Local Geology

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Jurupa Hills Geology

Rock formations and topography tell a long complex story.; Often the story stretches so far back in time that measurements are expressed in tens to hundreds of millions of years. For this reason geology has been called the science of earth’s deep history. The iconic landscape of Southern California, with its steep mountains and broad alluvial valleys, reveals just such a complex, dynamic history.

A short walk up the trail behind the Nature Center takes one to vistas overlooking some of these natural features. On the far side of the San Gabriel Valley, along the northern edges of the cities of San Bernardino and Fontana, runs the San Andreas Fault, perhaps the most famous in the world. Here two great Continental Plates - the Pacific and North American - meet, with each slowly slipping past the other.

To the northeast rise the rugged San Gabriel Mts. with Ontario and Cucamonga Peaks towering over the valley. Relatively young in geologic time, the mountains contain ancient Proterozoic rock – Augen and Mendenhall Gneiss, both dated at well over a billion years.

About 5 - 7 million years ago a block of earth’s crust containing this very old rock, along with other igneous and metamorphic rock, began rising along local fault zones to form our present-day San Gabriel Mts. Powered by the same tectonic forces that generate our southland earthquakes today, the area continues to be very active. These young mountains are being thrust up at a rapid rate, and would be much taller if erosion were not paring them down.

The Jurupa Hills, behind the Nature Center, are a product of these same geologic forces. Marble formations in the hills point back to a time when an ancient ocean covered the area. Shallow enough to support coral reefs, evidence of geological deposition also indicates that regional mountains were eroding into this same region.

As part of this coral reef sank sediment flowed in upon it. Over time the weight of the sediment transformed the calcium formations of the reef into limestone. As more pressure came to bear on the limestone it metamorphosed into marble, a low-grade metamorphic rock. About the same time as the marble was being formed, magma rose from beneath it to form the granitic rock we find in the hills today.

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A World Famous Mineral Locale

 As this granitic magma intruded into the calcium-rich limestone deposits it created an environment rich in the formation of certain minerals. Approximately 200 minerals, prized by collectors and studied by scientists, have been collected from the Crestmore Quarries.

 Rock from Jurupa Hills quarries has also been used in more functional ways – as material to build the Long Beach and San Pedro breakwaters during the first half of the 20th Century.

A Levitating Rock

When Michael Heizer, the artist behind the Los Angeles Museum of Art’s (LACMA) Levitated Mass exhibit, discovered a 340 ton boulder in Riverside County’s Jurupa Hills that he wanted to use for his sculpture at LACMA, transporting the megalith from its location to Los Angeles became a national event. In June of 2012 it took eleven days for the 206-wheel trailer cradling the granitic monolith to creep along a carefully planned route to the campus at LACMA. Sightseers and camera crews lined the route, and international media monitored the progress. At one time, before our inland valley became a sea of development, the area was home to a system of low-lying sand dunes. These Delhi Sands had been created over time by Santa Ana winds blowing down through the Cajon Pass, picking up bits of geological material and dropping it at the foot of the hills. At one time this system stretched many miles through what is now Loma Linda, Colton, and Fontana.

A Sand-Loving Endangered Species

One of the local natural residents adapted to life in this unique landscape. Indeed the Delhi Sands Flower-Loving Fly adapted so exclusively to this dune environment that extensive development in the area has imperiled the future of the species. Local populations are currently estimated to be in the hundreds.

Hovering like a hummingbird the fly has a long proboscis to feed on the nectar of native flowering plants growing in the dunes. The adult female lays its eggs in the sand. Larvae develop underground. The endangered Delhi Sands Flower-Loving Fly is a classic example of the degree to which natural organisms are intimately attached and in tune with their geological or natural foundation.