Category Archives: Guide

NPS National Historic Trails Site

A reader sent a link to this site and I thought it would be of interest to all of you. This site, produced by the National Park Service (NPS), has detailed maps showing the routing of various national historic trails. Of interest to California, it shows the National Old Trails Highway (US 66), El Camino Real (US 101 for the most part), Old Spanish Trail, and a few others. Come check it out!

National Historic Trails – NPS

Sightseeing Faults in the Los Angeles Area

Southern California has many faults, some hidden, some very obvious. One of the more fun aspects of geology is to see where the land moves. To witness the power of what Earth can do can be quite awe inspiring. Some features are easily recognizable, such as the San Gabriel Mountains which have somewhat rapidly risen due to faulting. Other features are less apparent in the modern urban environment, such as “setbacks” along major fault zones. This page will showcase some of the more visible fault features throughout Southern California.

NOTICE: Some of these locations are either along major roadways or in residential areas. Please use caution when visiting these sites and be respectful of those living there. Parking regulations vary at each site so be aware before you park.

Maps used are courtesy Google Earth and USGS Quaternary Faults layer – adapted by Michael F Ballard.

Recent faulting and Fault Scarps

San Fernando Valley

On February 9, 1971, an M6.6 earthquake struck early in the morning in the northwestern San Fernando Valley. This quake, known as the 1971 Sylmar / San Fernando Earthquake caused a great deal of damage from collapsed buildings, pipeline breaks, downed powerlines, and collapsed freeways. The damage was gradually repaired but some evidence remains today.

1971 Earthquake Scarp Location
Deflected curb along the south side of Cometa Ave.
Abrupt rise from the curb to the yard is evidence of the fault scarp from 1971.

A fault scarp along with a deflected curb can be seen quite readily on Cometa Ave, just east of Fernmont St. The front yard at 12676 Cometa Ave abruptly rises from street level at an angle. Looking west from this location, you can see a slight bend in the curb on the south side, another reminder of the 1971 quake.

Imperial Valley Scarps and Offsets

Numerous earthquakes in the Imperial Valley have produced a significant amount of surface rupture. Evidence of these ruptures can still be seen today in many parts of the valley. Two areas of interest can be visited. One in the western end of the valley and another southeast of Brawley.

Western Imperial Valley

Western Imperial Valley – Imler Road site

Earthquakes in 1968 and 1987 produced a small amount of surface rupture along the Superstition Hills Fault Zone, which is an extension of the Elsinore Fault Zone. This is visible today as a low “mole track” which follows the fault line and cracks in the roadway on Imler Road where it makes a big bend at Huff Road. This track can be followed for many miles across the desert but be cautious as the area surrounding the roadway is a military bombing area.

Imler Road Mole Track, looking south

Southern Imperial Valley

Southern Imperial Valley Fault trace

On May 18, 1940, an Mw 6.9 earthquake struck in the Imperial Valley along the Imperial Fault. The quake caused a lot of damage to buildings and infrastructure. It produced a significant amount of surface rupture as well, which offset canals and roadways. Additional events in 1966 and 1979 increased the offset. The fault zone can be followed quite easily today by tracking the broken and offset pavement as well as shifted canals.

McCabe Road canal shift
McCabe Road fractures
Chick Road western canal shift
Chick Road eastern canal shift
Roadway damage on Chick Road
I-8 between Meloland Road and Barbara Worth Road prior to pavement replacement. Offset can clearly be seen in the distance along with newer asphalt paving over the concrete.
Cracking as a result of the various earthquakes along the Imperial Fault at old US 80. This section has since been resurfaced but the telltale cracks are still visible.
En-Echelon cracks forming along Old State Highway 111 just north of Aten Road (near Imperial Valley College)

Pre-Holocene Events

Older seismic events in Southern California also leave their marks. A fault scarp along the Sierra Madre Fault is visible in the foothills above Etiwanda.

Etiwanda Area Scarps

At the northern end of Etiwanda Avenue and a short hike later, you can walk along the scarp of the Sierra Madre Fault. This section hasn’t broken in a long time but is still quite active, as the freshness of the scarp suggests. This can be traced along the foothills of the San Gabriel Mountains for many miles where it hasn’t been erased by development or erosion.

Out in Rancho Cucamonga inspecting the Sierra Madre Fault.

Of course, no fault tour of Southern California is complete without the San Andreas Fault. The fault has numerous and visible features.

Near the San Andreas Fault crossing at Ave S.

San Andreas Fault Tour

Alquist-Priolo Zoning Act

San Gabriel Fault Zone through central Santa Clarita

Not all faults need to produce recent surface rupture to be considered a problem. In 1972, the Alquist-Priolo Zoning Act was passed by the California legislature which required setbacks and additional building requirements surrounding fault zones. How wide a zone was dependent on how large an earthquake could be expected from that fault. Any fault that was capable of causing surface rupture got special attention. An example of these setbacks can be seen in Santa Clarita along Newhall Ranch Road between Hillsborough Pkwy and Dickason Dr / Ave Tibbets. A large shopping center called Bridgeport Marketplace was constructed atop the San Gabriel Fault Zone. A large setback was required for construction, which can be seen by the very wide separation of structures and the more unusual location of the parking lot. Normally such centers have the parking adjacent to the roadway. In this case, it is behind the structures.

