Field Trip - Santiago Oaks handout
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School
Orange Coast College *
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Course
A110
Subject
Geology
Date
Jan 9, 2024
Type
docx
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7
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Physical Geology
Name Santiago Oaks Field Trip
Background:
Santiago Oaks Park, located in the Orange Hills just a few miles from our campus, contains excellent exposures of the rock outcrops that make up the Santa Ana Mountains. Santiago Creek, which runs through the park, will give us the chance to apply some of what we’ve learned about streamflow, sediment transport, and sediment deposition in a stream valley. We’ll also try measuring the discharge of the creek itself. Finally, the submerged dam located slightly upstream will remind us about the considerable effects humans can have on water direction, landscape, and the surrounding ecology.
The North American West Coast showcases some of the complex geology that develops along a plate boundary. The rocks we see in these mountains did not form where we see them today. Geologists believe the original sources were island arcs, like those making up Japan or the Philippines; deep sea sedimentary deposits; ancient igneous lavas extruded on the sea floor, and bits and pieces of displaced continental fragments! The whole area, including the Santa Anas, is thoroughly faulted, folded, uplifted, and undergoing intermittent but significant erosion.
The mountains that make up the Transverse Ranges are unusual in their east-west orientation, as most of the mountain ranges in North America are oriented north-south. This east-west alignment is part of their long geologic history, including the slow movement northward from their distant points of origin, and their accretion as terranes.
Geologic history is always recounted starting with the oldest rocks in the area. This would be the Santa Monica slates, metamorphic rocks that formed as deep-sea muds on an ancient seafloor about 150 million years ago, an unknown distance away from their current location, probably to the south and west.
What caused the metamorphism? Intrusion of molten material that baked the old sea floor shales into slates. This intrusion, which became the Hollywood granites, took place probably between 110 and 80 million years ago.
This event was followed by millions of years of slow uplift and erosion of the overlying formations that exposed part of the granites and the slates. The eroded sediments, which consist of rocks, pebbles, and sands, became a deposit of conglomeratic sediments that covered much of what has become the Santa Monica and Santa Ana mountains. Over the next several millions of years the land subsided, the sea flooded over the old land surfaces and deposited various marine sandstones and shales on top of the terrestrial conglomerates.
Today, the mountains are slowly rising and slowly rotating clockwise due to tectonic forces, as the entire range apparently turned 90 degrees after deposition of the Modelo formation. The Channel Islands apparently rotated along with the rest of the Transverse Range mountains, as seen in their present east-west alignment, similar to that of the Santa Monica Mountains and, to a lesser extent, the Santa Anas.
At the park:
1. At multiple locations around the park, you will see signs like these:
In what ways do humans contribute to erosion by walking off of the designated trails?
2. Take a closer look at the path we’re walking on.
Embedded in the trail you can see pieces of what is
typically called road metal
: material that is added to a
trail or road to help stabilize it and reduce erosion.
a. Is the road metal material natural, artificial, or both?
b. What does the addition of road metal do to the degree
of sorting of the material that now makes up the path?
c. What does that in turn do to the porosity of the
material that makes up the path? d. So how does adding road metal help stabilize a road or path from continued erosion? At the outcrops:
Along the path and farther up the hills, you will see some examples of the local outcrops that make up the geology of this area.
3. What class (igneous, sedimentary, metamorphic) and type (e.g. basalt, sandstone, slate) of rock does
this appear to be?
Class: Type: 4. Describe the clasts in this rock in terms of the following variables:
Size (include estimate of particle size range) Shape _________________
Color Degree of sorting Likely means of deposition 5. Are the outcrops exclusively located at the level of the path/stream? Yes No
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6. Where else do you see outcrops? 7. Therefore what is the minimum thickness of this deposited layer (estimate)? At the dam:
The Santiago Oaks Submerged Dam, a
pioneering water project for its time, was first
built of clay in 1879 and formed one of the first
reservoirs in what was to become Orange
County. After flood damage in 1884 and 1888,
it was rebuilt with rock and concrete in 1892.
