Th El Dorado Railway Cut, Where Terranes Collide EarthCache

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Hidden : 3/6/2013
Difficulty:
1.5 out of 5
Terrain:
2 out of 5

Size: Size: other (other)

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Geocache Description:

An abandoned railway cut along the El Dorado Trail near Shingle Springs, CA, reveals four different major rock types of the Sierra Nevada foothills and hints at how the diverse materials of the North American continent came to be assembled on its western coast.

The western slope of the Sierra Nevada is a marvelous geological history book if you can just get past the cover, the green cover of plant life that is.  Except for the massive granite batholith that is now exposed along and immediately to the west of the crest of the range, much of the western slope is old and much eroded, transformed into soils in which all manor of plant life takes hold.  This makes for very nice forests and wild life habitats, but these plants do tend to hide the rocks.  So to the rescue of us rock heads comes mankind’s preference for transportation corridors which are relatively straight and of modest gradient which leads in turn to railway and highway cuts.  And to exploit these tidy cross sections through time we are fortunate to have a very handy guide in the book Roadside Geology of Northern and Central California by David Alt and Donald W. Hyndman hereinafter referred to as RGNCC.
Since at least the middle Paleozoic era roughly 400 Mya (Million years ago) the continental crust that has become North America has moved westward riding up and over the Pacific Plate which is expanding eastward toward the continent. This subduction collision has a number of physical consequences that work synergistically to assemble this fascinating detective story.  The collision itself while pushing down the Pacific Plate causes the continental crust to be thrust up, to fold and wrinkle, and to form massive mountain ranges.  The process would seem to be somewhat episodic so that the ranges are often separated by valleys, the so called basin and range structure so evident in an east-west cross section through Death Valley National Park for which, by the way, there is an excellent series of Earthcaches. The friction between the subducted plate and the continental crust melts large quantities of the material at that interface which then makes its way toward the surface creating both intrusive and extrusive igneous formations, e.g. the aforementioned batholith and volcanic mountains and lava flows respectively.  Over the eons both wrinkle ranges and volcanic mountains are eroded by the rains and snows that the mountains themselves produce by blocking the easterly flow of moist air from the Pacific.  Water and gravity carry the eroded material down to the Pacific where it forms vast off-shore fields of sediment.  The relative motion of plate and continent then plows up this sediment and adds it to the western edge of the continent.  And as if this east-west processing weren’t complicated enough, there is in addition north-south motion along interfaces between the blocks (sometimes called terranes) created in the east-west episodes as well as along cracks (faults) in the terranes themselves. So with that brief and doubtless overly simplistic outline of 400 million years of geologic history, let us move on to one specific location where several of the above processes and their products show themselves in space of just a few tens of meters.
Running from Folsom on the eastern edge of California’s central valley eastward through Placerville and on to Camino, roughly paralleling the modern US Highway 50, there is an abandoned railway which is slowly being converted into a hiking and bicycle path, the El Dorado Trail.   From the large overpass of Ponderosa Road 4.1 km ( 2.5 miles) northeast along US 50 is a huge, new cut made for the off ramp to the Red Hawk Casino to the north of the highway.  In this cut you can see much of what we will describe below, but the bad news is  that you’ll be zipping past on a freeway, the cut has been made boringly smooth, and besides there’s no safe way to get access on foot to the cut faces. The good news is that just 20 meters south of the off ramp is the corresponding railway cut, fully separated from and undisturbed by the highway work, and much more roughly and thus interestingly hewn by the early 20th century railroad engineers.  The less good news is that to reach it you need to do a short hike.  You can do it one of two ways.  Approaching from the west just 2 km from the Ponderosa Rd. overpass get off US 50 onto Shingle Springs Dr.  Turn right and proceed 0.85 km southward to the tracks.  Just a few meters beyond the tracks you can turn into the parking lot of a church where you can park most of the time.  You then hike roughly 2 km east to the cut.  Alternatively you can go 1.7 km past the casino overpass to the Greenstone Road exit.  The exit is beyond Greenstone road and loops you back around to the right to Greenstone Rd.  Turn left onto Greenstone Rd and proceed 1.7 km to the tracks.  Near the tracks is an auto repair shop where you might be able to park, but we usually go on south about 250 meters to the road side parking lot of American Legion Post 119.  It is then a 2.1 km hike northwestward along the tracks to the cut.  Coming from the east, Greenstone Rd and Shingle Springs Drive are 4.5 and 7.5 km west of the Missouri Flat over pass on the west side of Placerville. An added bonus is that along either 2 km hike there are several regular Geocaches should you be interested in them.
Now that we’ve arrived at the railway cut, let us set the stage and introduce you to the cast of characters you will meet.  The map on page 37 of RGNCC identifies the rock formations along US 50 from Sacramento to Carson City.  California’s capitol city is located in the middle of the alluvium of the Central Valley.  At Folsom you begin a climb up into the Sierra foothills and enter the Western Jurassic (145 to 208 Mya) terrane.  From Shingle Springs to a bit east of Placerville you are in what is known as the Calaveras complex which dates from late Paleozoic era (245 to 286 Mya).  East of the Calaveras is the Shoo Fly terrane which was accumulated from sediments onto the continent during the Mississippian period of the middle Paleozoic, 320 to 360 Mya.   East of the Shoo Fly terrane is the grand Sierra Nevada batholith. The railway cut of this Earthcache is at the juncture of the Western Jurassic terrane and the Calaveras complex.  On the RGNCC p.37 map we see at this location the confluence of four different major rock elements, dark green serpentinite, gabbro,  metamorphic rocks, and marine sedimentary rocks. It is these that you will be looking for in the cut.  To describe each a bit more completely we turn to the Audubon Society’s  Field Guide to North American Rocks and Minerals by Charles W. Chesterman. We will also draw upon numerous individual pages in the Science Portal of Wikipedia.
 

