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Collections Close-Up: Beautiful Benitoite

Not all that glitters is gold — sometimes it’s benitoite! So discovered prospector James Couch, when in 1907 he encountered some sparkling specimens of what would one day become the California state gem. While gold looms large in the story of our state, the unique geology of California has gifted us many other magnificent rocks and minerals. From cinnabar to serpentinite, we are delighted to share these with the public once more in our classic California minerals exhibit. We are fortunate enough to have one specimen of rare benitoite on display, and for this month’s close-up, we’ll zoom in on our only other specimen, safely secured in storage.

Benitoite specimen from the Museum’s collections.

Our second benoite specimen is hosted by a chunk of blueschist that amounts to the size of a large hamburger. Brilliant blue sparkles alert us to the presence of benitoite, or barium titanium silicate (BaTiSi₃O₉). These are nestled alongside black sprinkles of neptunite in a white, almost fuzzy looking layer of natrolite. Blue, black, and white, each of these substances is its own mineral, a naturally occurring inorganic element or compound with a characteristic structure. 

These minerals and their blueschist host rock come from the guts of the southern end of the Diablo Range. Millions of years ago, unusual combinations of hydrothermal fluids seeped into the cracks of the blueschist found within metamorphosed serpentinite, forming a variety of rare minerals including benitoite, neptunite, natrolite, joaquinite and others. And while benitoite is found in a few places around the world, this section of the California Coast Range geologic province in southeastern San Benito County is the only place in the world where gem quality benitoite crystals are found.

It’s also the first place where benitoite was found. There are some complex claims to the initial discovery, but the most widely accepted story begins with failed melon farmer James Couch prospecting outside Coalinga on behalf of investor R. W. Dallas. In December of 1907, Couch noticed some blue sparkling stones he thought to be sapphire. In the subsequent months, Dallas looped in a few dealers and gem cutters, who offered different identifications, including an expert in Los Angeles who thought it was volcanic glass (perhaps because of it’s conchoidal fractures). By the time a sample made its way to San Francisco, a lapidary who thought the stone was spinel showed it to a friend, who sent it to UC Berkeley mineralogist Dr. George Louderback

Dr. Louderback investigated the sample, finding it too soft to be either spinel or sapphire. Upon examination, Louderback identified the specimen for what it was – a new substance to science, and named it Benitoite, after the San Benito river which ran through the area of the mining claim. He soon journeyed to the Dallas Mine, where operations had already started, to study benitoite’s geological context. The paper he published describing benitoite’s mode of occurrence also includes the first published photos of benitoite and early photos of the mine.

Benitoite Mineral Specimen and Gems (Louderback, 1909)

The paper describes a lot of the properties that make benitoite exciting – including the high refractive indices and strong dispersion that make it especially sparkly (although you have to go elsewhere to learn about benitoite’s stunning fluorescence). It is still described by many sources as one of the “finest” descriptions of a new mineral species to be written. It’s worth noting that benitoite, while new to science, was nonetheless the embodiment of a scientific prediction that had been made decades prior. It exhibits a ditrigonal-dipyramidal crystal habit, a shape that looks sort of like someone glued the bottoms of two triangular pyramids together, within the hexagonal crystal class. This was the first time such a form was found naturally occurring, one of only thirty-two possible classes of crystal shapes mathematically predicted by Leipzig crystallographer J.F.C. Hessel in 1830.

Open Cut Benioite Mine 1908 (Louderback, 1909)

Even as the true nature of benitoite was unfolding, commercial operations quickly emerged at what was first called the Dallas Mine. Over the decades the mine has seen varying techniques and levels of productivity. In the earliest years, folks were after the largest possible gems and they wanted them fast. This meant hacking off a lot of big knobs of crystal that were initially covered, due to the way the mineral veins had formed within the blueschist, in a fine layer of natrolite. Waiting to dissolve the natrolite in acid, which could happen with no harm to the benitoite, took too long. This meant that a lot of well crystalized mineral specimens within their original rock matrix were broken up and made into gems. Fortunately, some whole specimens remain, and the museum purchased the specimen featured in our Collections Close-Up from San Francisco based J. Gissler in the late 1930s.

Our display specimen was also purchased in the 1930s. This specimen has been exhibited since 1985, the very year that Californian’s made benitoite their state gem to celebrate its beauty and uniquely California story. And though the history of this mineral is part of it’s charm, mineral discovery is not a thing of the past. Scientists continue to discover new minerals as the result of current field work, or even through fresh understandings of preserved museum specimens.

