This tale has been told before (at my website edmedley.com) but it is not a bad story and can bear re-telling…

My life and career lurched abruptly  in early 1989, when I first encountered melange.  Melanges are heterogeneous mixtures of strong rock blocks embedded within weak rocks, often sheared shale. Actually: melange is shorthand for “melanges” which tend to be different rock masses in different places.  After 1989  my research and a large part of my geoprofessional focus  became the engineering characterization of melanges and other bimrocks (block-in-matrix rocks: learn more here), which are complex geological mioxtures of hard blocks of rock surrounded by weaker matrix rocks. Bimrocks include a wide variety of geologically complex mixtures: fault rocks, melanges, lahars, and weathered rocks. But melanges can be the most intractable.

I was the on-site geotechnical engineer for the Terrabay project, a high-class residential development that had taken years and tears to final fruition. The development was much opposed because the rural nature of San Bruno Mountain, cherished by many local folk. I actually became a local folk since the house I rented abutted the development – of a morning I would hop over my fence to hike the mountain before strolling into the on-site field office.

The Mountain was home to the Mission Blue butterfly, one of three rare species of fauna. The Terrabay project was allowed to go forward only after many years of controversy, and was subject of the the first Habitat Conservation Plan authorized under the Endangered Species Act Incidental Take Permit provisions.

Such was the oversight and controversy over the development, that the city of South San Francisco retained an eminent geotechnical engineering firm as Geotechnical Engineering Reviewer. The firm was led by Dr. F~, an internationally famous engineer. He was a Brit, as I, but we did not get along at all. Dr. F~’s field staff were very smart folk, PhDs all, with many opinions and criticisms, some of which were valid; others not so. I had a lowly first degree in Geological Engineering, which had been sturdy enough for me for many years but seemed puny in comparison to the several PhDs.

The melange was uncovered in the early in the project, during earthwork excavations. It looked like faulted rock; indeed, it is “faulted rock” in the sense that faults are broken terrain. But the word “fault” is to geologists and engineers a bold red flag, since there are many regulatory ramifications related to potential earthquake movements along active faults. So soon there was much debate about unexpected faulting.

Besides melange and what looked like a myriad of faults, the footslopes of the Mountain were underlain by several landslides, some of which only became evident when the bulldozers and scrapers started to exhume them. Confusingly, the landslide deposits looked like melange, which is not surprising since they were composed of original melange bedrock. The nature, geometry and cost of remediation of these unexpected landslides prompted considerable tension on the project. Indeed, there was much argument, debate and angst between myself, the firm I worked for, Dr. F~’s firm, the developer, the contractor, and the civil engineering designer - often while we were gathered in exploratory trenches.

The Terrabay Project was profoundly disruptive to me. I had left Hawaii for a few weeks to start the project, and stayed on it nearly a year. I was never to return to Hawaii to live. Melanges, lurking landslides, a cabal of PhD reviewers, and seemingly ceaseless technical debate left me feeling more ignorant than I had at any time since I had left school in 1978. So, at age 42, I decided to refresh my confidence by entering the University of California at Berkeley to study for a Master’s degree in Geotechnical Engineering. That small but critical lurch, inspired by first encountering melange,  led to the much larger one of continuing to a PhD. Working with Professor Richard E Goodman, I pioneered research into the characterization of melanges and similar bimrocks.

So:  I have much to be grateful for first encountering melange on San Bruno Mountain [5].

[NOTES]

[1] The acute accent above the é  in mélange (pronounced may-LAWN-juh) is often neglected in the United States. It is acceptable to pronounce melange as mell-AHN-juh.

[2] Medley and Melange are connected even more: medley is an English word loosely meaning mixture for which the French is mélange.

[3]: In 2008 I organized a Field Trip for the American Rock Mechanics Association Conference in San Francisco. The tour had to be canceled, but many folk were so disappointed about that cancellation, that they asked me if I could lead a “private” tour. So, off on a small bus we went and had a jolly day looking at small bites of Franciscan Complex melanges. If you are interested in Franciscan melange and and tourist chatter, then look at the informal Field Trip guide that I prepared for that frolic. John McPhee does a good job describing the Franciscan with his prosaic book Assembling California.  Also: the annotated presentation “An Introduction to Bimrocks“ is a useful review on bimrocks, as is my most recent contribution: Geopractitioner Approaches to Working With Anti-Social Melanges, a chapter in the book in Special Paper 480: Mélanges: Processes of Formation and Societal Significance, by John Wakabayashi and Yildirim Dilek, published by the Geological Society of America. Wakabayashi and Dilek’s  full-text introduction to the volume is available at the GSA website and is a good place to start if you want to learn more about melanges.