 
Figure 1  
Metamorphic Rock is a term applied to a huge and extraordinarily diverse collection of rocks which owe their character to the transformation of other more basic rocks called the protolith.  The transforming agency may be elevated temperature or pressure or both and the presence of other components such as water or carbon dioxide (CO2) may be important.   The protolith may be either sedimentary or igneous.  In the case of the El Dorado railway cut the two metamorphic rocks that this Earthcache has been able to identify are closely related.  Serpentinite, (Figure 1) which just happens to be the California’s state rock and is often called “Greenstone”,  is a member of the serpentine group of ultramafic  hydrous magnesium iron phyllosilicate  minerals  (Mg, Fe)3Si2O5(OH)4).  Briefly dissecting that mouth full: ultramafic refers to rocks very low in silica (SiO2 less than 45%) and high in magnesium (Mg) and iron (Fe) oxides MgO and FeO (more than 18%).  Hydrous signifies that the metamorphosis occurs in the presence of water.  Finally a phyllosilicate rock is one which forms in sheets (from the Greek φύλλον, phyllon, i.e. leaf) of silicate tetrahedra (Si2O5­ ).  The layered nature of this serpentinite can be seen under magnification, but the rock does not delaminate easily, unlike greenschist.

 
Figure 2
Greenschist (Figure 2) is also a mafic metamorphic rock, but the term “schist” implies a medium grade of metamorphosis and the presence of much more obvious lamination or foliation. The greenish color of these rocks comes from the presence of mineral components such as chlorite and actinolite , iron-magnesium silicates with additional aluminum and calcium content.
 
Figure 3  
Gabbro (Figure 3) refers to a large class of coarse grained intrusive mafic igneous rocks similar in chemical composition to basalts (lavas) but which solidified under higher pressure and more slowly.  Most gabbro is “equigranular” in texture, that is the crystals of the various minerals are similar in size and usually quite small.  In some cases, and this applies in the case of the Railway Cut gabbro, some larger “phaneritic “, that is visible to the unaided eye, crystals may be present and are frequently white and made of plagioclase feldspar.  Furthermore in some gabbro the iron is in a chemical state that allows the iron to transform into iron oxide and results in the reddish brown color of surfaces that have been exposed to air for protracted periods.  This is also the case at this Earthcache.
 
Figure 4  
Sandstone (Figure 4) is the archetypical sedimentary rock and is constructed of sand grains cemented together.  The grains may be the result of other rocks ground up or may be crystals of a single mineral.  Their sizes range from 0.0625 mm to 2 mm (smaller it’s silt, larger it’s gravel).  The most common minerals in sandstones are quartz and feldspar. The materials that bind the sand grains together are calcite, silica, and clay.  The sedimentation may occur from either water or from air.  It is often episodic and separated in time by other effects so that layering is very obvious and large in scale. Once you arrive at the cut, walk up and down the cut trying to identify each of the four main types of rock formations and their relation to each other.  There are many small features too, such as veins of quartz.
 
 
 
HOW TO CLAIM THIS CACHE:
To claim the cache, go to the posted coordinates which are at the east end of the cut. We STRONGLY RECOMMEND that you take with you a printed copy of the descriptions and photos of the rock types. You will be walking west and should be able to identify each of the four main types, not necessarily in the order described above. Answer the following four questions:
  1.  At the posted coordinates, on both sides of the tracks, there is a small accumulation of  _______________.
  2.  Walking west for about 55 meters you are in a massive outcropping of ________________.
  3.  You then arrive at a narrower (c.12 meters) band of __________________.
  4.  Beyond #3 you pass through another massive formation of __________________ which contains several bands of #1 (perhaps hinting at a common protolith?) Suggestion: Be sure to inspect the freshly exposed surfaces of small rock fragments, especially in areas 2 and 3, since the weathered surfaces of these two types can appear quite similar.
If you can answer the questions, that is great! If you can't answer them, but at least tried, that is OK too. No need to send me the answers! This Cache is for your own knowledge and enjoyment. As long as you read all the descriptions and feel you learned something, feel free to log the Cache. REFERENCES
Roadside Geology of Northern and Central California by David Alt and Donald W. Hyndman, Montana Press Publishing  Co. Missoula, Montana, 2000 (ISBN 0-87824-409-1)
 
National Audubon Society Field Guide to North American Rocks and Minerals by Charles W. Chesterman,  Alfred A. Knopf, New York, 1979 ( ISBN 0-394-50269-8).

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