For more on benitoite and it’s geological and cultural history, join our Collections Close-Up conversation with Professor Hilde Schwartz on June 10.

Collections Close-Up: Benitoite with Hilde Schwartz

There are few things more Californian than benitoite, a mineral formed within the low temperature, high pressure environment of subduction zones and sparsely sprinkled throughout serpentinite landscapes. While the mineral exists in isolated locations globally, gemstone quality material has only been found in California — one of the reason’s it was named our State Gemstone in 1985.

Learn about the geologic and cultural history of this mineral with Museum Collections Manager Kathleen Aston and Dr. Hilde Schwartz, lecturer in the Earth and Planetary Sciences Department at UC Santa Cruz, during this installment of our member-exclusive Collections Close-Up series.

Resources

About the series

Zoom into the stories, secrets, and science of our collections during monthly webinars with Collections Manager Kathleen Aston. This live event is an extension of our monthly Collections Close-Up blog, with added insights and intrigue. Members are invited to participate in this program before it is made available to the general public as well as ask questions directly of Kathleen.

Not yet a Member? Join today!

Your support helps us steward our collections and offer educational programs that connect people with nature and science. Memberships start at just $15/year.

Rock Record: A Closer Look at a Few of the Great Geologic Landmarks of the United States

Written by Gavin Piccione

The rocks on the surface of the Earth are shaped and transformed by the boundless forces of nature, creating a vast and ever-changing arrangement of formations for humans to observe and ponder. Even though the terrain of the U.S. comprises less than 2% of the Earth’s surface, the wide variety of environments found here gives rise to a diverse set of geologic wonders. In this installment of Rock Record, we will take a closer look at the mechanisms that formed some of the most distinctive and interesting of these geologic landmarks.

Delicate Arch in Arches National Park, Utah. Photo credit: Wikipedia user Palacemusic.

The Arches of Arches National Park

Unlike the natural bridges here in Santa Cruz, the arches of Arches National Park are not the result of localized erosion from waves. So how did these enigmatic features form?

The famous arches are a marvelous geologic coincidence, stemming from three key processes that unfolded over the past 300 million years. The first factor leading to the arches was the deposition of massive salt layers by an inland sea 300 million years ago. In the ensuing time, denser, stronger sandstones were deposited on top of these salt layers and the weight of the overlying rocks, combined with tectonic forces in the region (the second arch forming factor), caused the salt layers to bulge and push to the surface. The combined effects of the tectonic forces and the underlying salt layers created a massive anticline, or convex rock fold, in the overlying sandstones (see the schematic below).

Schematic of the geologic processes leading to arch formation.

As this bulge in the Earth was eroded away, folded sandstone layers were then exposed at high angles at the surface. Because the sandstone layers were less erodible than some of the surrounding rocks they were left behind, jutting out of the earth surface in formations known as “fins” (see photos below). It was from these fins that the arches were eventually formed.

Schematic of Arch formation from Sandstone “fins”. Image courtesy of the National Park service.

The final arch forming factor was localized fracturing from faulting within the fins. Tectonic processes caused faults along small, weak shale layers within the sandstone columns, leading to highly fractured zones. Over time, wind and rain plucked these fractured zones from the fins, leaving behind the arches we see today.

Photo of Mono Lake tufa towers. Photo credit: oakdaleleader.com.

The Mono Lake Tufa Towers

Above the quiet waters of Mono Lake, the tufa towers stand like a peculiar shrine to the geologic processes operating in this area east of the Sierra Nevada. The conditions that created the otherworldly tufa were an intersection between the realms of chemistry, geology and hydrology, forming tufa towers throughout the ~760,000-year life of Mono Lake.

To understand the formation of the tufa, it is easiest to start by analyzing the setting of Mono Lake: Mono Lake lies in a basin that allows water to flow in, but not out. Meaning that all of the dissolved rock particles that flow into the lake stay there, causing the lake waters to become very salty and to have a very high pH (e.g. Acids like vinegar and lemon juice have low pH, while bases like baking soda and ammonia have high pH.) These lake conditions are conducive to high levels of the molecule carbonate (CO3), one of building blocks for “carbonate” minerals.

