Welcome to Black Mountain
As the top of Black Mountain, you’ll encounter a pale gray limestone outcrop that is strikingly different from the sandstones and shales that dominate nearby ridges. These rocks formed far out in the tropical Pacific Ocean more than 100 million years ago and tell the story of underwater volcanoes, drifting tectonic plates, and how ocean life left behind a record in stone. This EarthCache will guide you through observing the limestone at the summit and help you understand how it formed, how it moved thousands of miles to this spot, and how weathering and tectonics continue to shape it today.
The Geology Behind the Limestone
Limestone is a sedimentary rock made mostly of calcium carbonate, which forms in warm, shallow seas where corals, shellfish, and plankton produce carbonate skeletons that accumulate on the seafloor. Over millions of years, these remains compacted and hardened into rock, now known as the Calera Limestone. At Black Mountain, this limestone originated as part of a seamount in the Pacific Ocean more than 100 million years ago, built up with the shells of microscopic plankton called foraminifera. Plate tectonics eventually carried this seamount eastward until it collided with the edge of North America, where pieces of it were scraped off and added to the Franciscan Complex. That is why limestone from a tropical marine setting now sits at the summit of a Bay Area mountain, where it stands out against the surrounding sandstones and volcanic rocks.
From Ocean Seamount to Mountain Summit
The journey of the Black Mountain limestone from the middle of the Pacific Ocean to its current position is a record of tectonic motion on a grand scale. The limestone was once part of a seamount built on the Farallon Plate, which moved eastward for tens of millions of years. As the plate was subducted beneath North America, parts of the oceanic crust and its sedimentary cap were scraped off, crumpled, and accreted to the edge of the continent, forming the Franciscan Complex. Later, activity along the San Andreas Fault carried the limestone northward, while regional uplift raised it to its present elevation of 2,812 feet. This means that when you stand on Black Mountain today, you are standing on a slice of ancient tropical ocean floor that traveled thousands of miles and was hoisted high above sea level by the forces of plate tectonics.
Karst Weathering at the Summit
The limestone at Black Mountain is not just interesting because of its origin but also because of how it weathers today. Unlike sandstone or shale, limestone readily dissolves in slightly acidic rainwater, which contains carbonic acid formed when rain absorbs carbon dioxide from the atmosphere and soil. Over time, this process creates features known as karst weathering, which can be seen at the summit as pitted and etched rock surfaces, solution grooves, and small hollows where water has gradually dissolved the calcite. These subtle but distinctive textures are evidence that even at the ridge top, chemical weathering is actively shaping the outcrops, influencing soil chemistry, drainage, and the types of plants that can grow there. Karst processes here are less dramatic than in famous cave systems but still provide a living example of how limestone interacts with water to sculpt the landscape.
Tasks for This EarthCache
To log this EarthCache, visit the site and complete the following tasks. Send your answers via Geocaching or email.
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Include “Limestone Outcrop - Black Mountain - GCBC7NH” on the first line of your message.
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Observe the limestone and describe its color and texture. What features help you identify it as limestone?
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Explain how this marine limestone might have traveled from an ancient ocean seamount to the summit of Black Mountain. What processes could have moved and uplifted these rocks over millions of years?
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Look carefully at the surface of the limestone for any visible fossils. If you see any, describe their shape, size, and color. If you do not see any, what might this suggest about the environment in which the limestone formed?
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Examine the outcrop for signs of karst weathering. Describe at least one feature you observe and explain how water could have produced it. Why do you think karst weathering is prominent at this location?
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In your log, attach a photo of yourself or a personal item with the limestone outcrop in the background. (Note: photos predating the publication of this EarthCache are not accepted.)
Works Cited
https://en.wikipedia.org/wiki/Black_Mountain_%28near_Los_Altos%2C_California%29
https://baynature.org/article/peninsula-treasure/
https://pubs.usgs.gov/of/2005/1127/chapter7.pdf
https://en.wikipedia.org/wiki/Limestone
https://en.wikipedia.org/wiki/Franciscan_Complex
https://en.wikipedia.org/wiki/Karst