Month: May 2026

Interesting! I have not been there, but I have heard it described as gold mineralization localized in one of the low-angle thrust fault zones that extend south from the San Andreas Fault along the north side of the San Gabriels. Those faults help accommodate the compression created where the San Andreas bends more westerly north of the Coachella Valley, before resuming its more northwesterly course near the west end of the San Gabriels.

Could you see any visible geologic evidence of that on the ground? Fault gouge, sheared rock, quartz veining, altered zones, breccia, or repeated/tilted beds would all be interesting. Envious that you got there!

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Here is a more elucidated version:

Interesting. I have not been there, but the description makes geological sense. The gold mineralization has been described as localized within one of the low-angle thrust or reverse-fault zones that extend southward from the San Andreas Fault along the north side of the San Gabriel Mountains.

That setting is important because this part of the San Andreas is not a simple, straight strike-slip boundary. North of the Coachella Valley, the fault bends into a more westerly trend. That bend creates transpression: strike-slip motion combined with compression. The crust has to shorten and thicken, and some of that strain is taken up by reverse and thrust faults along the northern San Gabriels. Farther west, near the west end of the range, the San Andreas resumes its more typical northwest trend.

In such a compressional fault environment, mineralization can become focused along zones of weakness. Fault planes, fractured rock, breccia zones, and sheared contacts can all act as pathways for hydrothermal fluids. If those fluids carried silica, sulfides, and metals, they could deposit quartz veins and gold where pressure, temperature, chemistry, or permeability changed.

So I would be curious whether there was visible evidence of the structure on the ground: sheared or crushed rock, fault gouge, breccia, slickensides, repeated rock units, tilted beds, iron staining, quartz veining, altered zones, or a distinct contact where one rock package has been pushed over another. Even if the gold itself was not visible, the structural setting might be.

Envious that you got there. That is exactly the kind of place where the landscape may tell a larger tectonic story if you know what to look for.

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Interesting. I have not been there, but the description makes geologic sense, and it opens up a much larger story than just “gold in a fault zone.”

The gold mineralization has been described as localized within one of the low-angle thrust or reverse-fault zones that extend southward from the San Andreas Fault along the north side of the San Gabriel Mountains. That is an important detail. A low-angle fault zone is not just a crack in the rock; it is a broad zone of broken, sheared, crushed, and chemically altered rock where movement has taken place over time. In a compressional setting, one block of crust is being driven up and over another. Those zones can become major pathways for mineral-bearing fluids.

This part of the San Andreas is not a simple, straight, clean strike-slip fault. Farther south and east, the San Andreas trends northwest-southeast in the classic right-lateral fashion. But north of the Coachella Valley, the fault bends into a more westerly orientation. That bend matters enormously. When a right-lateral strike-slip fault bends the “wrong” way, the two sides do not simply slide past each other smoothly. They jam, shorten, and thicken the crust. Geologists call that transpression: a combination of lateral shear and compression.

That compressional zone extends along the northern front of the San Gabriel Mountains. The mountains themselves are partly a product of that tectonic crowding. The crust is squeezed, folded, broken, and shoved upward. Some of the strain stays on the main trace of the San Andreas, but much of it is transferred into adjacent reverse faults, thrust faults, oblique-slip faults, folds, and fractured belts. In other words, the San Andreas does not act alone. It produces a whole structural neighborhood.

The low-angle thrust zones extending south from the San Andreas are part of that neighborhood. They help accommodate shortening across the range front. In plain terms, the crust cannot slide smoothly through the bend, so it stacks, uplifts, and fractures. That creates the conditions in which fault-related mineralization can occur.

Gold mineralization in such a setting is usually not random. Hydrothermal fluids move through the most permeable pathways available. A fault zone provides exactly that: broken rock, fractures, crushed breccia, shear planes, and repeated openings caused by movement. Fluids rising through these pathways may carry silica, iron, sulfur, carbonate, and small amounts of precious metals. When conditions change–pressure drops, temperature changes, chemistry shifts, fluids mix, or the pathway narrows–minerals precipitate. Quartz veins, iron oxides, sulfides, clay alteration, and sometimes gold may form in and near the fault zone.