Each dam reached down to bedrock, forcing
ground water to the surface where it was
diverted for irrigation and domestic uses in this
newly-developed agricultural area. The dam
extends 110 feet across the creek and has a
maximum height of 19 feet. This photo was
taken sometime in the 1940’s after a recent
storm.
8. The diagram below shows what the landscape profile looked like in 1879, when the dam was first built. Based on what you have just read and what you can observe at the dam, sketch the following over
the schematic below as they relate to the present day:
a. Landscape profile b.
Surface water level
c.
Groundwater flow
Soil/fractured
rock
Bedrock
9. Take a look at the outcrops just below the dam. There is a lot to observe here! Sketch enough of this area (don’t forget to include the scale) to show evidence for the following:
-
Weathering
-
Folding
-
Faulting
a. What has caused the weathering you see here? b. What sort of forces have caused the folding? c. What sort of forces have caused the faulting? d. If your last two answers were different, how do you explain this? At the stream:
The velocity and discharge of a stream can be crucial information for anyone living near it. This exercise gives you experience in measuring the key variables that control stream behavior. You will be separated into groups and directed to an area of the stream with measurable flow.
-
First we will measure the cross-sectional area of the stream channel, using a tape measure (provided).
o
Work with your team to stretch the tape measure across the stream so that one of the first marks is at one
edge of the stream.
o
Record the width of the stream in Table 1 on the next page.
o
Think of the first mark on the tape measure as your starting point, your “0 distance” reference point. Note that the water depth at this mark (the stream’s edge) is zero. This data point has already been recorded.
o
Move to the next 10cm mark on the tape measure. Using a ruler (provided), measure the water depth from the water’s surface (immediately below the mark on the tape measure) to the stream bottom. Use centimeters as your unit. Call out your results for a teammate to record in the table.
o
Continue across the stream, recording the water depth as you go until you have collected depth information across the entire stream. Don’t worry if you have not used all the blanks in the table. If the flow is exceptionally high today, you can add additional data on the back of the handout if needed.
-
Now we will measure the average velocity of the stream. o
Measure some distance along the stream where there is substantial flow (using the tape measure). The trackway, once measured, will be marked with flags along the bank of the stream. You may need to clear leaves or other debris to ensure a continuous flow path. Record your distance in Table 2.
o
With one person timing, record the time it takes for your flotation device to travel this distance down the middle of the stream.
o
Repeat your measurement at least twice more and average your data in the table to generate a flow velocity (in cm/s).
Table 1. Stream depth data
Total width of stream:
cm
Distance
(cm)
Depth (cm)
Distance
(cm)
Depth (cm)
Distance
(cm)
Depth
(cm)
0 cm (first
mark)
0 cm
Table 2. Velocity data
Stream length (cm)
Travel time (s)
Stream velocity (cm/s)
Average stream velocity (cm/s)
Plot your cross-sectional area below using your depth data, choosing appropriate units along the x-axis and y-axis as needed to fit your data. Use the major gridlines to scale your stream data. Count all the full boxes in your stream’s cross section and multiply that value by the area of each square. Add any partial boxes, rounding as closely as possible. 0 cm
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10. a. What is your total cross-sectional area? cm
2
b. What is your average velocity? cm/s
c. From this information, calculate the stream discharge in cm
3
/s, then in L/s (1 L = 1000 cm
3
). Show your calculations:
discharge: L/s
11. Is your calculation likely to be higher or lower than the actual discharge at this time? Why?
Upstream from the dam:
If time permits, we will continue along the trail, up the stairs, then turn right to walk past the small pond behind the dam. After about 1/3 of a mile, a horse trail branches off to the right – we will take this
spur trail until it crosses the creekbed. At this location we will repeat the previous procedure to calculate the flow velocity.
12. Based on observations at this location, what is the approximate stream velocity?
13. Compare this value to the stream velocity below the dam. Are they the same? If not, how are they different?
14. If the flow rate at this location is lower, where is the water? Propose a hypothesis.