Saturday, April 15, 2017

St. Francis Dam Disaster, Part 1: Before and After

The March 12, 1928 failure of the "Saint Francis Dam" in northern Los Angeles County in Southern California was one of the worst "natural" disasters in U.S. history and resulted in the second greatest loss of life (about 450 lives). Only the 1906 San Francisco Earthquake and its associated fire claimed more lives.

My coverage of the St. Francis Dam is "broken" into two parts.  This first part concerns the physical aspects of the dam, and the second part (the next post) concerns the geology of the site.



This photograph (from Wikipedia.com) shows the completed dam in 1928, which was built in 1924 through 1926. The view is to the northeast. The dam was 185 ft. high (56 m). The length of the main dam was 700 ft. (210 m), and the length of the wing, on the west side of the dam, was 588 ft. (179 m). The reservoir was 3 mi. long, and the maximum water depth was 182 ft (55 m). The reservoir was at full capacity (38,000 acre-feet), just before the dam collapsed.


This photograph, upstream view, (also from Wikipedia.com) shows the remnants of the dam after its collapse on March 12, 1928. Only the central part and most of the wing dike (on the west side of the dam) remained intact. This dam, which was part of the Los Angeles Aqueduct system, collapsed just before midnight. The wall of water was initially 125 feet high, and its initial velocity was 18 mph. The 12 billion gallons of water roared down the Santa Clara River Valley to the city of Ventura, which is as the coast. The water traveled a total of 55 miles, and it took 5.5 hours to reach the ocean. The debris was a deadly mixture of mud, barbed wire, wood, etc. Nearly all the victims never knew what hit them.

All the subsequent photographs (in both of the posts) were taken by me in January, 2011 and in April, 2017.




This is all that is remains today of the main part of the dam. Although the central part did not collapse during the failure of the dam (see first picture), it was eventually dynamited in order to prevent climbers from attempting to climb it and possibly fall (i.e., liability issues).


Remnants of the "steps"which were on the downstream part of the face of the dam.


Rebar sticking out of large chunks of the concrete rubble. The staff is 1.5 m (in 10 cm increments). The flat surfaces were once part of the "steps," mentioned above.


Other sides of the same blocks shown in previous slide.


Piece of cable which was added to the concrete mixture. The concrete consisted of large angular chunks of cobbles and boulders of local rock types, including the Pelona Schist (e.g., gray angular piece in lower left side of picture, next to the cable).


This is the floodplain about 0.25 mi. downstream from the dam site. As you can see, there are building-size chunks of the dam that were broken up and transported by the wall of water suddenly being released from the reservoir behind the failed dam. Most of these chunks, some weighing as much as 10,000 tons came from the western side of the dam. Most of the eastern side collapsed in place and remained near the dam. Some of the transported chunks moved as far as 3,000 ft. downstream. For scale: Notice the cars on new highway along the left side of picture.



San Francisquito Creek once again flows freely, right through the middle of the collapsed dam.


Friday, March 31, 2017

San Andreas Fault Displacement of a Distinctive Granite

The San Andreas Fault is one of the most famous geologic features of the world. It is common knowledge that the sides of the fault, relative to each other, are moving. This post concerns a distinctive granite that has been displaced approximately 160 km (100 mi.) by this fault.
Map showing locations (in red) of the distinctive granite that has been offset by the San Andreas Fault in Southern California. The arrows show the relative sense of offset along the sides of the fault. Artwork is by R. Squires.
The rock that has been displaced is, indeed, a type of the igneous rock granite. To be more precise, however, it is a megaporphyritic monzogranite. "Megaporphyritic" means that the crystals are of vastly different sizes. A monzogranite is the commonest type of granite with roughly equivalent amounts of orthoclase and plagioclase, along with abundant biotite.


Megaporphyritic monzogranite hand specimen (20 cm length) from Mill Creek Canyon. The large pink crystals are orthoclase, the white ones are plagioclase, and the black ones are biotite. 


The orthoclase crystal just right of the center of the above picture show twinning. Crystal twinning occurs when two separate crystals of the same substance share some of the same crystal lattice. Instead of a normal single crystal, the crystalline structure appears doubled. There are several minerals that commonly have twinned crystals, and orthoclase is one of them.
An exceptionally large crystal (4.5 cm length) of orthoclase from the Mill Creek Canyon locale.
The rocks, which are found in the Liebre Mountain area in northern Los Angeles County and also in the Mill Creek Canyon area of San Bernardino County, are believed to have been part of the same intrusive magma body that crystallized during the Triassic, about 215 million years ago. The two rock masses, which were both derived from melted continental crust of Precambrian age, share identical chemical compositions and geologic age. They both also suffered a Late Cretaceous thermal event about 70–75 million years ago.

Geologists refer to localities that show precise offsets along faults as "piercing points." Liebre Mountain is 100 km northwest of the Mill Creek area. This distance of offset is less than the generally accepted distance of about 240 km of offset along the San Andreas Fault.

Several years ago, I noticed a boulder-size piece of granite in the office of one of my colleagues, Dave Liggett. Over the years, I kept admiring the rock, and, one day, he kindly gave me some nice representative specimens. These are the same specimens illustrated here, and they are from the Mill Creek Canyon locale. He also provided the background story of this rock. 


