Orestimba Rocks (9/10/17)

In the afternoon we went forward in the same direction, downstream, and after a league we got to the arroyo of Orestimac [Orestimba Creek], opposite the Apalamenes, the allies of the Tatives when the latter fought us.  This creek, which comes from the hills, is not flowing nor does it contain water, but it is known that in the rainy season it fills up and even overflows.  This spot is the least bad on this whole side of the river but even so it would not be suitable for a mission.  It has only firewood, river water, and much good fish. --- Second Expedition of Fr. Jose Viader and Second Lieutenant Gabriel Moraga, October, 1810

Orestimba Rocks from the north

Needless to say, much has changed since Fr. Viader rode through my neighborhood scouting the land and looking for runaway acolytes.  While the Padres weren’t particularly interested in the land along Orestimba Creek many others were; long before Spanish colonization, the bands of the Northern Yokuts understood the area’s value, and the earliest records of several missions mention the native rancheria of “Orestimac” as being a place of meeting for the native peoples. (Unfortunately, within a decade of Viader’s account the West Valley was effectively denuded of its native population largely through total destabilization of their lifestyle by Spanish incursions.) One of the things that has not changed, however, is the presence of the Orestimba Rocks, a large mass of stone that stands in silent testimony to the presence of the Yokuts and serves as a reminder of processes that make short work of the span of human history. While I found a bit of general stuff available on the Rocks as a cultural site, I had not been able to find any examination of them as a geological curiosity.  Since I had known of them for a while but had never investigated, it was time for a georun to see for myself.

 

The Orestimba Rocks are about 8km west of Newman, CA, at the very eastward edge of the Diablo Range along Orestimba Creek.  They are not difficult to find nor to approach, but they are on private grazing land so care must be taken to respect that.  My wife Karen decided to go with me on this one, so I threw my gear and some water-bottles in the Green Hornet and we set off.  Driving out Orestimba Rd., we crossed under I-5 and parked next to the east bank of the California Aqueduct (don’t block the service gates).  From here, it was an easy 1.12 km walk southwards along a paved (!) service road to reach the Rocks. 

The Rocks are pretty impressive, even from a distance.  There is nothing similar in the region around them, and nothing as eye-catching for some distance further west into the Coastal Range.  They are a large, seemingly-continuous mass of rock stretching along a low ridgeline, obviously more resistant to erosion than the material around it.  Using Google Map’s satellite function as my tool, they measure over 357m lengthwise, and are over 80m at their widest point and 64m at their narrowest.  Being on grazing land, they are also surrounded by three-strand barbed cattle fencing.  I had observed that the property was not posted as “No Trespassing,” so I assumed that trespass was tolerated at the Rocks so long as the cattle were not disturbed and nothing was left or damaged at the site.  At one point on the fence, a stile was constructed of two stout cargo pallets which enables the careful to cross the wires without injury or damage to the fencing, and as they appeared to have been there awhile it again implied a certain tolerance for respectful trespass, so I crossed the wires and began photographing and scouting for a way up the Rocks.  Karen, being somewhat less intrepid about such things (and also being leery of meeting Our Friend the Western Diamondback) chose to wait on the road. 

Erosion and weathering in various forms are present all over the Rocks

Given their location I had assumed that the Rocks were some variety of sandstone, and the signs and forms of erosion and weathering I observed as I approached the mass (including some shallow tafoni) confirmed this.  Unlike the majority of the sandstone rocks to be found in the immediate area, however, the stone of the Rocks is coarse, with generally light-colored grains easily discernable with the naked eye.  I was guessing arkose, but since my current hand lens is pathetic a good, close look would have to wait for the stereomicroscope when I got home; my Boreal is one of my favorite toys.

Much of the face of the Rocks is either very steep or sheer, with a way up to the top being only for those who enjoy mountain-goating on hot rocks a lot more than I do.  Although I neglected to take height measurements, I’m guessing an average of between 10-20m. Fortunately I found a V-shaped notch in the formation not far from where I crossed the fence, using it to ascend to the top with only minor difficulty.  It was well worth the effort. 

Looking north from the top of the Rocks.  In the distance can be seen the "grove of sycamores" along Orestimba Creek mentioned in mission records.  To the upper left can be seen the California Aqueduct.

The top of the formation is roughly flat, providing easy walking over most of its length, and there is much to see.  Here I found evidence of the presence of the Yokuts: mortar holes, used for grinding food, most likely acorns from the oaks that are prevalent in the Coastal Range.  I found four of them in a group, but there may be more elsewhere on the Rocks.

I also found many more examples of weathering and erosion, a surprising array of both dormant and active plant life, variations in color of the arkose and substantial bedding in the formation.

 Unfortunately for me, at this point my explorations of the Rocks were cut short when my thick-soled, “value-priced” hiking boots decided to morph into thin-soled moccasins.  Although they had given me good service for several years of low-pressure trail hiking, they couldn’t handle the rock; the rubber soles briefly flapped around like a trout out of water, then sloughed off altogether.  I was not happy, especially since picking my way back down off the rock and walking over a kilometer back to the car on soles of Thinsulate and surplus military boot sock did not appeal to me; a kilometer seems a far greater distance when you can feel every pebble under your feet.  In any case, I clambered back down off the Rocks and as I neared the wire I picked up some fragments of rock for examination later.

   

So ended my first georun to Orestimba Rocks.  Examination of the specimens under the Boreal back home appears to support my initial identification: the Rocks are a mass of arkose, most likely of the arkose-rich Great Valley sequence. There was far too much feldspar in the rock for standard sandstone; my unscientific “eyeball” count would say between 30-40%.  There was also quite a bit of calcite, as well as a smattering of other materials.

 

At the beginning of all this I set out to find the answers to several questions, and one of them remains unanswered: why do the Rocks stand out so prominently from the landscape around them?  Presumably they are not the only mass of arkose in this area, and indeed they may just be the upper portion of a larger mass as yet unrevealed by erosion.  But, why here?  And why are there no other exposures nearby?

I have a feeling I’ll be heading over there again in the near future...after getting some new boots.

Requiesque in pace

Sources

Bartow, J. Alan and Nilsen, Tor H., 1990.   Review of the Great Valley sequence, eastern Diablo Range and northern San Joaquin Valley, central California.  Open-File Report 90-226: Dept. of the Interior, U.S. Geological Survey.

Cook, S.F., 1960.  Colonial Expeditions to the Interior of California: Central Valley, 1800-1820.  Anthropological Records 16:6, p. 239-292. Berkeley and Los Angeles: University of California Press.