On the Rocks: The Equinox Elixir

This recipe is part of our series On the Rocks: Exploring Science and Nature through Curated Cocktails.

The vernal equinox marks one of two moments per year when the sun is exactly above the equator and day and night are of equal length. This means that we exit the days of mostly darkness and enter a time of increasing light.

Spring is traditionally a season of transition and new birth: snow melts, seeds grow, and blossoms bloom. This year, the vernal equinox that marks the beginning of spring also represents one year since we began sheltering-in-place. Four seasons have passed and our community has experienced the ebb and flow of the pandemic in tandem with the natural cycles of the Earth.

As we prepare for this next transition, welcoming new growth in fire scarred areas and fresh nectar in our gardens, we also look forward to our own revitalization. The Museum is excited to reopen our doors this spring and host more programs outdoors and in nature than ever before. But we continue to straddle the line of branching out and keeping shelter as our community continues to be impacted by the COVID-19 pandemic. In addition to in-person opportunities, we will continue to offer virtual resources that bring the Museum to you.

And it is with this period of transition in mind that we bring you the Equinox Elixir: a curated cocktail that is equal parts dark and light. We used gin from Venus Spirits for this recipe, a local distillery that sponsored an event we were meant to host last year for the equinox, but had to cancel in an effort to stop the spread of the coronavirus. Today, we toast them and you as we prepare for brighter days ahead.


  • 1.5 oz of Venus Gin No. 01
  • 1.5 oz of cherry syrup (such as from a jar of Luxardo cherries)
The Equinox Elixir once mixed, combining the tart sweetness of the cherry syrup with the fresh botanical quality of Venus’s Gin No. 1.


A layered cocktail utilizes liqueurs of slightly different densities to create visual layers in the drink, with the specific gravity of the liquid ingredients increasing from top to bottom. Water has a specific gravity of 1.0, whereas our thick syrup has a specific gravity of 1.8 and our gin is around .95.

Pour your ingredient with the higher specific gravity first (i.e. cherry syrup) into a martini glass, taking care to pour straight down the center so that the syrup doesn’t coat the walls of the glass where you don’t want it to. Then, shake your gin with an ice cub in a cocktail shaker vigorously for a few seconds. Turn a tablespoon upside down and place it just over the glass. Slowly pour the gin over the spoon so that it slowly spills into the glass without mixing with the syrup.

Marvel at its dichotomy and then give it a stir to mix the flavors together and enjoy!

Resources for further exploring the equinox

Post by Marisa Gomez

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Collections Close-Up: The Life and Legacy of Randall Morgan

There are few names in our local naturalist community that are as universally revered as that of Randall Morgan. Also known as Randy or R, Morgan was a pillar of the local natural history community.

Though he passed away a few years ago, his influence on the natural world and those who celebrate it in Santa Cruz is evident from the the Sandhills that his activism helped to save, to the local chapter of the California Native Plant Society that he helped found, to the Santa Cruz Museum of Natural History where he worked as a taxidermist to pay for studying linguistics at UC Santa Cruz. His legacy also lives on in the collections of the Kenneth S. Norris Center for Natural History.

Join Kathleen Aston, Collections Manager at the Santa Cruz Museum of Natural History, and Chris Lay, Director of the Kenneth S. Norris Center for Natural History, for an exploration of Randall Morgans life and legacy, including his collections, taxidermy, and conservation efforts.

This month’s Collections Close-Up is in sponsorship of the exhibit Look. Act. Inspire., celebrating the naturalists of Santa Cruz County. It is presented in partnership between the Kenneth S. Norris Center for Natural History, the San Lorenzo Valley Museum, and the Santa Cruz Museum of Natural History.

Resources for further exploration

This program is part of a series in support of the exhibit Look. Act. Inspire. and is presented in partnership between:

Image result for san lorenzo valley museum

Rare Plants and Community Science in the CZU Burn Zone with Amy Patten

The area impacted by the CZU Lightning Complex Fires hosts a slew of rare plants. As we enter spring, the season of new growth, botanists will be paying close attention to these rare plants, but they’re not the only ones. A “community scientist” is anyone who makes and shares observations in an effort to contribute to scientific understanding — and we hope you will help us bring community science to the burn zone.

During this online training with Amy Patten, Rare Plant Treasure Hunt Manager for the California Native Plant Society, you’ll learn how you can search for and document rare plants as a community science volunteer. We’ll go over some of the fascinating and beautiful rare plants you can see in the burn area, as well as online tools you can use for survey efforts as part of the CZU Lightning Complex and Community Science Project.

