Alum Rock Exploration

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Difficulty: Moderate
Length: 4.0 miles / 6.4 km
Duration: 1-3 hours

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Overview: Just a few minutes outside of San Jose, you'll find a place to explore 100 million years of history. Alum Rock Park, created in 1872 as the first municipal park in California, offers both ancient rocks and new geologic changes (and lots of nice trails and wildlife, too). Despite its longevity and proximity to a populous urban area, it remains one of the less-visited jewels in the Bay Area's crown.

Special thanks to the City of San Jose’s Alum Rock Park Staff, and to YSI Staff John Harshman, Dorothy (DJ) Johnson, Bonnie LeMat and Susanne Mulcahy for their participation in this Exploration.

This Exploration created in collaboration with the Exploratorium.

Robin Marks and Lauren Sommer contributed to this Exploration.

Tips: 16240 Penitencia Creek Road
San Jose, CA 95127 map

408-259-5477

The lush Penitencia Creek, which bisects the Park, flows year-round. Once home to the Ohlone Indians, Alum Rock Park features rugged ridges with spectacular views of the Santa Clara Valley, geologic formations including mineral springs and evidence of seismic events, and a variety of native flora and fauna.

Park hours are daily from 8am until one-half hour after sunset.

Parking: $6

Youth Science Institute

Founded in Alum Rock Park in 1953, Youth Science Institute provides year-round school and public programs and interpretive services in the Park, thanks to a long-term partnership with the City of San Jose.

YSI is a 501(c)3 non-profit organization that partners with nature to teach science to 33,000 pre-school through 12th grade students throughout Silicon Valley each year. YSI school programs, after-school programs, and summer camps fill the gap that is present in science education today to inspire in students enthusiasm for science and love of learning, to develop the critical thinking skills they need for academic success, and to forge connections with nature that foster their personal health and earth stewardship.

YSI has two additional Science and Nature Centers at Sanborn Park in Saratoga and Vasona Lake Park in Los Gatos. Each Center is open to the public and features live animals and interactive exhibits, as well as year-round programs.

16260 Penitencia Creek Road

San Jose, CA 95127

Open All Year, Tuesday – Sunday, 12:00 noon – 4:00 PM or by appointment.

(Closed on Sunday from October-April)

Points of Interest

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Western screech-owl, other feathered friends

Alum Rock Park is home to a legion of critters, and hikers with attentive eyes are likely to spot some wildlife. The Western screech-owl (Megascops kennicottli), here nestled in the crook of a tree, is small by owl standards (only 8-10 inches high), but capable of taking prey larger than itself, like a rabbit or duck. Most of the time, though, these owls hunt insects or small rodents.

This tree's become a food storage pantry for local acorn woodpeckers. Living a communal lifestyle, a family of these birds will select a tree (or, in some cases, a utility pole) in which to stock the whole group's supply of winter acorns. Throughout the winter, the group remain near its granary tree, which may have as many as 50,000 acorns stowed in it.

It's easy to recognize these acorn woodpeckers by their red crowns and black-and-white clown faces.

Photo: Steve Ryan

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Vertical rock strata

Evidence of the park's volcanic past reveals itself along the hike. These layers of shale were once mud at the bottom of a body of water that once covered the region. Now, they're not only above ground, but are nearly vertical, owing to the pushing and plowing of the tectonic plates that continue to give Northern California its famous earthquakes.

Looking more closely at the rock, you can see the strata, and how the rock breaks easily along the lines along the layers.

Another set of rock layers that were laid down horizontally millions of years ago and have since been uprighted by tectonic forces. These rocks are a mixture of shale and a hard rock called chert. Chert is formed only at the bottom of deep oceans, made of the compressed silica skeletons of billions of tiny marine animals called radiolarians.

2c. Another kind of vertical strata (above the water) Photo

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Drips and lumps

Curious-looking lumpy rounds of rock near the creek are more evidence of past volcanic activity. They are lumps of flowstone, rock made from minerals, largely calcite, dissolved in the hot spring water. When that water reaches the surface, it cools and evaporates, depositing more minerals on the existing rock.