In addition to the surface lake waters, there are abundant groundwaters that flow into the basin from surrounding areas, which interact with the rocks as they flow towards the lake and become high in the element calcium. As these ground waters flow into the lake through underwater springs, the groundwater and lake water mix, causing the carbonate molecules and calcium from the two respective waters to bond, and form the tufa towers from the mineral calcite. This means that every tufa tower is a fossilized spring! Unfortunately, there is also a human induced component of the tufa towers we see today: when the city of Los Angeles diverted freshwater that once flowed into Mono Lake it caused lake levels to fall dramatically, leading to exposure of previously underwater tufa (see schematic of tufa formation below.)

Schematic of tufa formation from mixing of lake and ground waters (left and middle) and exposure through falling lake levels (right).
Aerial photo of Great Sand Dune National Park. Photo credit: The Denver Post.

The Dunes at Great Sand Dunes National Park

Sand dunes probably seem mundane for folks living in Santa Cruz who’ve likely seen the coastal sand dunes on the drive from Santa Cruz to Monterey. However, it may come as a surprise that the tallest sand dunes in North America are far from the ocean and are not associated with grand deserts like the Mojave either, but instead are found right in the middle of Colorado!

The dunes at Great Sand Dune National Park lie along the eastern edge of the San Luis basin between the San Juan Mountains to the west and the Sangre de Cristo Mountains to the east. This basin used to hold the massive Lake Alamosa, which drained around 440,000 years ago. Following the lake drainage, sediments from the lake bottom and the surrounding mountains began to build up in on the basin floor (see maps below).

Idealized maps of the San Luis Basin before (left) and after (right) Lake Alamosa drained. Map credit: National Park Service.

The location of this basin in the greater Rocky Mountains funnels wind from the southwest, causing the sand dunes to grow in a natural pocket in the Sangre de Cristo Mountains. During storms events, opposing winds are driven from the east, causing the dunes to grow to their great heights (see image below).

Aerial photo of the great sand dunes in their natural pocket in the Sangre de Cristo Mountains.

Learn more about our nation’s geologic landmark, including some closer to home, during May’s Rockin’ Pop-Up.


Rock Record is a monthly blog featuring musings on the mineral world from Gavin Piccione and Graham Edwards, PhD candidates in geochronology with the Department of Earth and Planetary Sciences at UC Santa Cruz. They also host our monthly Rockin’ Pop-Ups as “The Geology Gents”.

Rockin’ Pop-Up: Great Geologic Landmarks

Tourist season is upon us, the time when people pick-up and travel the world in search of new experiences, lasting memories, and unbelievable views. For this month’s Rockin’ Pop-Up, we’ll be taking a roadtrip through some of the great geologic landmarks of the United States, from Yellowstone to Yosemite, and Carlsbad Caverns to our own Pinnacles.

About the Series: Join the Geology Gents, Gavin and Graham, for monthly conversations about rocks live on Facebook. Each month we’ll explore a different geologic topic, from Santa Cruz formations to tips for being a more effective rockhound. Graham Edwards and Gavin Piccione are PhD candidates in geochronology with the Department of Earth and Planetary Sciences at UC Santa Cruz.

Submit your questions ahead of time by emailing events@santacruzmuseum.org and feel free to include pictures of rocks you’d like identified! Note: you do not need to have a Facebook account to be able to watch the program live.

Watch Past Pop-Ups
Read our blog Rock Record

Rockin’ Pop-Up: The Moon!

The earth rises over the surface of the moon.

Geology literally means the study of the Earth, so why are we digging into the Moon for this month’s Rockin’ Pop-Up? Well, as the Geology Gents put it, “The moon is basically the Earth.” Say what? Don’t worry, all will be revealed. This month, learn about the formation of our moon and how scientists study it.

About the Series: Join the Geology Gents, Gavin and Graham, for monthly conversations about rocks live on Facebook. Each month we’ll explore a different geologic topic, from Santa Cruz formations to tips for being a more effective rockhound. Graham Edwards and Gavin Piccione are PhD candidates in geochronology with the Department of Earth and Planetary Sciences at UC Santa Cruz.

Submit your questions ahead of time by emailing events@santacruzmuseum.org and feel free to include pictures of rocks you’d like identified! Note: you do not need to have a Facebook account to be able to watch the program live.

