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Alum Rock Exploration

Alum Rock Park, created in 1872 as the first municipal park in California, offers ancient rocks & new geologic changes.

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Rating: 5 out of 5 by EveryTrail members
Difficulty: Moderate
Length: 4 miles
Duration: 1-3 hours

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... more »

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


The lush Penitencia Creek, which bisects the Park, flows year-round... more »

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Points of Interest

1. 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,... More

2. 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... More

3. 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... More

4. 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... More

5. 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... More

6. 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... More

7. 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 ... More

8. 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... More

9. 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... More