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
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
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.
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.
A Sand-Loving Endangered Species
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.
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.
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.