That is why I would be especially interested in what was visible on the ground. The gold itself might not be obvious, and in many cases it would not be visible at all. But the structural and alteration evidence might be. I would look for a zone of rock that appears more shattered, softened, stained, or disrupted than the surrounding material. Fault gouge would be one clue: clay-rich, powdery, crushed material produced by grinding along the fault. Fault breccia would be another: angular fragments of older rock cemented or packed together within the fault zone. Sheared rock, where minerals or rock fabrics are smeared out into streaks or bands, would also be significant.

Quartz veining would be especially interesting. In a mineralized fault zone, quartz may occur as thin stringers, irregular veinlets, thicker veins, or stockwork networks. The veins might be white, gray, rusty, milky, or iron-stained. Rusty red, brown, orange, or yellow staining can indicate oxidation of iron-bearing minerals such as pyrite. That does not prove gold, but it is often associated with hydrothermal alteration. Greenish, whitish, or clay-rich alteration halos around fractures could also be meaningful.

A true thrust relationship might show itself in larger structural ways. You might see one rock type lying structurally above another in a way that does not match normal stratigraphic order. There may be repeated units, abrupt contacts, crushed zones along the contact, or beds and foliations tilted at unexpected angles. In metamorphic or plutonic terrain, the evidence may be less like neat layered beds and more like zones of mylonite, cataclasite, breccia, altered granitic rock, or sheared gneiss.

Slickensides would be another prize: polished or striated fault surfaces showing the direction of movement. On a thrust or reverse fault, the lineations may suggest upward movement of one block over another, though in this region movement could be oblique because the San Andreas system combines strike-slip and compression. That obliquity is part of what makes the San Gabriel block so structurally interesting.

The broader tectonic setting is the key. From the San Gorgonio Pass / Coachella Valley region westward along the northern San Gabriels, the San Andreas enters a restraining-bend geometry. The motion between the Pacific and North American plates is still dominantly right-lateral, but the bend forces compression into the system. That compression is relieved through uplift of the San Bernardino and San Gabriel Mountains, thrusting along range-front and internal faults, folding of sedimentary basins, and distributed deformation away from the main fault.

At the west end of the San Gabriels, where the San Andreas resumes a more northwesterly course, the geometry changes again. The compressional intensity and structural style are not uniform along the entire belt. Some areas are dominated by uplift and thrusting; others by oblique-slip deformation, strike-slip transfer, folding, or basin development. That variability could help explain why mineralization is localized in one fault zone rather than spread evenly everywhere. The right combination of structure, permeability, host rock chemistry, fluid source, and timing has to occur.

So the place you mention may be interesting for two reasons at once. Locally, it may preserve a mineralized fault zone where gold-bearing fluids used a low-angle thrust structure as a conduit. Regionally, it may be a small exposure of the larger tectonic machinery that built and continues to deform the northern San Gabriel Mountains. The mine or prospect is the local expression; the restraining bend of the San Andreas is the regional engine.

What I would want to know from someone who had been there is whether the mineralization looked structurally controlled in the field. Was the quartz confined to a shear zone? Did the veins parallel the fault fabric, or cut across it? Was the country rock pervasively altered, or only fractured locally? Was there a visible low-angle contact? Did the workings follow a seam, a vein, a breccia body, or a crushed zone? Were there iron-stained ribs or resistant quartz ledges standing out from softer fault gouge? Were the dumps full of milky quartz, rusty quartz, altered granite, schist, or breccia?

Those details would help distinguish several possibilities. It could be gold in discrete quartz veins occupying fractures related to thrusting. It could be disseminated or veinlet-controlled mineralization in a broader altered shear zone. It could be mineralization along a contact between contrasting rock units. Or it could be older mineralization later caught up in San Andreas-related deformation. The field relationships would matter.

