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1999 Field Season: Day 9
July 3, 1999
Today, the morning was spent attempting to resolve a particularly puzzling question of stratigraphy.
In its simplest form, stratigraphy is a simple thing to resolve. A few commonsense rules govern the entire discipline, such as layers deposited on top are younger than those deposited on the bottom. If you think about it this makes sense; if it is on the bottom, it had to be there first, and, hence, is older. Stratigraphy in the field is usually straightforward; however, faults, erosion, subtle changes in the appearance of the rock layers, all contribute to complicated patterns that can be downright baffling.
Take our Member 11 sand layer that we are attempting to reach at Site A11-10. It is full of fossils and artifacts. Both above it and below it, though, are other sand layers that have some artifacts and fossils, and which look passably similar to our target sand. We are very interested in following the target sand all over the basin, and reconstructing the ancient Member 11 landscape, similar to what we have done for the Member 1 paleosol.
Now, just to the north of Site A11-10, there is an outcrop that contains two layers of sand -- an upper and lower layer -- both of which have some fossils and artifacts. The question is which (if either of them) is our target sand layer? One way of solving this problem is to "follow the contact." That is, walk along the rocks and touch the sand layer every couple of feet until you reach the new outcrop. Then you will know which layer it is simply by having carefully followed it all the way. This method only works if you can follow the contact.
But at this study site, a cliff blocks our way, so it is not easy to follow the units. We can see that the lower sand layers to the north are not our target layer, while the upper layer does appear to be our target sand. But to confuse things, a drying event occurred in the past, that left the bottom of the lake exposed to the sun, causing polygonal mud cracks to form. A layer of these cracks occurs just on top of the upper sand layer. An event such as this can usually be tracked over an entire basin, and in fact several hundred meters away, we have evidence of a mud-crack layer two meters above our target sand.
Confused? Basically, we have contradictory line of evidence. One says the top sand layer cannot be the one we want to track. The other says that it probably is. We are probably going to have to return to this site many times in order to resolve the issue.
The afternoon was spent in much less controversial pursuits. We set up our laser transit system in order to begin placing a datum (point of known position and elevation) in Locality B. One of the innovations of this project is that instead of having a separate coordinate system for each excavation, there is one grid for the entire Olorgesailie Basin. This allows us to correlate any find in any site with any other. The laser transit system we have is precise to within 5 mm at distances of up to a kilometer away. Any point in the basin, where there isn't an obstruction to block the laser beam, can be pinpointed on our three-dimensional coordinate grid. We use the laser transit to record the exact position of every major artifact and bone discovery.
Another use of our transit system is measuring the amount of movement of faults in the Olorgesailie Basin. Over the years we have noticed slow but perceptible changes in the land. Additionally, we have received reports of minor earthquake tremors in the basin. As such, we have set up a series of transects (lines of datum points) across known faults. Over the coming years, we plan to use these to measure movement of these faults to within millimeters.