Limited space is also available for in-person trainings on March 27 and April 15.

About the Speaker

Amy Patten works in the Rare Plant Program at the California Native Plant Society’s state office where she manages the Rare Plant Treasure Hunt project, a community science project that documents rare plant populations throughout California. Amy lives in Santa Cruz and is passionate about protecting the plants and wildlife of the Central Coast.

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.


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


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.


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.


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”.

A Guide to the Fossils of Santa Cruz County

Santa Cruz County is home to marvelous wonders, from the shoreline to the summit — and every inch of this landscape was under the ocean mere millions of years ago. In an afternoon you can watch whales breach in the ocean and look at the fossilized remains of their ancestors on the beach (or high up in the mountains for that matter).

From mastodons to megalodons, this guide provides an overview of our local rock formations that feature fossils and how to dig deeper with the resources we’ve compiled.

Jump to: Fossiliferous Formations | FAQ | Additional Resources

Fossiliferous Formations

Before you can understand the fossils of Santa Cruz County, you need to dig a little deeper into the rocks of Santa Cruz County.

Purisima Formation (3-7 Ma)

This sandstone formation was deposited at shallow, near-shore conditions, which is why it has a coarser composition than the Santa Cruz Mudstone it followed. The blue-gray sandstone primarily consists of sediment deposited from rivers dumping into estuaries and bays.

A large fossil in grey rock on the beach.

If you find a fossil on a beach in Santa Cruz County, it is most likely from this formation. The Purisima Formation features dozens of species of invertebrate fossils, especially mollusks, as well as cetaceans and pinnipeds (i.e. whales and seals).

Though there are outcrops of this formation north of Santa Cruz, within the County this formation can be found from where Merced Avenue intersects West Cliff Drive in Santa Cruz down to the cliffs of Seacliff State Beach. The best way to find fossils from this formation is at low tide on the beaches below Depot Hill in Capitola, between Capitola Beach and New Brighton Beach. Look to the cliff walls and through boulders and smaller rocks littering the sandy shore.

Santa Cruz Mudstone (7-9 Ma)

While the Purisima Formation formed from shallow-water sediment, the Santa Cruz Mudstone formed farther out at sea where the sediment consists of finer silt and clay. This formation has a more yellow tone and is often patterned with rusty red cracks, caused by methane seeping through the rock while it was still under water.

Finding a fossil in the Santa Cruz Mudstone formation is a much trickier task than the Purisima Formation that overlays it (in parts), but there are fossils to be found. While most are small bivalves (i.e. clams) and echinoids (i.e. sand dollars), O. megalodon teeth have been found in this Formation.

The arch at Natural Bridges State Beach is Santa Cruz Mudstone, as are all of the cliffs up the coast from there until just before Año Nuevo. Read our Guide to the Swift Street Outcrop to learn how to distinguish this mudstone from the Purisima Foundation sandstone.

The arch(es) at Natural Bridges State Beach have eroded over time so that now just one remains.

Santa Margarita Formation (10-12 Ma)

The Santa Margarita Formation is a marine deposit of Miocene sandstone and conglomerate. It is visible in areas of the southern Santa Cruz Mountains and you can spot it by its rough, chunky, and sparkling white appearance.

Some of Santa Cruz County’s most magnificent fossil finds have been unearthed from the Santa Margarita Formation. According to Frank Perry, “Fossils of at least 20 species of sharks and rays are present, as are remains of bony fishes, marine mammals such as sea cows and sea lions, and invertebrates including mollusks and sand dollars.”

Features from this formation on display in our exhibits are a cast of a fossil sea cow, an O. megalodon tooth, a jaw bone from a baleen whale, and a dig-box of sand dollars. We also have many more examples in our collections storage, such as the ones seen here.

There are outcrops of this formation in the lower parts of the Santa Cruz Mountains all the way up to Año Nuevo, but in Santa Cruz County we find it mostly north of Santa Cruz, through Scotts Valley and up to Boulder Creek. The rare Santa Cruz Sandhills habitat consists of sediment from this formation.

