On First Encountering Melange

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 a melange.

Franciscan Complex melange- Terrabay Project  1989 (photo E. Medley)Melanges are heterogeneous mixtures of strong rock blocks embedded within weak rocks, often sheared shale. Actually: melange is shorthand for “melanges” which like most other rock-types tend to be different-looking 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) which are complex geological mixtures 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 are the most intractable and puzzling. They are also horrible for engineers and engineering geologists to work with.

Bear with me a few paragraphs for a short excursion.

In 1919, the British geologist Edward Greenly first described the chaotic Gwna Melange of Anglesey in North Wales as autoclastic mélange (see Note [1] at bottom). (That factoid is pleasing to me: my heritage is Welsh – I lived in North Wales as a child and apparently spoke some Welsh [2].) But melanges are also found in over 70 other countries, usually in mountainous areas, near recent or ancient tectonic subduction zones. The Franciscan Complex of Northern California  (“the Franciscan” for short) is geologically infamous as such a tectonic, regional-scale jumble of shards of earth’s crust, interspersed with some of the most spectacular melanges and bimrocks in the world [3].

The details of melange formation are controversial. There are thousands of papers on melanges, a good proportion focused on those of the Franciscan. But only few of those papers, which include some of my own contributions, address the troublesome geopractitioner aspects of melanges [4].

Back to the story: I first encountered a melange in 1989 when I visited the San Francisco Bay Area from Hawaii for a short job, intended to be a few weeks duration. The melange underlies the lower slopes of San Bruno Mountain in South San Francisco, and is part of the Franciscan Complex. I had never before seen such a complex mess of rocks.

South flank San Bruno Mountain, So. San Francisco, CA; mass grading underway for Terrabay project; 1989 (photo: E.Medley) 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.

Geotech consulants in trench at Terrabay project - 1989 (photo E. Medley)Besides melange and what looked like a myriad of faults, the foot slopes 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, the now-Dr. Eric Lindquist and I pioneered research into the engineering characterization of melanges and similar bimrocks [5].

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


[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.  My website bimrocks.com has lots of information including my review-style 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.

[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] Curl up for good. longs reads with the PhD dissertations of Dr. Eric Lindquist and Dr. Ed Medley…


Add a Comment