In a place like that, the visible clues are often subtle. A casual visitor might see only broken rock, brush, mine dumps, and a scar in the hillside. But a geologist would ask: why here? Why did the miners follow this exact line? Why is the rock crushed here and not there? Why does the staining follow a zone? Why do the veins have this orientation? Why does the ridge or drainage line up with the structure? Those questions are often more revealing than the mine opening itself.

That is why I said I would be envious. It sounds like the kind of locality where a small mining feature might sit directly on top of a much larger tectonic story: the San Andreas bend, compression along the northern San Gabriels, low-angle faulting, fractured rock, hydrothermal fluid movement, and localized gold deposition all intersecting in one place.

outline

I. Main point

The locality is interesting because the gold occurrence may not be isolated or random. It may be tied to a larger structural system associated with compression along the San Andreas Fault and the north side of the San Gabriel Mountains.

II. Regional tectonic setting

A. The San Andreas Fault is not straight in this region.

B. North of the Coachella Valley, the fault bends into a more westerly trend.

C. This bend creates a restraining-bend geometry.

D. In a right-lateral fault system, that geometry produces compression as well as lateral slip.

E. The resulting deformation is called transpression: strike-slip motion plus shortening.

III. Effect on the San Gabriel Mountains

A. The northern San Gabriels occupy part of this compressional zone.

B. The crust is shortened, thickened, fractured, and uplifted.

C. The mountains are partly a result of this tectonic crowding.

D. The San Andreas does not carry all the deformation by itself.

E. Strain is distributed into nearby structures, including:

1. Reverse faults
2. Thrust faults
3. Oblique-slip faults
4. Folds
5. Shear zones
6. Fractured belts

IV. Low-angle thrust faults

A. Some fault zones extend southward from the San Andreas along the north side of the San Gabriels.

B. These may be low-angle thrust or reverse-fault zones.

C. In such structures, one block of crust is pushed up and over another.

D. These faults help relieve or accommodate compression from the San Andreas restraining bend.

E. They form broad zones of crushed, sheared, fractured, and altered rock.

V. Relationship to gold mineralization

A. Gold mineralization may be localized within one of these low-angle fault zones.

B. Fault zones are natural conduits for hydrothermal fluids.

C. Mineral-bearing fluids can move through:

1. Fractures
2. Breccia zones
3. Shear planes
4. Crushed rock
5. Reopened fault surfaces

D. Gold and associated minerals may precipitate where conditions change, including:

1. Pressure drops
2. Temperature changes
3. Chemical shifts
4. Fluid mixing
5. Changes in permeability
6. Narrowing or sealing of fluid pathways

VI. Possible mineral indicators

A. Visible gold may not be present.

B. More likely clues would include:

1. Quartz veins
2. Quartz stringers
3. Stockwork veinlets
4. Iron staining
5. Rusty red, brown, orange, or yellow oxidation
6. Sulfide remnants or boxwork textures
7. Clay alteration
8. Silicified rock
9. Altered granitic, metamorphic, or sedimentary host rock