Watch Past Pop-Ups
Read our blog Rock Record

New Perspectives on the Past Through Freshly Exposed Rocks

In this edition of Rock Record, the Geology Gents unearth a few key examples of how newly exposed outcrops have led to important geological insights, as well as some geologic exploration into freshly exposed rock and sediments exposed by the CZU Lightning Complex Fires. 

By Graham Edwards and Gavin Piccione (aka the Geology Gents)

To reconstruct Earth history, geologists rely on the rock record: the accumulated rocks that, through their accumulation and formation, are relics of ancient geologic processes spanning geologic history. Such rocks provide a spyglass with which to peer into geologic history. But our view through this spyglass is limited to rocks that are both exposed at the Earth’s surface and have survived the effects of erosion.

As geologists, we often rely on Earth processes to expose new rocks and provide us fresh glimpses into Earth’s history. Since exposing fresh rock requires a lot of energy, natural disasters or extreme natural events can expose clues to this history through fresh rock surfaces. Human activities, such as construction or mining, can also expose new geological wonders.

In this edition of Rock Record, we’ll go through a few key examples of how newly exposed outcrops have led to important geological insights, as well as some geologic exploration into some freshly exposed rock and sediments exposed by the CZU Complex Fire.

Roadcuts

Roadcut in Maryland exposing large folds. Image credit: Joel Duff, Naturalis Historia

The construction of roads often requires the removal of large sections of rock, leaving sheer rock faces on the sides of the road. Some of the most famous rock outcrops are ones exposed in roadcuts and these unique locations are a frequent destination for college geology classes.

As undergraduates, the Gents (i.e. Gavin and Graham) explored roadcuts in the Northeastern US, and learned about tectonic motion through the faults and folds exposed in roadcuts (like the one in the image to the right), about metamorphic rocks via roadcuts in Maine, and about large deep-sea sediment avalanches (called turbidites) from roadcuts in upstate New York. Rocks exposed on the sides of roads can also be significant for geologist’s understanding of the sequence of events in an area.

For instance, a roadcut in Owens valley (see image below), settled a longstanding debate amongst geologists about whether the Bishop Tuff was deposited before or after the first glaciations in the area. The exposed rock showed the Bishop Tuff sitting on top of the Sherwin Till glacial deposit, meaning that the tuff must have been deposited after the till.

Roadcut in Owens valley showing the Bishop Tuff overlying the Sherwin Till. Original image taken by James St. John

Fire

Forest fires are, in many cases, an important natural event for the health of a forest because they clear the forest floor of brush and dead vegetation. Through this process, fires also expose large portions of rock that would otherwise not be visible. For this addition of Rock Record, the Gents explored some areas of the Santa Cruz mountains that have been burned in the CZU Lightning Complex fires last August.

Taking Empire Grade North, areas of Cretaceous (145-66 million years before present) igneous and metamorphic rocks that were previously covered by vegetation are exposed in the burn zone of the recent fires.

Weather

Sometimes extreme weather events can expose new outcrops or geologic features. For instance, the Frijoles Fault of a previous Rock Record post, The Faults that Shape Santa Cruz, was hidden behind trees and shrubs until a powerful storm event in the 1970s drove enough coastal erosion to expose the fault in the sea cliffs. Even more recently, heavy rains can cause landslides on the steep topography of the Santa Cruz Mountains and Santa Lucia Mountains of Big Sur. Each of these landslides exposes new surfaces that allow geologists and geomorphologists to study what causes landslides and the ways that massive amounts of Earth can be rapidly moved down hillslopes.

The exposed Frijoles fault contact, which was long hidden behind trees and shrubs before it was revealed by a storm in the 1970s.

Meteorites 

Some rocks are truly out of this world! Rocks that formed beyond Earth and arrive on Earth are called meteorites. Most meteorites come from the asteroid belt, a ring of rocky debris that dwells between the orbits of Mars and Jupiter, while some meteorites come from Mars and the Moon. As any Earth dweller knows, meteorites are incredibly rare, but they are important samples of other celestial bodies and leftovers from planet formation that we can study in close detail here on Earth. So, a meteorite fall is an incredibly exciting event for planetary scientists and geologists, alike!

One of the most important meteorites ever to land on Earth was the Allende meteorite, which landed in 1969 near the town of Pueblito de Allende in the state of Chihuahua in northern Mexico. The stone broke into pieces before it landed on Earth, but the collected chunks of this meteorite total >4,000 pounds with more pieces still found today! Because there was so much meteorite to go around, many scientists have studied it, and since the Allende meteorite is made of some of the most ancient material in our solar system it has provided an invaluable window into the earliest moments of our solar system just after the Sun formed!