Monterey Formation (17.5-6 Ma)

The Monterey Formation is a Miocene deposit rich in organic material. While it might not reveal fossils of charismatic megafauna like the Santa Margarita formation that followed, the Monterey formation has other interesting biotic features. Monterey Chert, used for tools by Indigenous peoples along the coast for thousands of years, is a feature of the Monterey formation. Chert is extremely diatomaceous (contains high quantities of organic material from plankton), and under other conditions could have become oil. Regularly occurring controversies regarding drilling for oil in the Monterey Bay are due to the presence of the diatom-rich Monterey Formation.

All of the above notwithstanding, there are micro-fossils to be found. Fossils of diatoms are only visible under a microscope, but fossils of some fish fragments and mollusks are (a little) easier to find. The example from our collections on the left is a fossil pea crab.

There are outcrops of this formation throughout the Santa Cruz Mountains — and throughout California. Oil drilling operations off the coast of Southern California and even inland are removing oil from the Monterey Formation. In Santa Cruz County, you can explore this type of rock on parts of Ben Lomond Mountain, along Lompico Creek, and at Majors Creek Canyon.

A variety of Awaswas stone points from present day Santa Cruz County, featuring chert.


I think I found a dinosaur bone — did I?

That cool thing you found in Santa Cruz is undoubtedly cool, but we gotta tell you — it’s not a dinosaur. Our landscape in Santa Cruz was still millions of years away from forming when the dinosaurs were alive in the Mesozoic era, 248 to 65 million years ago.

So what did I find?

It could be a fossil bone or shell, or it could be a uniquely weathered stone. Explore the resources listed here or email a photo and detailed description to us at info@santacruzmuseum.org to help determine the identity of your interesting find.

May I collect fossils?

When collecting anything from nature, always practice the “Know Before You Go” philosophy. Determine who manages the land you are on and their laws. For instance, State Parks do not allow collecting of any kind (plants, fossils, etc.), while National Forests do to an extent. Never collect without permission.

Is it a bone or a stone?

There are a few things to consider when determining if the object you have found is a (fossilized) bone or a stone.

  • Where was it found? If it was in your lawn, it’s probably a rock. Consider what rock formations are around you and how old they are.
  • Look at the texture. A rock will either be made up of packed sediment or crystalized minerals, whereas a fossilized bone will likely show evidence of the canals and webbing featured in actual bone.

Is it a modern bone or is it a fossilized bone?

Discarded bones have canals and webbing within them that are hollow. If the bone has fossilized, this texture will likely still be in evidence, but it will have been “filled” by mineralization. This also means that fossilized bones will likely feel heavier. Depending on how the bone fossilized, it may also have an altered color. BUT dark coloration does no necessarily mean it is a fossil — recent bones can also turn dark just by being under deep sand where the environment is anoxic.

Dig Deeper

Fossils at the Museum

On exhibit at the Museum

  • Cast of a fossil sea cow (Dusisiren jordani) excavated from the Santa Margarita Formation at a Zayante sand quarry in 1963.
  • Fossil skull of an American Mastodon (Mammut americanum) discovered in March 1980 by Aptos resident Jim Stanton. He spotted the giant molars protruding from a gravel bank along Aptos Creek.
  • Fossil jaw bone of a baleen whale from the Santa Margarita Formation in Scotts Valley.
  • An array of shark teeth (including Megalodon), bivalves, plants, and the skulls of a fossil dolphin, walrus, and sea lion, as well as microfossils.
  • Garden fossils: Take a stroll around the Museum’s Garden Learning Center and see if you can spot our large whale fossils.
  • Activities for kids: Multiple dig boxes features Santa Margarita Formation fossils of sand dollars and casts of a fossil sea cow.

Bring fossils home

Explore our other online fossil resources

Local Paleontologists: Then & Now

Books and Papers

Online Resources

Other Resources

Have a question? Email us at info@santacruzmuseum.org.

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.

Collections Close-Up: Picturing Nature

From rediscovered family photos to contemporary takes on unprecedented times, pictures taken for all kinds of purposes illuminate our collective understanding of the changing world around us. This month we investigate and celebrate the capacity of photography to shape our relationship with nature, from our foundational collections to our current exhibits.


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!

On the Rocks: The Solstice Sip

This recipe is part of our series On the Rocks: Exploring Science and Nature through Curated Cocktails.

A cocktail in a jar garnished with a sprig of incence cedar leaves in front of a decorated tree.

The winter solstice in the northern hemisphere marks the moment when the northern pole of the Earth’s axis is directed farthest from the sun. There is more darkness on this day than any other, but there is also the promise of new light.

