Stone River is a wall-like serpentine sculpture set in about three acres of land to the northeast of the Iris & B. Gerald Cantor Center for Visual Arts. It is about 3 1/2 feet high and about 4 feet wide at its base. It is made of more than 6,500 stones, including about 700 triangular coping stones weighing between 20 and 50 pounds each that top the sculpture. Each coping stone was individually shaped at a different angle to fit the wall precisely. The total weight of the piece is about 128 tons.
The stone at Stanford was once sand, and through the process of erosion will become sand once more. It was compressed into rock that was quarried and made into buildings, which were then damaged by earthquakes, pulled down, and stored. Movement and change are part of the material’s nature, and are qualities that I have drawn out in the making of Stone River. It is not a representation of a river—it is the river in stone. My use of the stone has, in some ways, returned the stone to the earth—another leg of its journey.
-- Andy Goldsworthy, Stone River Diary
When he heard about the campus "boneyard" -- a repository of salvaged historic materials including sandstone from buildings toppled in the 1906 and 1989 earthquakes -- he knew he'd found the right material. "The idea of stone that was once a building returned to the ground, back into the earth, for a work that is about flow, movement and change, it was perfect. It was really perfect.
"The reason why I put it into the ground is to weld it, and to give it the feeling that you've just scraped away one small portion of something that may be much, much larger. While we were making it, it felt like an archeological dig -- some people came along and thought it was an archeological dig," he said. "I had the idea that I was just revealing something that was already there."
Sandstone Basics
Sandstone is a category of rock made from sediment (a sedimentary rock). The sediment particles are clasts, or pieces, of minerals and fragments of rock, thus sandstone is a clastic sedimentary rock. It is composed mostly of sand, which means particles of a medium size, so sandstone is a medium-grained clastic sedimentary rock. More precisely, sand is between 1/16 millimeter and 2 mm in size (silt is finer and gravel is coarser). Sandstone may include finer and coarser material and still be called sandstone, but if it includes more than 30 percent grains of gravel, cobble or boulder size it's classified instead as conglomerate or breccia (together these are called rudites).
Sandstone has two different kinds of material in it besides the sediment particles: matrix and cement. Matrix is the fine-grained stuff (silt and clay size) that was in the sediment along with the sand whereas cement is the mineral matter, introduced later, that binds the sediment into rock. Sandstone with a lot of matrix is called poorly sorted. If matrix amounts to more than 10 percent of the rock, it is called a wacke ("wacky"). A well-sorted sandstone (little matrix) with little cement is called an arenite. Another way to look at it is that wacke is dirty and arenite is clean.
Mature continental sediment consists of a handful of surface minerals, and sandstone therefore is usually almost all quartz. Other minerals—clays, hematite, ilmenite, feldspar, amphibole and mica— and small rock fragments (lithics) as well as organic carbon (bitumen) add color and character to the clastic fraction or the matrix. A sandstone with at least 25 percent feldspar is called arkose. A sandstone made of volcanic particles is called tuff.
The cement in sandstone is usually one of three materials: silica (chemically the same as quartz), calcium carbonate or iron oxide. These may infiltrate the matrix and bind it together, or they may fill the spaces where there is no matrix. Depending on the mix of matrix and cement, sandstone may have a wide range of color from nearly white to nearly black, with gray, brown, red, pink and buff in between.
Sandstone forms where sand is laid down and buried. Usually this happens offshore from river deltas, but desert dunes and beaches can leave sandstone beds in the geologic record too. Sandstone does not usually contain good fossils because the energetic environments where sand beds form don't favor their preservation. When sand is deeply buried, the pressure of burial and slightly higher temperatures allow minerals to dissolve or deform and become mobile. The grains become more tightly knit together, and the sediments are squeezed into a smaller volume. This is the time when cementing material moves into the sediment, carried there by fluids charged with dissolved minerals. Oxidizing conditions lead to red colors from iron oxides, while reducing conditions lead to darker and grayer colors.
Stanford Sandstone
The sandstone making up Stone River and the nearby quad buildings is a local rock type, mapped as part of the Chico Formation of Late Cretaceous to Early Tertiary age; it is now interpreted as an unnamed Early Eocene arkosic sandstone (58 to 37 Ma). 