What mysteries do you suppose are hiding all around you, covered by trees, houses, or soil?


Rock Record is a monthly blog featuring musings on the mineral world from Gavin Piccione and Graham Edwards.

Graham Edwards and Gavin Piccione are PhD candidates in geochronology with the Department of Earth and Planetary Sciences at UC Santa Cruz. They also host our monthly Rockin’ Pop-Ups as “The Geology Gents”.

Ten Years Since the Tsunami with the UCSC Seismology Lab

Santa Cruz Harbor after the tsunami (Karen T. Borchers/Mercury News)

The March 11, 2011 magnitude 9.1 Tohoku-oki earthquake and accompanying tsunami was devastating to Japan and affected regions all around the Pacific Ocean, including here in Santa Cruz.

On the tenth anniversary of the earthquake and tsunami, Heather Savage and Kristina Okamoto of the UC Santa Cruz Seismology Lab discussed how and why the earthquake occurred and the lessons learned by earthquake scientists from this event. We also discussed how Japan has recovered since the earthquake.

About the speakers

Heather Savage | Associate Professor, Department of Earth & Planetary Sciences, UC Santa Cruz

Professor Heather Savage’s research focuses on earthquakes and faults. Using both laboratory experiments and field studies, she works on questions regarding the strength and stability of faults in order to improve our understanding of when and where larger earthquakes occur. She uses rock deformation and friction experiments at pressures and temperatures relevant to the seismogenic zone to study in situ fault conditions where earthquakes start. Heather uses field observations of fault structure, particularly mapping earthquake slip and fault damage zones, to provide windows into the processes that occur during earthquakes, such as heat production and chemical reaction, that affect fault zone mechanics. She has worked in a variety of geologic settings, studying faults in California, Nevada, Oklahoma, Alaska, Wyoming, Japan and New Zealand.

Kristina Okamoto | Graduate Student, Department of Earth & Planetary Sciences, UC Santa Cruz

Kristina Okamoto is a graduate student in the seismology lab at UCSC. Currently, she’s studying an induced earthquake sequence in Prague, Oklahoma in order to analyze the mechanics of earthquakes. She also uses laboratory experiments to explore the physics of friction at conditions relevant to earthquake depths.

Rockin’ Pop-Up: Biogenic Geology

When we think of geology and rocks, living things rarely jump to mind unless we’re talking about fossils. And when we think of fossils, we usually think of mineralized bones and shells or tell-tale impressions within sedimentary rocks. Some rocks, however, are made up entirely of the fossilized remains of once living creatures. These “biogenic” sedimentary rocks are an important part of the solid Earth and more common than you might think! The Geology Gents are no biologists, but they nonetheless explore biogenic sedimentary rocks and the incredible geologic histories they record.

About the Series: Join the Geology Gents, Gavin and Graham, for monthly conversations about rocks live on Facebook. Each month we’ll explore a different geologic topic, from Santa Cruz formations to tips for being a more effective rockhound. Graham Edwards and Gavin Piccione are PhD candidates in geochronology with the Department of Earth and Planetary Sciences at UC Santa Cruz.

Submit your questions ahead of time by emailing events@santacruzmuseum.org and feel free to include pictures of rocks you’d like identified! Note: you do not need to have a Facebook account to be able to watch the program live.

Watch Past Pop-Ups
Read our blog Rock Record

Rock Record: The Faults that Shape Santa Cruz

In this installation of Rock Record, we explore how faults shape Santa Cruz. First, a little about faults.

By Graham Edwards and Gavin Piccione (aka the Geology Gents)

Image: San Andreas Fault. Credit: Kate Borton, David Howell, and Joe Vigil.

Faults are flat fracture surfaces within rock where portions of the rock move past each other. These features are some of the most prominent ways geologic processes shape the surface of the Earth. Motion along faults is responsible for the creation of most of the Earth’s mountains and valleys; faults significantly impact how and where rocks are eroded; and motion across faults causes earthquakes. In California, the most iconic fault is the continent-scale San Andreas Fault, but faults in this area occur at a wide range of sizes and often create familiar landscape features.

