includes all of the following:
   • Title
   • Author and date
   • Scale
   • North arrow
   • Geologic unit contacts
   • Strike and dip symbols
   • Structures: fold axes and faults
   • Explanation of symbols (legend)
   • Age and description of geologic formations
   • Location of cross-section
8. On a separate sheet of graph paper construct a north-south cross section (A –B) at a location of your choice that you believe will best illustrate the structure (syncline and faults). Be sure to show line of section on your map.
9. In addition to the map and cross section, write a geologic report. This report will include 5 paragraphs: (1) Introduction that includes why the area was studied, what you did and the geographic setting; (2) Methods that describes how you did your research; (3) Rock Units or Stratigraphy where you describe the rock units in order of decreasing age, thicknesses, description of contacts, fossils (if observed), and origin; (4) Structure where you describe folds and faults; and, (5) Geologic History to discuss the sequence of geologic events recorded by the rocks at Rainbow Basin.