Geology Word of the Week: Alluvium

def. Alluvium:
1. Loosely-consolidated sediment transported or re-worked by freshwater.
2. What you put down on a geologic map when loose crap is covering up interesting rocks.
3. Very interesting sediment with key climate information that will save the world from global warming and/or help the world grow more food. (In Quaternary science only).

I think that I learned the word “alluvium” during my first mapping exercise as part of my undergraduate geology field camp. I learned that “alluvium” or more specifically “Quaternary Alluvium” is what you sketch on your geologic map when recently-deposited, poorly-consilated, partly re-worked by water soil and sediment (*cough* crap *ahem*) is covering up the consolidated geological layers of rock (limestone, sandstone, granite, whatever it may be) that you are trying to map. This recent alluvium (deposited in the last ~2.5 million years or in the Quaternary) hasn’t had enough time to consolidate into a sedimentary rock.

It can be frustrating to encounter alluvium at a key contact. Geological mapping is, when you think about it, a little bit crazy. I still remember the teaching assistant telling me to “guess” where I thought a particular contact lay underneath a wide expanse of alluvium. Guessing is, to an extent, acceptable in geology. As a novice geologist, I found the tolerance for guesswork remarkable– when you make a geologic map, you take measurements wherever you can see a particular rock layer and wherever you can’t see it– where it’s obscured by other layers or alluvium– it’s okay to guess where you think it may be. You can even make reconstructive geologic maps where you draw where layers used to be before they were eroded!

As a geologist, I suppose that I have become good at “thinking below the alluvium” or guessing. I am not afraid to make an educated guess. You have to in geology. You take what evidence you have– a small outcrop sticking out of the alluvium or that annoying biological cover; a sliver of a deep layer exposed in a fault; a diamond from the deep; a single, enduring zircon grain that’s survived 4 billion years; whatever you have– and you do your best at interpreting the geology. In academia, you guess the best you can and the different flavors of guess can lead to decades of back-and-forth discussion. In industry, you guess and the difference between a good guess and a bad guess can be millions of dollars. You guess where to drill for oil, where to mine for diamonds, where to prospect for gold. You are often wrong. If you are wrong too often, you are fired. To survive, you have to become good at guessing. You have to accept that guesswork has a place in science. You have to learn to guess as scientifically and accurately as possible.

I suppose that “thinking below the alluvium” is an example of where intuition and art, even, enters science. Experienced geologists and those with a natural intuition are better at guessing, better at figuring out where a layer runs underground or where to drill for oil. Certainly, geologists have many tools and data available to them to help them guess. As we develop our geological toolbox– both geophysical and geochemical tools– we have to guess less. Or we have more ways in which to narrow down our guesses to the most likely guess.

When I was in fieldcamp working on that first mapping exercise, I remember something that our instructor said to a few students (myself included).

A few years before, this instructor had been a lecturer at well-respected University X. His first year of teaching at the university he noticed that a large number of financial recruiters made rounds of the geology department, sweeping up recent geology grads for positions in banking and investment. He found this befuddling– many of the geology students who were recruited had never even taken an economics course. When the recruiters returned the next year, he cornered one of them in the hallway and asked why they were recruiting in the geology department. Did they simply recruit from all departments because students at University X were known for being smart?

The recruiter replied that, yes, students from University X were known for being smart but that the geology department was targeted in particular. The instructor asked why this was and the recruiter replied something like this:

“Because geologists are not afraid to make confident decisions based on extremely limited data. This is a very useful skill for finance. We can train new recruits in finance, but it’s difficult to train new recruits to make decisions.”

I have never thought about going into finance (even typing this makes me shudder… I plan to leave day-to-day finances and taxes and investments and such to my soon-to-be husband), but I suppose it is true that some skills translate. There is merit to studying geology– and science in general– because of the ways in which you learn to think. The ways you learn to analyze data and test hypotheses and make decisions. I’d argue that a scientific background of any sort has important skills that translate into other fields. There are many reasons to study science, even if you never intend to become a scientist. Personally, though, I hope that I’m always able to make my way as a geologist. If only because I much prefer jeans and t-shirts to skirts and suits.