The Creation of the Santa Cruz Mountains

Faults of the Bay Area. Credit: UC Berkeley Seismology Lab.

Plate tectonics drive fault motion, creating large fracture surfaces in the Earth’s crust as the plates move apart, alongside, or crash into each other. Along the San Andreas, this plate motion is primarily horizontal (side-to-side), as the Pacific plate moves North and the North American plate moves South.

Major faults like the San Andreas are not actually one single fault, and instead make up fault zones, or networks of parallel faults that take up portions of the overall motion. Regional scale plate motion, like that responsible for the San Andreas, is often taken up by branching fault networks, instead of by one single fault zone. In the San Francisco Bay Area, the North-South motion of the Pacific and North American plate is taken up by four major faults: San Andreas, San Gregorio, Hayward, and Calaveras (right).

A careful look at the trace of the main San Andreas Fault reveals that this fault is not perfectly straight, but rather curves and wiggles a little bit. This complicates the motion of the fault. It is easy to slide two flat blocks past one another, but if you add bumps to these blocks, they are much harder to move. This is because when these bumps run into each other, they catch on one another. When these bumps are small (like sandpaper) it makes the fault hard to move. When these bumps are large, like the bend of the San Andreas Fault just north of Santa Cruz, the landscape in that bend can get squeezed together or pulled apart. Given the left-to-right motion of the San Andreas, the bend North of Santa Cruz squeezes the landscape together, thickening the crust and pushing the Earth upward to form the Santa Cruz Mountains.

The Faults in our Backyard

The Ben Lomond fault is the largest fault running through Santa Cruz, starting offshore in the Monterey Bay and weaving its way up into the Santa Cruz Mountains. Geologists know this fault was last active over 85,000 years ago, since it cuts through the older Purisima Sandstone and Santa Margarita Sandstone but does not disrupt the younger marine terrace deposits. The Ben Lomond fault is responsible for the path of the San Lorenzo river, which took advantage of rock that was fractured and weakened by motion along the fault.

Geologists from UCSC mapped the fault through Santa Cruz in the 1980’s, and for this installation of Rock Record we tracked the areas where this fault is visible from West Cliff drive up through Felton.

West Cliff Outcrop

The Ben Lomond fault first outcrops (geologist jargon for becomes visible in rock) on West Cliff Dr. in Santa Cruz, at the end of Woodrow Avenue, just west of Mitchell’s Cove Beach. This outcrop is a subtle, tight notch in the cliff. When compared to other rocks along the sea cliff, the rocks in the Woodrow Ave. outcrop are much more fractured, which is a result of more intense rock deformation caused by motion along the fault. Note, the large, dark boulders along this section of the cliff were placed there to prevent erosion.


Escalona Dr. Outcrop

Because the Ben Lomond fault is younger than the marine rocks that make up most of Santa Cruz, evidence of its existence does not appear in many areas South of Felton. One exception is along Escalona Dr., where the fault creates a small notch in the north side of street on private property.


Fault Scarp on UCSC campus

While an outcrop of the Ben Lomond fault is not visible between Escalona Dr. and Felton, evidence of motion along the fault can be seen along Coolidge Dr. on the UCSC campus. On the west side of the road, there is a steep drop down into Pogonip park. This ledge, termed a fault scarp, was made by motion along the Ben Lomond fault, where the east side of the fault was push upwards and the west side downwards. 


Outcrop Along the San Lorenzo in Henry Cowell State Park

North of UCSC, the Ben Lomond fault runs parallel (and sometimes through) the San Lorenzo River. In Henry Cowell State park, just north of the “Garden of Eden” swimming hole, the Ben Lomond fault can be seen along the west bank of the river. While subtle, this outcrop has some of the tell-tale signs of fault activity. Similar to the West cliff outcrop, there is pronounced fracturing of the rock in this area. Additionally, topography on the west side of the river reaches a low-point, which geologists term a saddle, where the rocks are more eroded.

