Geology Word of the Week: A is for Alluvium

A geology student on some alluvium, Western USA, Fall 2005.

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.