Sunday, October 5, 2008
Well, that's not exactly the case. This is what I mean. In geology it's fairly common to divide major rock units or formations into sub-units, especially when dealing with large scale maps that show a lot of detail. Sometimes the units or sub-units are formal parts of the stratigraphy of a formation; sometimes they are informal units made up by people working in an area where detailed stratigraphy isn't broadly recognized.
When dividing a section or formation into units or sub-units, it is standard practice for geologists to number the units from the bottom up, that is, with smaller numbers at the bottom or older parts of the sections and larger numbers at the top or younger parts of the section*.
For example, I might start mapping some large section of limestone in Nevada, maybe one that is fairly similar throughout but not entirely, and maybe one that hasn't previously been divided into subunits. Or maybe I'm working in some volcanic terrane where units and even major formations haven't been subdivided at all, but I need some way to distinguish rock formations and type on my map, especially so I might be able, then, to figure out the structure.
I'd start at the bottom of some hill, presuming that the section is right side up with youngest on top and the oldest on the bottom, and I'd call the first unit, Unit 1. (Not very original, but it works.) So the first unit, a light gray, cliff-forming, sandy limestone is Unit 1. The next identifiable rock formation I run across as I walk up the hill is a buff-weathering, slope-forming shaly limestone. I call that Unit 2. Unit 2 is younger than Unit 1. That's the way it is. We number things from the bottom up. Unit 3, above Unit 2, just happens to be a gray, cliff-forming sandy limestone almost identical to Unit 1, but Unit 3 has a few scattered brachiopod fossils in it, and Unit 1 doesn't. And so it goes, all the way up the hill - and down the next hill if need be - and all the way across the area, going from older to younger, with the unit numbers getting larger and larger.
The frustrating thing I've had to deal with recently is a particular rock formation that has two already-described stratigraphic sections. One is a section with numbers for the sub-units; one section has letters for the sub-units. Both the numbering and lettering systems are upside down. I can choose from using a numbered section that starts at the top of the formation with Unit 1 and works it's way down-section into older rocks with Unit 2 on top of Unit 3, on top of Unit 4, on top of Unit 5, and so on into however many subunits this rock formation has. Or I can choose a lettered section that starts with Unit A at the top, older than the Unit B beneath it, which is on top of Unit C, on top of Unit D, and so on to Unit Z. (I kid you not about the Z.)
I have a somewhat difficult time keeping track of numbers that start at the bottom of some formation with Unit 8 and work their way up-section, younger, often uphill (not always) to Unit 1. I find it almost impossible to start at the bottom of the formation with Unit Z and work my way up-section into younger rocks, often uphill, with Unit Y on top of Unit Z, with Unit X on top of Unit Y, and so on.
Perhaps the obvious the thing is to start at the top of the section, in this particular case and work downward into older rocks - and that's fine, until I have to start thinking about the rocks I just looked at. I keep having to say the alphabet over and over again, sometimes near the beginning, sometimes in the middle somewhere, and sometimes near the end, just to keep track of these upside-down letters.
At least they are upside down to me, even though the stratigraphy is right side up.
Maybe these numbered and lettered measured sections were from some area where it turned out that the formation in question was upside down when they described the section. Maybe. Then, maybe, when they started at the bottom of the hill and measured there way up, they started with 1 or A, which turned out to be the youngest unit of the formation because everything was overturned sometime back in the Paleozoic (a long, long time ago - more than 250 million years ago!). Well, maybe.
*Geologic convention is to number or letter units from oldest to youngest because the first layer deposited will then be called Unit 1 (or A). It was the first, it is the oldest, in a right-side up section it will be on the bottom or underneath all later and younger units.
NOTE: The section in question was described using letters A through W, with A at the bottom of what turned out to be an upside-down section (a section overturned by a regional-scale fold). Read a little more here and here.
Updated on 17Jun16.