A Detour North: Año Nuevo Faults

While the faults around Santa Cruz can be subtle or hard to get to, you don’t need to travel far to see faults and evidence of their activity. Año Nuevo State Park lies on the land where the trace of the San Gregorio Fault strikes dry land again (south of here it follows the coast just offshore). The San Gregorio Fault continues on its right-ward progress, dragging the westward/seaward portions of Año Nuevo to the northwest. Looking down at Año Nuevo State Park from the air, you can see the work of the San Gregorio Fault plainly. Año Nuevo Creek, which drains out of the steep mountains here, passes right by the visitor center and creates a pleasant, sheltered beach where it meets the ocean. However, over geologic history, the last leg of Año Nuevo Creek kept getting pushed to the northwest. Eventually it got pushed so far out of the way of upper Año Nuevo Creek, that the creek abandoned the distant creek bed and found a new, more convenient route. Geologists call this streambed hopping avulsion. But the abandoned creek beds to the northwest of today’s creek show us that the San Gregorio Fault has been shifting everything to the right of their neighbors across the fault. 

With the benefit of this birds’ eye view, let’s look a little bit closer and see what’s going on at human-scale. If you go from the visitor center (Marine Education Center) down along the Steele and New Years Creek Trails, you will arrive at the southern edge of Cove Beach, where Año Nuevo Creek reaches the ocean. If you continue southeast a short distance you come across a beautiful fault slicing across the sea cliff (below). This small little fault is not the San Gregorio Fault, but actually related to the nearby Año Nuevo Creek Fault which slices up the valley the creek flows down. As faults damage the rocks they cut through, they weaken the rock there, making it easy prey for an erosive mountain stream to chew into.

First creek fault.

We can even do a little geologic sleuthing. The fault clearly slices through the ancient sandy beach deposits on the top of the cliff, so we know the fault must be younger than those. Those sandy deposits are less than 100,000 years old, so we know this fault has moved in the last 100,000 years.

Let’s return to the creek, and look southeast toward Santa Cruz. Here you can see two different rocks pressed together There are light-colored pebbly rocks that meet with the familiar brown sandstone of the Purisima Formation along a jagged, sloping line of contact. The zig-zags of the contact are old stream banks where the ancient creek chewed into the rocky bank. Those pebbly rocks above the old stream banks are ancient deposits of Año Nuevo Creek. This means that today the Creek is now eroding back into them! If we get up close to the rocks, we can even find a variety of rocks including some charcoal that came from ancient fires and was washed away with rainwater before getting caught up in these streambank deposits. 

Some of these chunks of charcoal have been dated using radiocarbon and give an age of around 10 thousand years old (unpublished data from this report). Since these deposits have 10,000 year old charcoal in them, they cannot be any older than 10,000 years. That is super young in geologic terms.

If we travel to the northwest toward Pescadero on Cove Beach we can follow these young creek deposits on the cliff and even see a few faults that cut through the rocks of the cliffs. As you come level with the “Pond” a few trees tower at the edge of the cliff. If you look closely here, you can see that the light pebbles fade into a messy zone of jumbled rock and eventually back into familiar Purisima sandstones. That messy zone of jumbled rock is none other than the Frijoles Fault. Since this fault slices through those 10,000 year old stream deposits, the fault must be younger than 10,000 years. Once again, in geologic terms, this is a young and active fault.

The sea cliffs along Cove Beach are a great place to do geologic detective work. Just remember that these sea cliffs are zones of active erosion and rockfalls may happen unexpectedly. When looking at rocky cliffs, always be careful, aware, and safe!


Rock Record is a monthly blog featuring musings on the mineral world from Gavin Piccione and Graham Edwards.

Graham Edwards and Gavin Piccione are PhD candidates in geochronology with the Department of Earth and Planetary Sciences at UC Santa Cruz. They also host our monthly Rockin’ Pop-Ups as “The Geology Gents”.

Rockin’ Pop-Up: The Varieties of Volcanoes

This month, the Geology Gents explore how varied volcanoes can be. Graham Edwards and Gavin Piccione, PhD candidates in geochronology at UC Santa Cruz, put on blast how lava type affects the appearance of volcanoes and the way those volcanoes erupt.

About the Series: Join the Geology Gents, Gavin and Graham, for monthly conversations about rocks live on Facebook. Each month we’ll explore a different geologic topic, from Santa Cruz formations to tips for being a more effective rockhound. Graham Edwards and Gavin Piccione are PhD candidates in geochronology with the Department of Earth and Planetary Sciences at UC Santa Cruz.

Submit your questions ahead of time by emailing events@santacruzmuseum.org and feel free to include pictures of rocks you’d like identified! Note: you do not need to have a Facebook account to be able to watch the program live.

Watch Past Pop-Ups
Read our blog Rock Record