Opened in 1891, Stanford University was built in the style of California's Spanish missions using the sandstone quarried from the northern end of the Santa Teresa Hills, south of San Jose. A special railway line was built just to haul about 800 cubic feet of stone daily to Stanford form the Greystone Quarry. These sandstones represent some of the oldest stratified sediments overlapping the highly deformed deep water sediments, basalt, and ultramafic rocks of the Franciscan Complex, which underwent deformation during subduction of sea floor beneath California.
This sandstone represents a moderately deep-water subsea fan and channel deposit that is lithologically and stratigraphically identical to rocks of the same age in the Sierra Azul, near Loma Prieta to the southwest. When these sediments were being deposited, you could have sipped margaritas on the beach at the foot of the Sierra Nevada and to visit what is now Stanford would have required a submarine... Fluctuating sea levels and uplift of the land later combined to raise these marine rocks into a "high and dry" position in the modern Santa Teresa Hills
How It Formed
Rivers brought particles of quartz and feldspar that eventually became Stanford’s sandstone down from the mountain range we now know as the Sierra. At that time, and for millions of years afterward, the ocean met the mountains on their western slope; that was the “coast.” Rivers draining the slope cut through granite, picked up chunks of rock, cobbles, and pebbles that broke off in seasonal freezing and thawing, and carried them into the ocean. Over millions of years, the sedimented beds of this sandy material piled on top of each other and became many kilometers thick. The pressure of the upper layers compressed those below, forming sandstone rock.
Geology Professor Elizabeth Miller and her colleagues have done a bit of detective work on our sandstone and know in fairly fine detail where it came from and when. To begin with, they can date it approximately, because there is one (and maybe only one) fossil-like impression of an ancient creature in the thousands of tons of stone quarried for the Quad. This is a Turitella, a common sea snail that has been around for millions of years. Geologists now agree that this one comes from the early Eocene Epoch, somewhere around 34 million years ago, give or take. Geologists can also locate the geographical source of our stone. They infer that it must have been in the mountains around what is today Sequoia and Kings Canyon national parks, because they know that since the Eocene, the Pacific Plate has migrated 175 kilometers north, bringing its sediments with it. This migration was an almost unimaginably slow process—the plate moved at the speed at which fingernails grow. Not all the chapters in our stone’s life were so slow, however.
Miller looked at thin sections of it under a microscope and could see that the grains of quartz it contains are angular and not well-sorted. This is evidence that the sand is what is called “first-cycle”— meaning that it came down from the mountains fast and was deposited almost immediately into a deep marine canyon, like the one under Monterey Bay. In other words, it was not washed on beaches and smoothed by waves.
For more than 25 million years, our sandstone lay in its marine canyon, carried calmly north by the migrating Pacific Plate. Then, approximately eight million years ago, the plate began to slip under its neighbor, the North American Plate, folding and lifting its shelf sediments above water and forming the series of parallel ridges and valleys of the Coast Range as we know it today—including the Santa Cruz Mountains, Mount Tamalpais, and the Santa Teresa Hills in San Jose, which became the source of Stanford’s sandstone.
For the next 8 million years or so, our sandstone (shown in light blue in the above to the right, labeled TLS) lived a fairly quiet geologic life, undisturbed by humans or even animals for most of that time. Ice ages followed temperate spells, and eons of vegetation grew and died back, laying down many feet of soil on top of the stone. Now, thanks to Andy Goldsworthy's design, the stones that once stood high above Stanford Campus quietly return to the Earth from which they came.
Logging requirements:
(You can log your find as soon as the email is sent, but logs with insufficient answer may be deleted)
Send me a note through my caching profile with:
1. The text "GC69V21 Stone River" on the first line
2. The number of people in your group (put in the log as well).
3. Take a walk along the winding river. How long is it from end to end?
4. Look at the stones. What colors do you see? What does that tell you about its mineral make-up?
5. Feel the stones. Describe the texture. How would you describe the grain? It is coarse or fine?
Would you consider this a relatively hard or soft stone? Why?
6. Optional (yet encouraged) Post a photo of you and your party at (or in!) the Stone River.
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Sources:
Sandstone & Tile, Stanford Historical Society Fall 2010, Volume 34, Number 3
Stanford Report Stanford Report, January 23, 2002
Stanford's Signature Sandstone KQED Science
California Rocks! GEO 116, Continuing Studies, Stanford University
Sandstone Basics Geology at about.com education.