Monday, March 20, 2017

Epidote (the coating form)


Ever since I started collecting minerals, I noticed green splashes of a material coating various pieces of granite and other various rocks. I learned later that the coating consists of the common mineral epidote (pronounced “ep-i-dote”).

Epidote is a calcium, aluminum, iron, hydroxyl-silicate mineral typically found in metamorphic-rock areas where alteration or replacement took place in association with hydrothermal fluids. Epidote is especially common in fractures or joints. Fibrous crystals of epidote can be dark-green, black, or even yellow.

The epidote I find, however, has a very distinctive pistachio or pea-green color. It occurs primarily as surface coatings on cobbles and boulders of biotite-rich granite, which weather out from sedimentary rock conglomerates, as shown below. The name “epidote” is derived from a Greek word meaning “increase,” in reference to its crystalline shape.

Epidote coating a clast (maximum dimension 5 cm) of granodiorite
found on a hiking trail in the Santa Clarita area, Los Angeles
County, Southern California.

Wednesday, March 8, 2017

Preferred orientation of Turritella



Fossil shells belonging to the shallow-marine gastropod Turritella are prone to have been preferentially aligned by waves and currents because their shells are long an straight. I used the first slide below in one of my earlier posts (July 24, 2014) on the subject of "Taphonomy of Mollusks Shells." Taphonomy is the study of post-mortem processes (waves, currents, bored by other organisms, etc.) that affect shells.

Eocene Turritella andersoni lawsoni shells in the Llajas Formation
of Simi Valley, Ventura County, Southern California. The
longest shell is 6 cm long. These shells occur in situ, in
a bed of silty fine-grained sandstone.

More Eocene T. andersoni lawsoni shells from the Llajas Formation
of Simi Valley. The longest shell is 6 cm long. This slab is a piece of
loose rock ("float") from the formation.

Thursday, February 23, 2017

Green fluorite

One of my more recent posts deals with a locality where corundum (sapphire, ruby) can be found in Southern California. This new post deals with another Southern California mineral locality, and it is where green fluorite can be found.

Flourite consists of calcium fluoride. It is a common mineral and used as an indicator of a hardness of 4 on the Moh’s Scale of hardness from 1 to 10. Flourite can come in a wide variety of colors (especially purple), but green fluorite is a relatively less common color.

The green-flourite locality is called the “Felix Mine” locality, which is just north of the city of Azusa, California in the foothills of the San Gabriel Mountains. The mine, established in 1892, is no longer accessible because of urban sprawl, and the vein which yielded the green fluorite has long been mined out. The specimen shown below was recently kindly donated to me by a collector.

Green fluorite (maximum dimension 2.3 cm) from the Felix Mine, Southern California.
The black material is the mineral galena (iron sulfide).


The largest crystals ever found of green fluorite at the Felix Mine were reportedly about 8 cm long. Most of the crystals, however, were very small to small size. The fluorite occurred in numerous subparallel veins cutting through decomposed granite. The mineral galena is commonly associated with the green fluorite.

Thursday, February 9, 2017

Trigonarca californica

This post concerns a common Late Cretaceous bivalve (clam) that lived in California approximately 92 million years ago (Turonian time). It is Trigonarca californica Packard, 1922, which is known from northern California (Siskiyou County) to southern California.

The specimens shown below are from the Santa Ana Mountains of Orange County, and they were collected from the Baker Canyon Member of the Ladd Formation. As this locale, where specimens can be abundant, this species lived in sandy, warm, shallow-marine waters. A collector recently kindly donated these specimens.

Right-hand valve of Trigonarca californcia Packard. Length 4.4 cm.

          
This unusual specimen shows the somewhat separated valves of a formerly closed-valved specimen
of Trigonarca californica Packard. The hinge with its distinctive teeth are nicely preserved. Length  of the left-hand valve (at the front of the photograph) is 4.3 cm.

The sturdy shell of this species has the shape of a rounded triangle. Its teeth (dentition) are distinctive and consist of numerous, relatively heavy, short, straight teeth along its hinge.

Genus Trigonarca, which belongs to family Glycymerididae, was widespread, with occurrences in North America, Europe, South Africa, and India. Trigonaraca is of Late Cretaceous age.

Saturday, January 28, 2017

Corundum crystals from Southern California

The mineral corundum, which is second only to diamond in terms of hardness, consists of aluminum oxide (Al2O3). Corundum comes in a variety of colors, depending on the trace amounts of other minerals (e.g., rutile = titanium oxide) it contains.

The color can be red, blue, yellow, brown, green, or purple to violet, and some crystals contain color zones. Pure corundum is white. If the color of corundum is red, it is called rubyIf the color is blue, it is called sapphire.

A friend recently gave me some corundum crystals from Cascade Canyon, San Gabriel Mountains, about 2 miles southwest of Mount Baldy, which is near the town of Upland in Los Angeles County, Southern California


A hand specimen (4 cm wide) containing small, scattered
 crystals of corundum. The color is between ruby and sapphire.
Most collectors would most likely refer to these crystals as ruby.
Close-up of the left-middle side of the hand specimen shown above.
The lenticular crystal in the lower right side is 4 mm long.

The corundum at the Cascade Canyon locality formed when complexly deformed sedimentary rock (of Paleozoic age) was contact metamorphosed (heated up) by small granitic intrusions (of Cretaceous age). 

If you want to see outcrop pictures and more information about this locality, just Google the phrase:  Cascade Canyon ruby