Baths and batholiths
Deborah Painter (USA)
“Look over there!” I exclaimed as I stood on the grounds of a manufacturing plant and stared across the tracks of the Atchison, Topeka and Santa Fe Railroad to the east of the plant. I was pointing at several mountains a few kilometres in the distance. “That mountain is glowing!”
Standing alongside me was James, the plant’s maintenance supervisor. “I guess because I’ve seen this for the past 14 years, I don’t even pay attention anymore” was his reply. The mountain was not glowing due to any internal source but because exceptionally light toned granites captured and reflected rays of sun streaming from behind a December cloud cover (Fig. 1).
The mountains looked like this for most of that chilly day and the glow shifted from mountain to mountain (Fig. 2).
The granitic mountain cluster was in Perris, a city in Riverside County, California in the USA. I have had the good fortune to visit this county twice recently on two separate and unrelated trips a few years apart. And my friend, Mike Ramsey, had been with me on both trips to this same county. He was with me and a friend late in November when we visited another friend in nearby Moreno Valley, and he was with me again in this early December trip, seeing a film at a Perris cinema and exploring the shopping centres while I was working.
This time, he went along to help me with the driving and to visit our mutual friends in the Hollywood motion picture industry once my environmental tasks were done at two industrial plants in this county and in nearby San Bernardino County. Moreno Valley and Perris are both in Riverside County, located in what is known as California’s “Inland Empire”.
These two cities are within the Perris Block in southern California in the USA. The Perris Block is known among geologists for three things: its modest gold deposits; several mountains, among them the Bernasconi Hills (Fig. 3); and its thermal springs.
The Bernasconi Hills surrounding the Lake Perris reservoir and most of the others are not especially high in elevation, averaging 670.5m above mean sea level, but they rise dramatically from a flat, uplifted peneplain, an area that can be said to be virtually featureless except for an occasional hill or mountain. (A peneplain is a level land surface produced by erosion over a long period, undisturbed by crustal movement.) The Perris Block and the city of Perris were named for Fred T Perris, the chief railroad engineer for the California Southern Railroad connecting the remote desert town of Barstow with San Diego.
The astoundingly fast-growing city of Perris was once small and little known outside Southern California. It was built in the late 1800s in response to the new depot. The town’s continued existence depended on the quantities of gold found in the hills, the hot springs, which created a tourist industry as people visited “baths” to partake of the hot mineral baths, and the fertility of the soil in this arid region. The Crescent Bath House in nearby Elsinore is an old bath house constructed in the 1880s and listed in the National Register of Historic Places (Fig. 4).
All but the Elsinore and House of Siloam Hot Springs baths are closed and out of business now, as American tastes changed and technologies for artificially heated hot tubs improved. However, natural thermal springs bathing still has its enthusiasts despite the hydrogen sulphide odour in the spring water here.
Moreno Valley is another phenomenally fast-growing city. As the costs of housing in Los Angeles continue to be higher than in nearby counties, many residents are moving to Riverside, approximately an hour’s drive from the metropolis. And the whole Perris and Moreno Valley area is sprouting shopping centres and new homes the way a garden sprouts cabbages.
The geology
The area is atop a great batholith with faults that are responsible for the thermal springs near Lake Perris (a man-made lake used as a reservoir) and nearby Lake Elsinore several kilometres to the west. Extensive faulting is unusual for granitic batholiths in eastern Canada and the eastern United States, but not for this one and others in the western part of the United States and western Canada. The Bernasconi Hills have a companion hill, which is unnamed. It resembles a volcanic cone (Fig. 5) but, like the Hills, is part of a mass of granite that was exposed due to erosion.
Granite needs little introduction. It is the most common rock of the continental crust. It is an intrusive igneous rock, meaning it is plutonic in origin and it cooled from magma very slowly, while buried beneath the surface. Granite is composed of a wide variety of minerals but always consists of the primary minerals quartz and orthoclase feldspar. The quartz and feldspar generally give the rock a light colour, ranging from pinkish to grey and white. The grains are large since granite, being an intrusive rock, cooled so slowly. Granite occurs in large areas called plutons, exposed at the surface as a result of significant erosion (Figs. 6 and 7).
The Perris Block is the central block of three large fault bounded blocks. It lies between the San Jacinto Block to the east and the Santa Ana Block to the west. The Perris Block is comprised of Cretaceous and older granitic rocks of the Southern California Batholith and metasedimentary basement rocks. To the north are the Cucamonga Fault Zone and the San Jose Hills Fault. To the south, the Perris Block is bounded by the San Felipe Fault Zone. Now the block is stable, but it went through crustal movement during the Pliocene and Pleistocene epochs. Deep isostatic flow in the aesthenosphere caused oscillation as the Los Angeles basin sank and the San Jacinto Mountains rose.