VII. Structural evidence to look for

A. Fault gouge: soft, clay-rich, crushed material along a fault.

B. Fault breccia: angular broken rock fragments in a fault zone.

C. Sheared rock: stretched, smeared, or foliated material produced by movement.

D. Slickensides: polished or striated fault surfaces showing movement direction.

E. Repeated rock units: the same unit appearing more than once because of thrusting.

F. Abrupt contacts: sudden boundaries between rock types.

G. Low-angle contacts: shallowly dipping fault surfaces.

H. Tilted or disrupted bedding or foliation.

I. Linear ridges, saddles, gullies, or drainage alignments following the fault zone.

VIII. Field questions for someone who visited

A. Was the mineralization clearly confined to a shear or fault zone?

B. Did the workings follow a vein, seam, breccia body, or crushed zone?

C. Was there a visible low-angle fault contact?

D. Were quartz veins parallel to the fault fabric, or did they cut across it?

E. Was there iron-stained quartz on the dump?

F. Was the country rock altered broadly or only along fractures?

G. Were there slickensides, gouge, breccia, or sheared rock?

H. Did the mine follow a structurally obvious line?

IX. Alternative interpretations

A. Gold may have formed directly in the thrust-related hydrothermal system.

B. Gold may occupy fractures caused by compression.

C. Gold may be related to an older mineralizing event later deformed by San Andreas-related faulting.

D. Gold may be localized along a contact between contrasting rock units.

E. Gold may occur in discrete veins rather than a broad mineralized fault zone.

X. Larger significance

A. The site may be a local mining feature.

B. It may also expose part of the broader tectonic machinery of Southern California.

C. The mineralization, faulting, uplift, and San Andreas restraining bend may all be connected.

D. The key question is whether visible field evidence supports that structural interpretation.

XI. Closing sentiment

A. The locality sounds worth seeing because it may combine mine history, structural geology, mineralization, and regional tectonics in one place.

B. The most interesting question to ask on the ground is: why did mineralization happen there, along that exact zone?

if I were to return to the site (Big Horn Mine, Angeles National Forest) what would be the top three pieces of evidence to look for?

I will ground this in the Big Horn Mine setting rather than speak only in generalities, and I will separate geologic clues from mine-safety/access issues.Thought for 15s

For Big Horn Mine specifically, I would make the top three targets these:

1. The Vincent Thrust contact itself

The main thing to look for is not just “a vein,” but evidence that the mine sits in the Vincent Thrust zone. The published mine description places Big Horn along the trace of the Vincent Thrust, with mineralization in a northeast-striking, low-angle zone dipping roughly 15 to 30 degrees northwest. It also describes the thrust as the contact between upper-plate San Gabriel Gneiss and lower-plate Pelona Schist. That is your number-one field question: can you see the structural contact, or at least the contrast between the rocks on either side? USGS/MRDS summaries describe the gold as occurring in a structurally favorable position along the Vincent Thrust zone.

In practical terms, look for a low-angle, laterally persistent zone rather than a steep, narrow fissure. Look for a change from more massive gneissic or amphibolitic rock to more foliated schistose rock. The Pelona Schist near the mine is described as well-foliated, medium- to dark-gray or black schist; the host interval between thrust strands is described as dominated by crushed amphibolite, an important host for the gold mineralization.

2. Crushed, sheared, gougy, or brecciated rock in the fault zone

The second thing to look for is mechanical damage: the ground preparation that made the rock permeable. The deposit description says favorable preparation included crushing and increased permeability within the Vincent Thrust zone, with impermeable gouge along upper and lower thrust surfaces helping confine the fluids. That is a very useful clue because it tells you the ore was not merely “near” a fault; it depended on the fault zone’s internal architecture.

On the ground, I would look for angular breccia fragments, crumbly clay-rich gouge, slick or polished shear surfaces, smeared foliations, shattered amphibolite, and zones where the rock changes from competent to rotten over a short distance. A mine dump may be as useful as an outcrop: pieces of crushed amphibolite, gneiss, schist, gouge, or veined breccia would tell you what the miners were actually following underground.

3. Quartz-pyrite-gold and calcite-pyrite-gold veinlets, especially where they follow fractured rock

The third thing is the mineralization style. The USGS resource study summarizes the district’s gold as being associated with small amounts of sulfides and occurring in quartz in fracture zones. The Big Horn mine description is more specific: it describes quartz-pyrite-gold and calcite-pyrite-gold veinlets formed from hydrothermal fluids.

In the field, look for white to gray quartz veinlets, calcite veinlets, rusty iron staining after pyrite, boxwork textures where sulfides weathered out, and quartz or calcite concentrated along fractures rather than randomly distributed. The key observation is orientation: do the veinlets parallel the thrust fabric, occupy sheared fractures, or occur between strands of the fault zone? If the veins and alteration are confined to the crushed thrust interval, that supports the structural-control interpretation.

My field hierarchy would be: first find the structural zone, then the damaged rock, and then check whether quartz/calcite/sulfide alteration is localized within that damaged zone. That sequence would give you the strongest evidence that the gold mineralization is tied to the Vincent Thrust rather than merely occurring somewhere near an old mine.