These pointy formations have the familiar look of cave stalactites, and indeed, they're formed in the same way. Rain that's fallen on the top of Alum Rock's mountains flows through rock layers, picking up minerals. Further down the mountain, the water flows out, depositing minerals as it drips to the ground. The "ceiling" it drips from develops pointy tips that focus the drops of water below them, creating the rounder lumps of rock pictured previously.

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Hot spring

Ah, now this is why people began coming to this area in the first place--hot springs! When you find them, inside a set of small shelters along the South Rim trail, put your hand in for a nice warmup.

The water in these springs has fallen on the top of the nearby hills and flowed through layers of rock still warm with residual heat of volcanoes that were active here tens of thousands of years ago. The air carries a whiff of sulfur, one of the minerals picked up by the water during its travels.

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Mineral deposits

The water that flows through this volcanic landscape leaves minerals everywhere--as rock formations, as a scent in the air, and here, as dust on rocks nearby the hot springs. The fine particles, mostly sulfur, are left behind by the warm, mineral-rich water as it evaporates.

Getting a closer look at the mineral deposits by scraping some of them off with a penknife.

You'll find the dust deposits just a few feet from the shelters over the hot springs. The white blotch near the ground to the left of the spring entrance is rock strata covered with minerals.

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History: a spring entrance

Not long ago, the hot springs at Alum Rock drew crowds of people from across the country who believed the minerals in the water had a healing capacity.

In its previous incarnation as a spa, Alum Rock was overrun with tourists visiting the baths, a huge swimming pool, amphitheater and restaurant.

Near the ends of this bridge, you can still see remnants of the smaller pools and the benches built in them for bathers.

In the 1970s, the fanfare of amenities at Alum Rock ended; most of the buildings were taken down, the large man-made swimming pool demolished, and the park became a preserve, returning to a more natural state. You can still see many relics from the old days, like this mineral-encrusted door, along some of the trails.

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Mountain Lion

Perched in suspended animation, a mountain lion bares its teeth to visitors to Alum Rock's Youth Science Institute. Also known as a puma or cougar, this large cat can lithely leap 40 feet in pursuit of prey. Much of the state of California (and many other states) is cougar habitat, though human development is constantly encroaching on the puma's environment, resulting in dwindling populations in some areas.

Look out for this aptly named insect lurking at the bottom of creeks and lakes, waiting to strike potential prey. The toe biter (officially known as Abedus indentatus) delivers a more painful chomp than any other insect. The digestive saliva it injects can liquefy muscle tissue. The round balls attached to the bug are eggs, which the female cements onto the back of the male.

Image Source: NoiseCollusion

Ground squirrels are common throughout California and Alum Rock is no exception. This one is hunched in its burrow, but these fetching rodents can often be seen sitting upright on their hind legs. When they feel threatened, they let out a metallic-sounding screech to warn its other family members.

- As in many northern California parks, there's no shortage of deer to be seen at Alum Rock. Look for their trails in the park, which often run between where the deer eat to where they rest.

Photo courtesy of zygzee

A more unusual sightseeing treat in Alum Rock is the bobcat. Bobcats are the most common wildcat in the United States. Their distinctive ear tufts make them easy to recognize. Because they're nocturnal, they're rarely spotted during the day.

Photo: www.wildphotos.com

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Differing habitats of Alum Rock

The steep slopes and varied topography of Alum Rock afford it several different habitats. Here, north-facing canyon walls that get little sunlight sport ferns, large trees, and other lush greenery.

In contrast, high up on the sunnier, south-facing slopes is a chaparral environment, with scrub, bushes, and grasses.

A wide variety of plants dot the riparian corridor next to the park's creek.

You'll also find stalks of horsetail growing near the creek. These ancient plants are unusual in that their stems are covered with silica. They are also commonly known as scouring rush, because they were once used to clean cooking pots.

8b. Differing habitats of Alum Rock Photo

8a. Differing habitats of Alum Rock Photo

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River shapes the rocks

Sandstone near the creek is worn smooth by rains and passing water. This rock is most likely a traveler from the top of a nearby hill, washed down into the canyon by winter storms.

Follow the trail and the stream disappears beneath your feet. In this spot, it's flowing through porous sandstone below the trail.

A little further down, the underground stream encounters a barrier of non-porous shale, and it's forced back to the surface again.