As our region hunkers down into a new Stay-at-Home order, we invite you to cozy up with this Solstice Sip. Powered by the evergreen qualities of the conifers around you, whether from a neighborhood redwood tree or a douglas fir you’ve recently brought inside and covered in twinkle lights, this classic twist on an old fashioned will remind you of the light to come and the light that still flickers, even in these dark times.

**The example shown here features incense cedar (Calocedrus decurrens) harvested from a Christmas tree, but other options readily available in Santa Cruz are coast redwood (Sequoia sempervirens) and Douglas fir (Pseudotsuga menziesii). The fresher the growth of the sprigs, the richer the flavor. Make sure to properly identify your tree before harvesting because some coniferous may be toxic if ingested (i.e. ponderosa pine or pacific yew).


Evergreen Simple Syrup:

  • A few sprigs from a conifer** (about 1/2 cup)
  • 1 cup of water
  • 1 cup of sugar

The Solstice Sip cocktail:

  • 1 1/2 oz Bourbon or Rye whiskey
  • 1 tsp Evergreen Simple Syrup
  • 2 dashes Angostura bitters


To make the Evergreen Simple Syrup:

  1. Add water, sugar, and conifer sprigs to a small pot and heat over low for 15 minutes.
  2. Remove from heat and let sit for another 15 minutes.
  3. Strain out sprigs and store in a jar in the refrigerator for up to two weeks.

To make the Solstice Sip cocktail:

  1. Combine simple syrup and bitters in a glass.
  2. Fill glass halfway with ice, then stir about a dozen times.
  3. Add more ice, enough to fill the glass.
  4. Add whiskey.
  5. Stir just until cold, about a dozen times.
  6. Garnish with a sprig of your chosen conifer.

Post by Marisa Gomez

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Rock Record: Guide to the Swift Street Outcrop

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

Santa Cruz is an ideal place to explore marine and coastal geology, with millions of years worth of geologic history exposed along its sea cliffs. One of the Gent’s favorite outcrops in Santa Cruz is along the cliff face on West Cliff Drive, at the end of Swift Street.

Getting to the outcrop

Park at the end of Swift Street and cross West Cliff Drive. Take one of the paths through the ice plant and walk down onto the coastal platform. Be careful, in some areas the path down to the outcrop can be steep.

To find the Swift outcrop on a map, the latitude and longitude are:
36˚56’58.72” N
122˚02’49.22” W

Download our West Cliff Rock Walk guide to help you get there!

A guide to the rock formations

The Swift Street outcrop contains over 9 million years worth of geologic history of the coast of Santa Cruz. Familiar formations found at Swift Street include the Purisima sandstone and the Santa Cruz mudstone, along with younger beach deposits that make up the top layer of the outcrop (pictured right). Each of these layers are separated by sharp erosional contacts (geologists call these disconformities) that represent missing time and material in the rock record.

Watch our Rockin’ Pop-Up on Santa Cruz Formations.

Ancient Methane seeps within the Santa Cruz mudstone

The bulbous, light-colored features found at and near the Swift Street outcrop are the geologic remnants of methane seeps, also known as “cold seeps” (pictured below).

Bulbous fossilized “cold seeps” at the Swift Street outcrop.

These formed while the Santa Cruz Mudstone was still mud in deep waters off the coast of California between 7-9 million years ago. The rock accumulated as sediments, including the bodies of perished sea critters, fell to the sea floor. As the bodies of phytoplankton and other marine microorganisms decayed in this mud, they released gases that slowly worked their way up to the surface. As these gasses followed cracks in the firmly packed sediment, they gradually widened these conduits and cemented the walls with carbonate minerals (the same thing limestone, chalk, and marble are made of), creating a sort of chimney to release these gases and fluids out of the seafloor.

Diagram of how the fluids and methane seeped their way from deep below the seafloor up through mudstones like the Santa Cruz mudstone. The original research on these fossilized methane seeps was done by UC Santa Cruz researchers and students! (Image source: http://citeseerx.ist.psu.edu/viewdoc/download?doi=

Seeps like these that bring methane gas and fluids from deep below the seafloor can be found today out in the deep regions of Monterey Bay. In millions of years from now, these same chimneys may find themselves on a new coastal outcrop!