One caution: observe from outside. Current hiking guidance warns against entering the mine and to be careful around old boards and metal; the route is also seasonal because CA-2 can close in winter.

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Working source set:

Caballeria, History of San Bernardino Valley: From the Padres to the Pioneers, 1810-1851 – mission-era foundation, Guachama, Politana, San Bernardino naming, La Placita, Agua Mansa, and transition to Mormon settlement.

Ingersoll, Century Annals of San Bernardino County, 1769 to 1904 – detailed San Bernardino County annals, Mormon colony, county formation, pioneer memory, illustrations, local biographies, railroad/citrus/irrigation development, and the 1904 historical frame.

Brown and Boyd, History of San Bernardino and Riverside Counties, Vol. I – broader county synthesis, San Bernardino and Riverside institutional history, towns, agriculture, irrigation, transportation, mining, education, courts, and later regional development.

Master timeline using all three files:

1542 – Cabrillo reaches San Diego Bay, beginning the Spanish coastal frame that Caballeria uses as background for San Bernardino Valley history. Caballeria’s table of contents begins with Cabrillo and Viscaino, then moves to the missions and San Bernardino Valley proper.

1602-1603 – Viscaino surveys the California coast and reinforces the Spanish naming pattern that later appears inland in mission-era place names.

1769 – Spanish occupation of Alta California begins under the Portola-Serra expedition. Ingersoll’s title frame begins San Bernardino County history at 1769, while Brown/Boyd and Caballeria both treat the mission system as the institutional background for the valley.

1770 – Mission San Carlos Borromeo is founded at Monterey, after which the Spanish occupation of California was considered complete in the mission narrative.

1771 – Mission San Gabriel Arcangel is founded. This becomes the parent mission for the San Bernardino Valley activity. Caballeria’s structure moves from San Gabriel directly toward Politana and the valley mission outposts.

1774-1776 – The inland Anza route makes the San Bernardino Valley part of the practical travel corridor between the Colorado River, the San Gabriel, and coastal California.

1810 – Politana is established at or near Guachama as the first Christian settlement in the San Bernardino Valley. Ingersoll places “Mission Settlements in San Bernardino County,” “Politana,” and “San Bernardino Mission Station” in his Spanish-era chapter.

1812 – Earthquakes and Native resistance disrupt the first Politana settlement. This marks the first failure of the San Gabriel mission foothold in the valley.

1820s – Mission activity resumes in the San Bernardino Valley, leading to the establishment of the San Bernardino mission station, agriculture, stock raising, and zanja irrigation works.

1830s – Secularization breaks the mission system and shifts land, labor, water, buildings, and herds toward Mexican civil and rancho control. Caballeria treats secularization, abandonment, land titles, and Mexican grants as the hinge between mission and rancho history.

1842 – Rancho San Bernardino is granted to Antonio Maria Lugo and his sons. Ingersoll also notes that the Lugos offered lands near Politana to New Mexican colonists.

July 4, 1842 – Daniel Sexton raises the American flag in San Gorgonio Pass, an early symbolic American act before formal U.S. control.

1843 – Lorenzo Trujillo and others settle at Agua Mansa, according to Ingersoll’s annals.

1845 – A second party of colonists under Jose Tomas Salazar removes from La Politana and founds Agua Mansa. Caballeria says Agua Mansa means “gentle water,” names Ignacio Moya as the first alcalde, and says Louis Rubidoux later succeeded him.

1846 – The Battle of Chino occurred during the Mexican-American War. Ingersoll places it in the Mexican-era chapter alongside Rancho Santa Ana del Chino, the San Bernardino Grant, Indian troubles, and the rancho order.

April 12, 1847 – A detachment of the Mormon Battalion is sent to establish a military post at Cajon Pass.

April 1848 – A Mormon Battalion party passes through Cajon Pass with a wagon; Ingersoll calls it the first wagon to cross that route.

1849 – Gold Rush-era movement passes through the inland route, but Caballeria notes that the San Bernardino Valley itself remained distant from the earliest gold excitement, with Mexican settlers continuing their pastoral life.

June 11, 1851 – The first Mormon party reaches Cajon Pass.

