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Rock Record: Panther Beach, an Extraordinary Geologic Feature in Santa Cruz

Written by Gavin Piccione

One of the most exciting perks of having an appreciation for geology is the limitless possibility to find new geologic features, even on the most seemingly mundane trips outside. It may be an interesting pebble that catches your eye or a new outcrop that piques your interest, but one usually does not have to travel far to find thought-provoking rocks. However, within this vast collection of terrestrial curiosities, some features stand out as particularly exceptional or interesting.

On a recent trip to Panther Beach, I experienced the thrill of discovering one of these remarkable geologic marvels hiding just up the coast. In this installation of Rock Record, I cover why the rocks at Panther Beach in Santa Cruz are a world-renowned sedimentary outcrop, and I investigate how this truly unique part of the cliff was deposited here.

The Rocks at Panther Beach

At first glance the rocks at Panther beach may look very familiar to folks who have spent time in Santa Cruz, with cliff walls of the main section of beach made up of the crumbly, beige sedimentary rock called Santa Cruz mudstone. However, if you head to the south-end of the beach, and pass under the archway, the rocks change drastically. Instead of the highly uniform mudstone, the rocks on this side are wavy, laminated sandstones (see image 1). Below these peculiar sandstones there are darker rocks, called dolomite-cemented sands, that have bands that tilt down and towards the southeast. If one breaks or scratches these dolomite-cemented sands, they may smell the faint odor of petroleum. The contact between these two rock types is high irregular, looking almost as if the dolomite-cemented sands were bubbled up into the wavy sandstones like liquid in a lava lamp (see images 2 and 3). Capping these two sandstones is the familiar Santa Cruz mudstone that makes up the rest of the cliff (see full outcrop in image 4).

Image 1: Wavy sandstone at Panther Beach
Image 2: Contact between the wavy sandstone, and the dolomite-cemented sandstone. Image from Sherry, et al. 2012
Image 3: Contact between the wavy sandstone, and the dolomite-cemented sandstone. Image from Sherry, et al. 2012
Image 4: Panther Beach outcrop. Image modified from Sherry et al. 2012

Formation of the Panther Beach Outcrop: The World’s Largest Sedimentary Injection Deposit

When geologists see large sections of rock that are unrelated to the surrounding rock, we often think of some surface process like motion along a fault, that may bring two different rock types in contact. However, at Panther Beach there is no evidence for a fault that could have brought the sandstones to the surface. So how did this ~100m section of sandstone become emplaced in a cliff that is otherwise made of Santa Cruz mudstone?

To understand the formation of the Panther beach outcrop we must start about 1 kilometer below the seafloor. Around 9 million years ago off the coast of ancient Santa Cruz, 1000 meters of mud (the Santa Cruz mudstone) was deposited at the bottom of the ocean above a thick sand deposit (the Santa Margarita sandstone). Later, after heat, and pressure turned most of these sediments into rock, the only sediments left were small pockets of Santa Margarita sands that had not been lithified (i.e., turned to rock). Portions of these sand pockets were rich in oil, which created two distinct reservoirs: oil-rich sands and oil-poor sands. These two sand types were spatially separated because of the density difference between oil and water (see image 5).

Image 5: Sediments prior to injection. Schematic modified from Sherry, et al. 2012.

As more mud was deposited atop the Santa Margarita Sands, the pressure on the sand built. Then finally a geologic event, probably either an earthquake or a landslide, shook the sediments causing the slurry of both oil-rich and oil-poor sands to be injected, at a high velocity, into the overlying rock through fractures in the Santa Cruz mudstone. This type of formation is called a sediment injection deposit, where sediments from below are squeezed up into the overlying strata like a tube of toothpaste (see image 6). While the sands were injected, the oil-rich sand slurry would have traveled slower than the oil-poor sands, leaving the oil rich sands below the oil-poor sands. Injection deposits occur in other places on Earth, but the Panther Beach outcrop is the largest sedimentary injection deposit in the world!

Image 6: Sediments after injection. Schematic modified from Sherry, et al. 2012.

Getting to the Panther Beach Outcrop

Image 7: View looking towards the south through the archway in the cliff

Panther Beach is located off Highway 1 about 5 miles north of Santa Cruz. The parking lot for beach access is unmarked but can be found on google maps. The walk to the beach is about 50 yards down a narrow, steep trail (see the image at the start of this post for the view from the top of the trail). Once on the beach, turn left and walk under the arch in the cliff (see image 7). Be careful, this passageway may not be accessible at high tide, use caution when walking through and do not try to pass when water is high! The sedimentary injection deposit makes up the sea cliff beyond the south side of the arch.

The GPS coordinates for Panther Beach are: 36.994, -122.169


Much of the information for this edition of Rock Record was first published in the journal article: Emplacement and dewatering of the world’s largest exposed sand injectite complex, by Timothy Sherry and others.

Reference cited:

Sherry, T. J., Rowe, C. D., Kirkpatrick, J. D. & Brodsky, E. E. Emplacement and dewatering of the world’s largest exposed sand injectite complex. Geochemistry, Geophys. Geosystems 13, 1–17 (2012).

A Guide to the Rocks of Santa Cruz County

Santa Cruz is an area of geologic interest with a complex history of processes that shaped the coastline, bluffs, terraces, and mountains we see today! Wind, waves, earthquakes, fires, and other natural forces have changed and shaped the landscape for millions of years, though humans have only been able to document those changes in the recent past.

Jump To: Formations | Rock Types | Minerals | Resources


Geologic Formations

The above map shows the distribution of different rocks in Santa Cruz County. Each color represents a different kind of rock and, in turn, a particular age. Many of these rocks represent formations.