Evelyn is a geologist, writer, traveler, and skeptic residing in Cape Town, South Africa with frequent trips back to the US for work. She has two adorable cats; enjoys hiking, rock climbing, and kayaking; and has a very large rock collection. You can follow her on twitter @GeoEvelyn. She also writes a geology blog called Georneys.

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  1. Thank you for the Monday morning flashback to soil and float mapping.

    And I’m wondering what you would have to say to a Geology major to get them to switch to finance (at least at the undergrad level). Most of us were pretty much in love with Geology not just as a science, but because we also were people who enjoyed being outside (at least on the nice days ;) ).

  2. Whenever I hear the term alluvial plain I hear Richard Attenborough talking in my head saying something about the amazing variety of species on the vast alluvial plain. That man could have made a poem about spelunking hippopotami sound scientifically important. Confidence in communication is something more scientists should work on and this post is a great example of some good communication!

  3. Just the other day I was thinking: In many places in the world matter is continously being pulled out of the atmosphere to steadily increase the depth of soil. If left alone it eventually gets covered up and turns to coal, but where’s the thickest/oldest soil in the world? And how thick is it and how old’s the substance at the bottom?

    I was gonna post it on the Straight Dope, but now I’ll give you lot a try first. You can all be rockist bigots. ;)

  4. Now, now… don’t go diss’in alluvium too much there. Bury it and fill it full of water and it becomes the aquifers that my customer base (people) need to live.

    PS – love the article. We geologists are a bit different. It drives engineers to distraction that we can make confident decisions based on (what they perceive as) no data. They’re just jealous.

  5. @bgclo: I like my bridges built with lots of hard data and conservative conclusions myself. Then again I’m guessing bridge builders often ask geologists about the ground under the bridge and/or building to determine if there is adequate stability. Is there a different level of certainty required for a pre-build geology report as opposed to an academic geology paper?

  6. @bgclo:
    I’m jealous? Hmm. I will have to think about that one. I’m a structural engineer and while I do not design bridges I do design very big buildings. When I design foundations I do not look to geologists for information – I get reports from geotechnical engineers. I am not sure what the difference is in their education but one thing is for sure. They do not do a lot of guessing. What they do a lot of is boring. Holes that is. I suppose there is plenty of guessing and artistry in selecting the bore hole locations on a site but when they give you a finished report, it is based on real data and a pretty hefty safety factor.

  7. Whenever I hear the word “alluvial”, I think of that great scene in Tremors where the beautiful and brilliant geologist explains to the handsome but slightly dumb heroes how the monster travels through the alluvial layer.

    hbsd heroes: (blank stare)
    bab geologist: The dirt
    hbsd heroes: (the lights go on)

  8. @James & @Flame Test – I was posting with tongue firmly planted in cheek, of course. You are both quite right that the vast generalization I was using doesn’t hold up under scrutiny. Geotech engineering is exactly what it sounds like: engineering based on geologic info & principles. So, indeed, much specific testing goes into foundation or structural investigations. That way, we get buildings & bridges that actually stand up.

    The bigger picture, non-site-specific background work & regional studies involve more a “arm wavy” approach — mostly because regional work can’t rely on such small scale data collection (it’s too limited) and getting usable data at the larger scales is expensive.

  9. Thanks for all the great comments, folks! Keep them coming. I apologize that I can’t respond in detail to each one. I have limited time as a busy grad student, and I’d rather focus on writing new posts. But I’ll reply when I can.

    To make a general comment, geology isn’t all hand-wavy. There is lots of data that geologists can and do collect. But as some of you have mentioned there is also the practical side of things. For instance, I suppose you could use geophysics and ground-penetrating radar or even a geochemical tracer or- get this- you could just dig a big hole to figure out what’s under the alluvium. The question is- does it matter enough to do so? Is it worth the time and expense? Or is a less-expensive, quicker, educated guess of what’s under the alluvium based on field relations and a little intuition good enough? If you’re about to build a skyscraper on top, I say go with all the tests. Much of the time it may be fine to just guess :-).

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