Additional Resources

Finding Old Highways Presentation

On Tuesday, May 4, 2021, I hosted a Zoom presentation for the Historic Highway 99 Association of California on the topic of finding old highways. This presentation included information about survey monuments, pavement styles, bridge types, and more. A recording of the presentation is now available for viewing and download. Please follow the link below to view.

https://zoom.us/rec/share/n7u0NOlbPcCFBoiSPEj4umnsj2V7bVEqnFeiZnlqGOe-XZ4wLoAtk12tNRJpeckm.EEZtms4r3PG3DY5e?startTime=1620177032000

There will be an additional presentation in the near future to cover some additional topics that weren’t covered and to have a bit more detail in others.

Guide to Truss Bridges

The Historic American Engineering Record, a part of the National Park Service, produced a very detailed poster for identification of truss bridges. While these bridges aren’t as common in southern California, they still get plenty of use. Railroads are the most common user of these types of bridges, specifically the Pratt Truss.

Highway Tips #2 – Signs

Road signs come in many different sizes, shapes, and colors. Each one carries a specific meaning to help guide you down the road in a safe manner. This installment will discuss each of the types of signs and what they mean. Road signs come in a variety of colors. Each color has a meaning, which can be quite important to know. The colors run from red, yellow, white, blue, green, brown, and orange.

Old stop sign along Topanga Canyon Blvd in Los Angeles, CA.

Red signs mean something is prohibited. Only a few signs use this color – stop signs and wrong-way signs are the most common of these. Stop signs are also the only one that is octagonal. The color red was not the first choice, however. Stop signs used to be yellow with black lettering. Color might depend on where you were. Eventually, stop signs became standardized with a red background and white lettering. Yield signs and some no parking signs also use this color scheme. While yield signs don’t prohibit movement, they are there to warn you of opposing or side traffic.

High wind advisory sign along State 41 in central California/

Yellow signs are advisory or warning. Things such as upcoming intersections, curves, and other roadway hazards are generally marked with yellow signs. Curves are a good example of this. The speed signs at curves are advising you of a safe speed for the curve. They aren’t a speed limit for the curve. It is a generally good idea to travel at the advised speed. Depending on many conditions, you may be able to go faster that the posted speed. With this in mind, I have found that 25 mph seems to be a breaking point for curves. Curves posted at 25 mph or higher can generally be driven faster. When they are posted lower than 25 mph, slowing down is highly advised. I’ve seen speeds as low as 5 mph posted on some roadways. I’ve also seen speeds as high as 60 mph. A curve with that high a speed is not a problem, but may be in an area with few curves and it is nice to know you don’t have to slow for it. Upcoming intersections are also posted with yellow signs. These signs tell you not that one is coming up but what type it is. Some signs are customized to show specific intersections, especially if it is a blind intersection in a curve.

Signage heading southbound toward Solemint Junction. Hwy 14 used to turn here to head east on Soledad Canyon Road from 1963-1972.

Black and white signs are regulatory signs, meaning they show some sort of rule that must be obeyed. Speed limit signs, for example, use this scheme. This is different than the curve advisory signs, which also show speed, as these signs show the maximum limit, not the advised limit. Parking signs are also generally in black and white as they limit or restrict parking in an area. Other signs in this category include bike lanes, passing, and other signs restricting either the movement or type of vehicle allowed in an area or lane.

Next services sign in Utah, which are located in Hinckley.

Blue signs are very specific in use. They act as guides to hospitals, telephones, and where services can be obtained. Roadside services can include gas stations, food, and lodging. These signs can be quite helpful for roadway users that are unfamiliar with the area and are not sure what sort of services can be obtained at a particular exit. Use of these signs varies by state as well. Some states show specific businesses, where others are more general, e.g. “Gas, Food, Lodging – Next Exit”.

Signage at a roadway junction.

Green signs are also guide signs. These give directions, by use of an arrow, to locations such as cities and roadways. They also give mileage to locations along a route. On freeways they generally show distances to the next exit, where others show distance to the next three cities. They may also mark things such as bike routes, which are roadways where there is no marked lane specific for bicycles but may be easier for bicycling in general.

Historic US 6 signage at Acton Junction.

There are also brown signs which mark recreation and historic area, such as beaches and parks. Park lands, such as national monuments, are marked with brown signs as well as forest areas. Beaches, such as the various State Beaches along the coast are marked with these brown signs as well.

Another view of the HOV bridge with a new exit sign below over I-5 north.

The last signs covered here are orange. These are for construction areas. These signs perform a wide variety of tasks but always mark a construction area. These are to be heeded in part to ensure the safety of construction workers. Detour signs also use this color scheme and mark an alternate route in case the main route is closed. Always use caution in areas marked with orange signs. In addition to construction workers, roadway quality may greatly vary as well. Well-paved roads may be turned to dirt for a time during the construction and speeds may be greatly reduced from normal.

As you can see, road signs come on many shapes and colors. Each help roadway users get to their destination in all sorts of ways. So, next time you’re out on the road, keep this in mind and it may help you on your journey to wherever you may be going.