Pinecrest Lake Moraines and Stream Capture

Pinecrest Lake, a very popular recreation site in the Stanislaus National Forest, is located off of Highway 108, approximately 4 miles northeast of Cold Springs and 1 mile east of Strawberry, California (see Figure 1). The EarthCache site is on the dam that holds back the water in the reservoir at Pinecrest. You will need to find a place to park, then hike to the dam site. The hike is short, but the elevation is approximately 5,600 feet, so the terrain rating is = 2.5. Two of the geological features geologists have been curious about here are: 1) glacial moraines; and 2) stream capture, or stream piracy. You will learn about both concepts at this EarthCache site.

Figure 1: Location of the Pinecrest Lake Moraines and Stream Capture EarthCache.

During the last ice age, the Pinecrest area was geologically overrun by glacial ice. Ken Brunges, who was the naturalist and caretaker at the Bennett Juniper (located up Eagle Meadows Road, east of Pinecrest) for almost 3 decades, analyzed glacial erratics, or boulders left behind by glaciers in the area, and found that the ice thickness in parts of the Stanislaus River Canyon reached an incredible ~2,500 feet! Imagine standing at the dam at Pinecrest Lake with 2,500 feet of ice over your head – what a crushing sensation that would have been! The top of Dodge Ridge Ski Resort, nearby, is at approximately 8,200 feet elevation, which is a difference of 2,600 feet, so the very top of Dodge Ridge may have stuck up above the ice during the most intense of the past ice age events.

As the ice flowed down through the river canyons from the Tuolumne Ice Field, a roughly 50 mile long (north-south) accumulation of snow and ice along the crest of the central Sierra Nevada, east of Pinecrest, it would have formed large U-shaped valleys as the silty, sandy, gravelly, cobbly bottom eroded the bedrock it scraped over, like a belt sander, slowly sanding away at the granitic rock beneath it. This is how glaciers behave. They don’t bulldoze material downslope, rather, they freeze sediments into their “soles”, or bottoms (like the sole of a boot), then the weight of the glacial ice, combined with the frozen sediments embedded into the flat-bottomed glacier, grind and polish away at the bedrock. All of that sedimentary debris has to end up somewhere and it ends up being deposited at the base, or toe, of the glacier, downslope where the glacier terminates, or ends. The lower in elevation a glacier gets, the warmer it generally is, at these latitudes, and the glacier ends up melting away, leaving a berm of sediment at its terminus. Geologists call these structures moraines (see Figure 2).

Figure 2: Lateral and terminal moraines of a valley glacier, Bylot Island, Canada. The terminal moraine is at the end of the glacier, while the lateral moraines are along its edges. The terminal moraine at Pinecrest Lake (though older and more worn down) sits where the parking area and campground are located.

The parking area at Pinecrest is built upon a glacial moraine. The loose sediments are indicative of glacial till, the material deposited by glaciers that form moraines. Till is composed of silt, sand, gravel, cobbles, and even boulders brought downslope, in the sole of the glacier as it worked its way downhill over the bedrock. To back up a bit and understand glaciers, let’s define what one is. To form a glacier, more snow has to accumulate annually, than melt away. The surplus snow then compacts into ice, which can actually flow, once the layer is greater than approximately 100 feet thick or so. As more and more snow and ice accumulate, it flows downhill into the river valley below, freezing sediment into its sole, sculpting the landscape, slowly over time, then melting away, leaving its sedimentary load at its base, or terminus. These glaciers, that flow down mountain valleys, are called alpine glaciers, one of the two types on Earth. (The other type is called a continental glacier and covers whole continents, like Antarctica). When the glaciers recede and melt back, they can leave multiple terminal moraines at their ends, if they reach equilibrium and stay in place for a while. This is likely what happened at Pinecrest. A Terminal moraine was deposited and formed over time, damming the river valley it was in, which was the headwaters of the North Fork of the Tuolumne River. In the next canyon to the north, the South Fork of the Stanislaus River lied in wait, ready to do some geological pilfering and plundering…

Stream piracy, or stream capture, likely occurred here at Pinecrest Lake (Jay Power, pers. comm). When one stream “captures”, or “pirates” another, what happens is one stream or river drainage system or watershed is diverted from its own bed, and flows instead down the bed of a neighboring stream (see Figure 3). This can happen for several different reasons including:

1. “Headward erosion of one stream valley upwards into another; or
2. Lateral erosion of a meander through the higher ground dividing the adjacent streams; or
3. Within an area of karst topography, where streams may sink, or flow underground (a sinking or losing stream) and then reappear in a nearby stream valley; or
4. Glacier retreat.”

Figure 3: Stream Piracy or stream capture.

Most likely, the glaciers in the drainages of the North Fork of the Tuolumne and the South Fork of the Stanislaus rivers, retreated, then one river (the Stanislaus) captured the other (the Tuolumne; see Figures 4, 5, and 6). The rock type mapped by King Huber at the dam site is different than the surrounding granitic rock. Perhaps it erodes more readily and was a geological “weak spot” or “path of least resistance”. Further investigation of the details is needed to solidify this hypothesis. In the meantime, enjoy the hike out to the EarthCache, as well as some of the other EarthCaches in the area – the geology here can be spectacular!

