Claremont rocks, part one

They’re everywhere. Dig into the ground and you will uncover many. They are found in landscapes.  They frame monument signs. They are found as the facing on pilasters and porches. They make up the foundations for older homes, and the veneer on new ones. They are the stuff of fireplaces and chimneys and backyard barbecues. They were used in the early days to make curbs for streets. They were used to build entire houses.

The rocks that have washed down from the canyons have come to be known as “Claremont potatoes,” because ones digs them up as plentifully, although the sizes belie potatoes, when bowling balls (or an even a larger characterization) is a more apt description. They were here before man, building up over millions of years throughout the Pomona Valley.

Claremont and the surrounding communities of the Pomona Valley sit on large sloping alluvial fans within the Santa Ana watershed. The alluvial fans have been produced by two main creeks, San Antonio Creek and Cucamonga Creek, along with many smaller ones. The entire valley sits atop the accumulated sediments washed off the slopes of the San Gabriel Mountains.  Therefore, all those “potatoes.”

About 20 million years ago when the Pacific plate was scraping against the North America plate a piece of the continental crust broke off. The block of crust was trapped between the two plates. It turned 90 degrees clockwise as it was dragged to the northwest by the Pacific plate. Then around five to seven million years ago, part of the crust began rising as a mountain range along the Cucamonga and Sierra Madre fault zones. That range is the San Gabriel Mountains that stretch from the Cajon Pass to the Newhall Pass in the west.

The mountain range is one of what is called the Transverse Ranges, which begin at the southern end of the California Coast Ranges and lie generally between Santa Barbara and San Diego counties. They are named so because of their east-west orientation, making them transverse to the general north-south orientation of the costal ranges. The Transverse ranges include the San Bernardino, Santa Monica and Santa Susana mountains.

Since the event a few million years ago, the geographically active mountains have been thrust up rather quickly and have been worn down by erosion just as fast. Much of San Gabriel Valley, along with the Pomona Valley and the San Bernardino Valley, sits on top of the accumulated sediments that washed off the slopes of the San Gabriel Mountains.

The name San Gabriel is from the nearby Spanish mission, although until 1927, the name Sierra Madre, “The Mother Mountains,” was used as well. Both names came from the early Spanish missionaries until the US Board of Geographic Names acted on a petition from Pomona College professor Alfred G. Woodford to permanently adopt the name, San Gabriel Mountains.

Mr. Woodford petitioned the board on August 16, 1927, for the body to adopt the name and received a response from James McCormick, secretary, on August 24:

“The United States Geographic Board has rendered no decision respecting the name of these mountains. My own view, however, is that San Gabriel is the name that should be used. Sierra Madre is too pretentious for application to this little group. As intimated by you the US Geological Survey, the Geological Survey of California, and the California State Mining Bureau have used the name San Gabriel consistently for many years.

In 1914 Henry Gannett, as chairman of this board, undertook to list and define the mountains of the United States. He had the cooperation of geographers and geologists in all parts of the United States, but he died before the work was completed. 

A tentative list, however, was compiled and copies of this or parts of it were mailed to those cooperating, for examination and criticism. In this list I find San Gabriel Mountains defined as limited north and south by Mohave Desert and Los Angeles Valley; and east and west by Cajon Pass and Southern Pacific Railroad.

I’ll refer this case to the board for consideration and decision during the coming fall or winter.”


On October 21, 1927, Mr. McCormick sent a letter to Professor Woodford informing him that “On October 5, the US Geographic Board formally adopted the name San Gabriel Mountains (not San Bernardino, nor Sierra Madre)…”

Today, the only remnants of the once common usage of Sierra Madre Mountains are found in place names such as the city of Sierra Madre or Sierra Madre Villa Avenue and the Gold Line’s Sierra Madre Villa station in Pasadena, named after a tuberculosis sanitarium that once existed at the base of the mountains.

What is interesting in the argument from Mr. McCormick about the name Sierra Madre being “too pretentious for application to this little group” is that the Board’s interpretation is that such a small mountain range could not be the “Mother Mountains,” presumably to other mountain ranges.

The name Sierra Madre was most likely derived, not from a literal meaning, but a metaphorical one, based on what the mountain range provided the early Spanish settlers and the native people, the Tongva, before them.  It was their lifeblood. It nurtured them.

The range provided them with water for their crops and daily needs. It deposited the sediment that created fertile farmlands. It provided nourishment and medicine in the plants found there and in the animals they hunted. It provided respite from the heat in summer.

Just as the creeks like San Antonio and Cucamonga were sources of life, they could be destructive as well in periods of heavy rain. The flood of 1938 wiped out a large swath of Claremont and the flood of 1969 crested Cucamonga Creek and caused widespread damage. Those two creeks of the Pomona Valley are now tamed by flood control channels, and no longer are the alluvial fans created by them depositing sediments and rock, or in Claremont, “potatoes.”


Next month, Claremont rocks Part 2.


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