September 1851 – The Mormons purchase the San Bernardino grant. This is the hinge from Mexican rancho occupation to organized American colonization.

1852 – The Old Fort is erected; the Mormons build a grist mill and a road up Twin Creek Canyon.

1851-1852 – The Little Church of Agua Mansa is built and dedicated to San Salvador; Caballeria says its parish records preserved marriages, births, and deaths.

April 26, 1853 – San Bernardino County is separated from Los Angeles County. The townsite of San Bernardino was laid out the same year, and the Mormon Council House was erected.

April 13, 1854 – The City of San Bernardino is incorporated. The first stage of service between San Bernardino and Los Angeles began the same year.

1855 – Volunteers under Captain Andrew Lytle enter the desert in pursuit of Native raiders, highlighting continued frontier insecurity after county formation.

1856 – Conflict grows between Mormons and Independents.

1857 – The Mormon recall begins, effectively closing the Mormon colony phase. Ingersoll’s contents treat “The Recall” as part of the Mormon-era chapter.

1858 – The first Union Sunday School and first May Day picnic are recorded; the Butterfield stage route is established. Ingersoll marks this as the start of the “Between Period–1858-1875.”

1859 – The Ainsworth-Gentry fight occurs, one of the best-known local violence episodes of the post-Mormon years. Brown/Boyd describe this period as one in which Mormon-Gentile tensions, mining-camp lawlessness, Native raiding, and Civil War sentiment converged.

1860 – Gold is discovered in Bear and Holcomb valleys; the San Bernardino Herald appears as the first newspaper in the county.

1861 – A toll road through Cajon Pass is established; Camp Carleton is established on the Santa Ana River; C. W. Piercey is killed in a duel near San Rafael.

January 22, 1862 – The great flood destroys Agua Mansa. Caballeria says rain continued for fifteen days and nights, the Santa Ana became a raging torrent, and the village was washed away except for the church and Cornelius Jensen’s house. Ingersoll summarizes the event as “Agua Mansa swept away.”

1862 – The first county educational convention is held, and the first orange grove, four acres, is set out at Old San Bernardino.

1862-1868 – Flood memory becomes part of the region’s historical identity. Brown/Boyd preserve Mrs. Crafts’ account of the 1861-62 rains, describing families fleeing to higher ground, adobe houses melting, and neighbors sheltering one another.

1875-1890 – Ingersoll’s “Progression” period: agriculture, horticulture, city growth, transportation, and the boom era reshape the county.

1880s – Citrus, irrigation, railroad development, and new towns transform San Bernardino County from a former mission-rancho-frontier district into a modern agricultural and town-building region.

1890s – County consolidation continues through public buildings, horticultural organizations, transportation networks, electric power, and institutional development.

1904-1905 – Ingersoll’s Century Annals of San Bernardino County, 1769 to 1904, is prepared as a permanent county history and biographical record; one biographical note says Ingersoll began gathering data in 1898 and published after delays.

1922 – Brown and Boyd’s History of San Bernardino and Riverside Counties offers a later retrospective synthesis, folding Caballeria and Ingersoll into a broader county- and regional-history.

The combined structure is:

Spanish and mission foundation, 1542-1830s: coastal discovery, San Gabriel, Politana, San Bernardino mission station, zanja, stock, and agriculture.

Mexican rancho and New Mexican settlement, 1830s-1851: secularization, Lugo grant, La Politana, La Placita, Agua Mansa, stock protection, and Battle of Chino.

Mormon colony and county formation, 1851-1857: purchase of Rancho San Bernardino, Old Fort, townsite, county separation, incorporation, roads, mills, and the Mormon recall.

Between periods, 1858-1875: post-Mormon instability, flooding, mining, lawlessness, Civil War tensions, schools, stages, and slow recovery.

Progression and boom, 1875-1890: railroad, irrigation, citrus, Redlands, Ontario, Chino, Highland, and speculative expansion.

Modern county memory, 1890-1922: public institutions, electric power, forest reserve, county division, horticulture, biography, and the writing of formal local history.