The block is comprised of monadnock-like hills (that is, isolated hills, ridges or erosion-resistant rock rising above a peneplain) and mountains dotting an uplifted peneplain with hills such as the Box Springs hills of Moreno Valley approximately 13km north of Perris, and the Bernasconi Hills. The Box Springs Hills are higher in elevation than the Bernasconi Hills, 914m at their highest elevation. The Perris Plain is drained by the San Jacinto River watershed. Below this basin, the Perris Plain is bounded to the south by the Plains of Leon and the mountains to the south.
In hot water
Hot and warm springs have been used for well over a century for secondary water sources and for resorts dedicated to bathing in the warm mineral laden waters that aid in relaxation. Most in the Perris area are now closed and, since they are on private property, off limits to visitors. The exceptions are Elsinore Hot Springs, located at the Lake Elsinore Hot Springs Motel at 316 North Main Street, Elsinore, and House of Siloam at 215 West Graham Avenue. Both are still open for drop in customers, as well as those who are staying at the motel. The Crescent Bath House is now a gift shop, located at 201 West Graham Avenue, 33.41 degrees N, 117.19 degrees W. Elsinore’s hot spring is “hot”, meaning it is 520C. It is located at 33.67 degrees N, 112.325 degrees W.
Lakeview Hot Springs is at Lake Perris and is “hot”, at a temperature of 380C. These springs are located at 33.837 degrees N, 117.140 degrees W. Eden Hot Springs, at a hot temperature of 430C, are located at 33.896 degrees N, 117.057 degrees W in hills east of Lake Perris. All the hot and warm springs listed here trend along a fault that trends southwest to northeast.
Hot springs circulate because of convection. Most rocks have faults and fractures, and some have more than others. When cold groundwater and magma heated water meet, convection begins and can continue for hundreds of years. The chemicals in the water of a spring are a factor of the rocks through which they circulate. In regions where granite is the predominant bedrock, silica is the mineral occurring in thermal springs waters.
Why are there more warm and hot springs in the western United States than in the eastern United States? East of the Mississippi they number fewer than twenty.
According to Gerald A Waring in his 1965 paper, revised in 1983, hot springs arise predominantly from igneous rock, these rocks being geologically younger than either sedimentary or metamorphic rocks, and those igneous rocks featuring thermal springs are found in the states of South Dakota, Wyoming, Colorado, California and Montana. Arkansas, also west of the Mississippi, boasts Hot Springs National Park.
Some arise from geologically older sedimentary strata and metamorphic rock. Such rocks can be found in the Atlantic Coastal Plain. Hot and warm springs are also common where rocks of any age have been faulted and intensely folded. There are warm springs in Georgia, Florida, North Carolina and Virginia. Two such springs in the state of Georgia are located in the town of Woodbury. “Hot Springs” in the Great Smoky Mountains of North Carolina are only 370C, cooler than most.
Climate, wildlife and holidaymakers
Lake Perris, 480m above sea level, is open to the public and has a beach surrounded by the Bernasconi Hills (Fig. 8).
In Southern California’s ‘rainy season’ lasting from November to late April, rain and sometimes snow provides much needed moisture. May through October are drier and warmer, and those are the months when beachgoers throng the shores. The Bernasconi Hills support a variety of wildflowers, reptiles, small mammals and birds. It is inadvisable to eat the fish one catches here due to high levels of polychlorinated biphenyls (PCBs) and mercury (see box below).
| A note on pollution |
|---|
| PCBs are reasonably likely to cause liver, brain and bladder cancer in human beings. PCB contamination at Lake Perris results from improper disposal or leaching of PCBs from paints and surface coatings of windowsills manufactured in the 1970s and before, and internal lubricants leaking from transformers installed prior to the mid 1980s when PCB containing lubricants were replaced with mineral oil. Mercury contamination arises from multiple sources, including air pollutants from burning coal as well as runoff and groundwater contamination from mines. Five or fewer skinless filet servings per week of certain fish such as sunfish and inland silversides from the lake are less harmful than servings of common carp which concentrate more contaminants in the meat. |
The other hiking location within the Perris Block, which my friends Mike Ramsey and David Hawk visited to hike with our friend Debra Marshall, was Box Springs Mountain Reserve. There had been a snowfall the night before (Fig. 9).
Box Springs Mountain Reserve is a biologically more diverse and less developed area than Lake Perris, and we saw a sign warning us about avoiding interactions with mountain lions that might occur along the trails (Fig. 10). We saw some suspiciously large tracks in the mud, but it was a false alarm, at least for that morning. A half hour later, we met a family coming down a hill with their Great Dane in tow. He had made the large prints we saw.
Box Springs Mountain Reserve is mostly open to the public, with a few environmentally sensitive areas off limits (Fig. 11). It is located at 9699 Box Springs Mountain Road, Moreno Valley, California 92557. USA.
About the author
Deborah Painter is an ecologist and general environmental scientist specialising in defence facility, commercial and industrial development planning to minimize deleterious environmental impacts. She lives in the United States.
References
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https://www.pe.com/2018/03/21/how-and-why-southern-californias-population-grew-so-much-in-one-year/
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