Seafloor methane seeps like the ones preserved at Swift Street occur in the Monterey Submarine Canyon today, providing nutrients for mini ecosystems on the seafloor, like these red-orange microbes that form mats around the seeps. (Image from the Monterey Bay Aquarium Research Institute)

Layers of the Santa Cruz Mudstone

While the fossilized cold seeps tend to get a lot of the attention, the tough Santa Cruz Mudstone around them is itself a fascinating piece of rock. In this area, the Santa Cruz Mudstone hosts alternating layers of pale mud and blocky porcellanite (pictured right), a rock that gets its name from its close resemblance to unglazed porcelain. This rock is very similar to chert, a glassy rock formed at the seafloor from the accumulation of the glassy skeletons of diatoms. Porcellanite, like that found at the Swift Street outcrop, has a bit more clay and calcite (probably from critters that make chalky skeletons) giving it its more porcelain-like appearance.

The layers of porcellanites have a distinctively blocky texture. This results from the very brittle nature of the rock type. Just like pieces of porcelain, when these were squeezed and warped by tectonic pressure, rather than bending like the softer, more ductile mud layers, the porcellanite essentially shattered in response to those forces. Yet, even in its shattered state, the porcelanite rock is remarkably strong and durable. For this reason, Santa Cruz Mudstone with its rugged porcellanite layers makes up many of the flat bases of the sea cliffs around West Cliff as it stands up against the erosive force of the waves that more easily cuts into the sands and sandstones of the overlying cliffs.

The highest visible layer of the Santa Cruz Mudstone is a thick (almost 1 foot-thick) light-colored mud layer (pictured left), that has old clam burrows on its surface and is overlain by Purisima formation with large chunks of porcellanite from the mudstones below. This tells us that before the sands of the Purisima Formation were laid down atop the mudstone, it spent some time being eroded by waves. Those clam burrows are a testament to the time it spent as a rocky seafloor bottom over 6 million years ago.

The chunks of porcellanite just above the contact tell the story of the earliest history of the Purisima Formation as powerful waves broke down and churned up rocks that were incorporated into the first layers of the Purisima sands.

The Purisima Sandstone Formation

Above the Santa Cruz mudstone lies the Purisima sandstone, a rock formation known throughout Santa Cruz for its abundance of stark white fossils of ancient shells. Swift Street contains only a relatively small section of the Purisima Formation, but several areas within it exhibit amazing sedimentary textures. Up on the cliff, sections of the Purisima are a flat brown, with no visible fossils, and parallel “beds” or ancient sediment layers (pictured right). These areas represent long periods of constant sediment deposition, with no major storms or changes to the environment.

Elsewhere in the outcrop, shell-rich layers and features called “cross-beds” (pictured below) tell us that at other times between 7 and 2.6Ma, this area experienced large storms that created strong ocean currents. The jagged contact between the Purisima formation and the above Quaternary-aged sediments represents nearly two million years of lost time in the rock record. 

Erosion and deposition of sands on top in the last 100,000 years

The base of the uppermost layer of the Swift Street outcrop is made up of an unconsolidated matrix of fine sand surrounding large, cobble-sized pieces of the underlying sedimentary rocks (pictured right), as well as abundant shell fragments. Because this layer is made up of fairly loose sediments, as opposed to rock, we know that it has not experienced long periods of burial required to turn sediment into rock (or lithification in geologist jargon.) For large cobbles to be ripped-up from the underlying layers and deposited here, requires high-energy wave systems like those found on the modern coast. Therefore, the transition from the underlying Purisima sandstone to these sediments likely represents a time where the Santa Cruz coast shifted from deep water to a coastal zone, likely as a result of sea level fall and tectonic uplift. 

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”.

Naturalist Night: Santa Cruz Sandhills

The Santa Cruz Sandhills are a rare habitat in the Santa Cruz mountains that support a very specific niche of plants and animals. Many of our endemic species in the County are limited specifically to the Sandhills. Join us for an overview of this habitat’s geologic history, rare plants and animals, and the human impacts that have added to its scarcity. This is a great class for budding naturalists and those looking to revisit the basics.

We are excited to be joined this month by paleontologist, science teacher, and Sandhills-local Wayne Thompson who will help us dig deeper into the geology of the habitat by showing off his fossil finds.

About the series: Join fellow nature enthusiasts for monthly explorations of the biodiversity of Santa Cruz County. Each month, our Public Programs Manager Marisa Gomez will share the stories of a specific Santa Cruz habitat as we develop our skills as naturalists.

This series will feature a presentation as well as an interactive session and is in partnership with Santa Cruz Public Libraries. Come prepared to share and to learn alongside naturalists deep in their journey and just starting out.


Watch other Naturalist Nights