[4] The geological literature of melanges is also  complicated by many apparent synonyms for melange fabrics, including: olistostromes, argille scagliose (scaly clay), sedimentary chaos, mega-breccia, chaotic structure, complex formations, lenticular fabric, tectonic mixtures, friction carpets, Varicolored Clays and wildflysch. The confusion of geological names is part of the  reason I coined the non-geological word “bimrocks” at the beginning of my research – bimrocks is not a geological word, and is intended to focus engineers’ attention, not on confusing geological implications, but on the fundamental and common fabric of mixtures of hard blocks in weaker matrix rocks.

[5]: If you are interested you can watch a video which which focuses on the favorite geological spots of San Francisco Bay Area geologists, and includes a segment on how melange at San Bruno Mt. changed my life.  Just wait through the ad at the beginning of the video.

A student asked me if I had had a break-through moment in my PhD research. I told him I had – it had been a moment of “soul-soaring elation”. That was such an odd, poetic expression to tumble from me, that the geologist audience and I laughed. But I shall never forget the joy, the soul-soaring elation, I felt during my breakthrough research moment in 1994.  See if you can feel it too…

I was measuring the sizes of blocks in Franciscan Complex melanges from photographs and maps then plotting histograms of the sizes. Histograms are graphs of the numbers of somethings of certain sizes: in this case the number of blocks that fell into “bins” or “classes” of   ranges. My measurements ranged form millimeters to kilometers – seven orders of magnitude (or the largest measurements were 10 million times larger than the smallest measurements.) It is really impossible to show the data on conventional graphs so I switched to logarithmic plots. The plots thus became “log-histograms”.

When plotted as log-histograms much of the data assumed neat order – steeply peaked curves marched nicely across the graph paper, looking very similar. Indeed: the block size plots (we call them block size distributions) are “self-similar” meaning that if you do not have some way to know the scale there is no real way to tell them apart. The plots are also “fractal” which basically means the same thing. Fractal mathematics is pretty cool stuff and very sexy in geology. But I am not a mathematician and you likeley are not either, so let’s skip ahead…

The plots were so invitingly similar that I gnawed at finding some way to collapse the plots some more. I eventually tried dividing all the block size data by the square root of the area of the photo or map containing the individual data sets. Doing that was pretty easy- I just worked my way through the exercise using Excel.

When I hit the Excel graphing button , up came all the data, collapsed into one fluffy big plot, with many colored symbols, organized into a constellation of datapoints that grouped together looked just like the individual plots. That was a thrilling moment, seeing the constellation on my screen; an amazed moment of “Holy shit! – look at that!” –  which is what “soul-soaring elation” is, expressed in (my) words.

I had stumbled upon the “scale-independence” of the block-size distributions of the Franciscan Complex melanges I had measured. It did not matter at what scale, it seemed that the melanges would always have a similar-looking block size distribution. In practice that meant many, many small blocks and a decreasing number of larger blocks.

The collapsed plot gave me the insight to come up with a way to answer the pesky question “If there are seven orders of magintitude of block sizes, then what is block and what is matrix?” The answer: “It depends on your scale of interest”. That is not a throw-away answer; it has significant implications in engineering – I have written  about that significance.  The findings also provided a solid justification for the work of my research partner, now-Dr. Eric Lindquist. Eric was working on relatively small physical models of melanges in the laboratory. Since our joint problem was the characterization of melange under a dam, one valid critique could be: “What is the validity of laboratory results based on small models to the real-world behavior of a full-scale dam and its foundations?” The answer was that the small-scale laboratory melanges were actually much more like scale models of real conditions than is generally found in geotechnical engineering.

Not all PhD researchers have the same soul-soaring elation, the same “Holy shit!” moment that I had.  I consider myself lucky and privileged, and I hope one day you have one too.