A geological formation is a basic rock unit that geologists use to group rock layers. Each formation must be distinct enough for geologists to tell it apart from surrounding layers and identify it on a map. A formation can consist of a variety of related or layered rocks, rather than a single rock type. There are over 14 geologic formations in Santa Cruz County. Most of these formations were created through movement of the crust because of tectonic uplift at the subduction zone off the California coast.

Most of the county is underlain by granitic rock. It formed about 100 million years ago from molten rock which cooled very slowly at a depth several miles below the earth’s surface. Since then, this area has been covered by the sea much of the time. Sand, mud and other sediment was deposited on the seafloor and was eventually compressed and hardened into sedimentary rock which was uplifted to form the Santa Cruz Mountains. Many of the sedimentary beds, which were originally horizontal, have been tilted, folded or partly eroded away. In some areas major faults have offset the rocks.

A large fossil in grey rock on the beach.

3 formations are known for fossils in this region:
Purisima Formation (3-7 Ma)
Santa Cruz Mudstone (7-9 Ma)
Santa Margarita Formation (10-12 Ma)

Learn more in our Fossil Guide.

Rock Types of Santa Cruz County

The three basic types of rocks- igneous, metamorphic and sedimentary– occur in Santa Cruz County. All are composed of minerals. Some consist of primarily one mineral, as in the case of marble, while others are an aggregate of many different minerals, as in the case of granite and conglomerate. 

Each rock type in the Santa Cruz area represents a different chapter in this region’s geologic past, and each has its own unique story to tell. The rocks of this area are mostly covered by soil and vegetation, so geologists must rely on scattered outcrops in creek beds, quarries, road cuts and sea cliffs in order to piece together the geologic history. 

Granite, Empire Grade Road

Igneous rocks formed from molten rock called magma. Plutonic rocks, such as granite, gabbro and alaskite, cooled very slowly, solidifying deep below the earth’s surface. This provided time for larger crystals of quartz, feldspar, mica and other minerals to form, giving the rocks a coarse texture. Volcanic rocks, such as basalt, cooled quickly at the earth’s surface and are very fine grained.

Marble, UCSC Quarry

The metamorphic rocks of this area are a geological enigma. They predate the granite rocks and were originally sedimentary rocks such as limestone, shale and sandstone. These were respectively metamorphosed into marble, schist and quartzite by the intrusion of magma about 100 million years ago. How much earlier these rocks were laid down as sediment, however, remains a mystery.

Mudstone, HWY 1 North of Santa Cruz

Sedimentary rocks in the Santa Cruz area originated for the most part from sediment such as mud, sand and gravel that was deposited on the sea floor. Over millions of years chemical alteration and pressure from burial hardened the sediment into rock. These rocks overlie the igneous and metamorphic rocks of this region and are of a younger age. 

Minerals

Minerals are the naturally occurring crystalline substances that make up the rocks around us. Minerals such as quartz, feldspar, and calcite are the most common constituents of rocks in this area. Dozens of other mineral species occur here, but in small amounts. Large, well-formed crystals- the kind most sought after by collectors- are scarce. 

Several minerals in this region have proven to be of great economic importance. Cinnabar (the chief ore of mercury) has been mined extensively at the New Almaden on the east slope of the Santa Cruz Mountains. Calcite (in the form of marble) has long been quarried near Santa Cruz for the manufacturing of lime and cement. 

Benitoite is the California state mineral. This unusual blue crystal was first discovered in 1907 in San Benito County. While benitoite is found in a few places around the world, San Benito County is the only place in the world where gem quality benitoite crystals are found.

Learn more in our Collections Close-Up.

Resources

Explore other resources for better understanding geology, paleontology, and the landscape of the Santa Cruz region.

Geology at the Museum

On exhibit at the Museum

  • Specimens of common minerals from the region
  • Specimens of common rocks from the region
  • A detailed topographic geologic map of the county
  • Garden: Take a stroll around the Museum’s Garden Learning Center and see if you can spot fossils and other large rock samples.
  • Activities for kids: Multiple dig boxes features Santa Margarita Formation fossils of sand dollars and casts of a fossil sea cow.

Bring rocks home

Rent a kit to explore local rocks at home. Kit rentals are $10 per week and can be requested here (you do not need to be a teacher to request a rental).

Explore Online Rock Resources

Shop the geology and paleontology section of our online store

Books and Papers

Santa Cruz Top 5 Geologic Must-Sees

Santa Cruz is an area of geologic interest with a complex history of processes that shaped the coastline, bluffs, terraces, and mountains we see today! Use this map as you walk, bike or ride your way across the county and explore some of the geologic must-sees our area has to offer. Visit our online Guide to the Rocks of Santa Cruz County to dig even deeper into the geology of the region.

Rockin’ Pop-Up: What even IS North America?

Santa Cruz County is obviously a part of North America. Right?

Well, it’s a little more complicated than that. There’s the continent of North America, but there’s also the North American tectonic plate — where Santa Cruz County does not reside! While our neighbors in Los Gatos on the other side of the Santa Cruz Mountains are located on the North American Plate, Santa Cruz is located on the Pacific Plate.

There are lots of little (and really big!) geologic surprises across the continent. Join the Geology Gents for this North American road trip!

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: Great Geologic Goodbyes

Graham Edwards and Gavin Piccione with fossil

For over three years, the Geology Gents have been regular fixtures at the Museum — both online and in-person. For this month’s pop-up though, we say goodbye to Graham as he heads out to New Hampshire for his post-doctoral work at Dartmouth College. The good news is that he’ll remain a Geology Gent from afar and Rockin’ Pop-Ups will continue!

In honor of this milestone, Graham and Gavin will be exploring some “great geologic goodbyes” for this month’s pop-up — from extinction events to the dismantling of Pangea.

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

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