Figure 4: View of Pinecrest Lake from the northeast, down the watershed, where, during the last ice age, a significant glacier flowed over the landscape.

Figure 5: Image shows the current stream paths for the North Fork Tuolumne River and the South Fork Stanislaus River.

Figure 6: Stream capture illustrated. The North Fork of the Tuolumne River flowed downhill toward the left during the last ice age, while the South Fork of the Stanislaus River currently flows into Pinecrest Lake, then downhill toward the right in this image. This idea was proposed by geologist Jay Power (USFS, retired) in ~2018).

EARTH SCIENCE BIG IDEAS

The Earth Science education community put together 9 “Big Ideas” for the Earth Science Literacy Initiative (ESLI), shown below. Their purpose was to highlight the main concepts and ideas a person should understand to be literate in the earth sciences:

An Earth-science-literate person:

• understands the fundamental concepts of Earth’s many systems
• knows how to find and assess scientifically credible information about Earth
• communicates about Earth science in a meaningful way
• is able to make informed and responsible decisions regarding Earth and its resources

Which of these Big Ideas below do you think are most relevant to this EarthCache?

Earth Science Literacy Project:

1. Big Idea 1: Earth scientists use repeatable observations and testable ideas to understand and explain our planet.
2. Big Idea 2: Earth is 4.6 billion years old.
3. Big Idea 3: Earth is a complex system of interacting rock, water, air, and life.
4. Big Idea 4: Earth is continuously changing.
5. Big Idea 5: Earth is the water planet.
6. Big Idea 6: Life evolves on a dynamic Earth and continuously modifies Earth.
7. Big Idea 7: Humans depend on Earth for resources.
8. Big Idea 8: Natural Hazards pose risks to humans.
9. Big Idea 9: Humans significantly alter the Earth.

GEOPRINCIPLES

There are several fundamental principles, developed over time, that guide geological reasoning and critical thinking, listed below. Read each short description, then use your best judgement to determine which principle, or principles, best relate to this EarthCache.

7 Principles in Geology:

1. Superposition – the oldest strata are at the bottom of the sequence
2. Original Horizontality - layers of sediment are originally deposited horizontally
3. Lateral Continuity - layers of sediment initially extend laterally in all directions
4. Faunal Succession - fossils succeed each other vertically in a specific, reliable order that can be identified over wide horizontal distances
5. Law of Intrusive Relationships - the geologic feature which cuts another is the younger of the two features
6. Uniformitarianism - the assumption that the same natural laws and processes that operate in the universe now have always operated in the universe in the past and apply everywhere in the universe
7. Catastrophism - the theory that the Earth has been affected in the past by sudden, short-lived, violent events, possibly worldwide in scope

LOGISTICS AND SAFETY

This EarthCache is located approximately 0.25 miles on a paved trail at an visitor overlook and is closed for half of the year or more, depending on the amount of snow in the area. The area is rugged and without cell phone service. EarthCachers should take great care in terms of safety while enjoying this cache.

Visitors to this site should plan ahead and prepare by:

• Knowing the regulations and special concerns for the area you are planning to visit (obeying laws that prohibit collection or destruction of artifacts);
• Carrying a map and a GPS unit and/or compass;
• Staying on existing roads and trails;
• Staying away from any/all mine shafts and adits;
• Planning for extreme weather, hazards, and emergencies;
• Being aware that cell phones DO NOT usually work in the rural areas away from the major highways;
• Leaving your travel plans with a responsible party, including the date and time of your return;
• Being aware of any natural hazards associated with the region (e.g. poison oak, rattlesnakes, mosquitoes, cliffs/steep slopes, etc., etc);
• Carrying a full-size spare tire, extra food, water, and warm clothing;
• Following the “Tread Lightly" and "Leave No Trace” philosophy.

REFERENCES

1. Brunges, Ken, (2002), "Ice Depth of Glaciers At Column of the Giants, Middle Fork Stanislaus River, CA, A First Analysis" (unpublished paper), November 19. 2002, kbrunges@lodelink.com.

2. Guyton, Bill (2001), "Glaciers of California: Modern Glaciers, Ice Age Glaciers, the Origin of Yosemite Valley, and a Glacier Tour in the Sierra Nevada (Volume 59) (California Natural History Guides). January 19, 2001.

3. Huber, N. K. (1983), “Preliminary geologic map of the Pinecrest quadrangle, central Sierra Nevada, California”, U.S. Geological Survey, Series and Number: Miscellaneous Field Studies Map MF-1437,Map Scale: 1:62,500, https://ngmdb.usgs.gov/Prodesc/proddesc_316.htm.

4. Natural Resources Canada Unattributed photograph (2020), Copyright Terrain Sciences Division, Geological Survey of Canada).

5. Power, J., (2018), pers. comm., alnilsson@mlode.com

6. SlideServe, (2020), Chapter 13, 13.3, River Valleys, River Valleys, DigitalOfficePro, https://www.slideserve.com/tory/chapter-13.

7. Wenner, J. M. and Baer, E. M., (2020), “The Math You Need, When You Need It: Math tutorials for students in introductory geosciences”, National Association of Geoscience Teachers (NAGT) and National Science Foundation (NSF), Division of Undergraduate Education through awards DUE-0633402 and DUE-0633755, https://serc.carleton.edu/mathyouneed/slope/index.html.