Friday, March 7, 2014

Random Geology While in the Field

A fair bit of my geology work could be described as a bit random (sort of like my blog posts). My field work is dictated by where projects take me. There is no inherent focus on solving some great geologic puzzle. Just an accumulation of random observations that sometimes may coalesce into a bigger picture understanding. And some observations though interesting to me, may have no relevance to the reason I am at a location. These later observations allow me to do a little theory with no worries about being right or wrong. Such was the case at the mouth of a small river on the Olympic Peninsula.

Black horizons within the soil profile caught my attention at the top of the beach.   

The stratum suggests that this location is a sediment accumulation location with previous organic horizons having been buried. However, the site is currently eroding.

Its easy to see that this spot would be susceptible to erosion as this section of shore is open to the ocean via the Strait of Juan de Fuca.

So Why the organic soil horizons? Theories: 1) big floods on the river buried the site with sand and gravel, 2) large storm waves took delta gravel and built a berm over older poorly developed soil horizons on multiple occasions, 3) tsunamis, 4) some sort of anthropogenic actions in the past.

Lots of tests to do to figure out the problem and rule out the various theories. The answer? Not resolved like may of my observations. I had other things to do that needed to be resolved and did not have time to work through all the theory tests or even what the tests should be or even if I had enough theories.

That said I like the idea of past storm waves piling up gravel and sand in a berm on multiple occasions. The site is now being eroded due to a lack of big waves that push sediment across the mouth of the river or the available sediment that would be moved by the waves is no longer present. I went with no on tsunamis as it seems thin organic zones in the sand and gravel would with plenty of air would not last. That said this location would have certainly been inundated. The big flood idea was ruled out as there was lack of evidence of large enough floods up stream along the river. That part of the project was looked at closely. I could not rule out the anthropogenic part as I did find some historic references to this location that suggested a lot more went on here than today. But those activities were over 100 years ago and Why would there be multiple burials of soil?

Another observation sort related to the above was the presence of glacial till just below the veneer of river gravel and shore transported cobbles at the mouth of the is river.

This smallish river has not built up much of a delta as wave action readily transports gravel and cobbles along the shore to the east. Sort of a surprise to see and files away to perhaps become part of a greater understanding if I can connect this observation with something else.  


Geoffrey Middaugh said...

I'm not sure I would call them horizons. They seem more like "lenses". Any charcoal

Dan McShane said...

Point taken and you suggest by that statement a test that was not done - test pits landward to see if these thin black layers are continuous over any distance. I did not see ant charcoal and would note another study at the site unrelated to mine did not note any archaeological sites along the shore. Perhaps the area has been a bit too dynamic to preserve any camps or villages.

Anonymous said...

I am frequently trying to remind folks that "lenses" are likely only a visible remnant of a much larger interface. Lenses didn't just appear where they are. There was at one time a surface associated with them and it is that interface between that surface and what overlies it that begs for investigation. I think that the idea of independent lenses in the archaeological record is quite problematic.

Geoffrey Middaugh said...

I agree with both comments. A charcoal lens in the PNW may not be cultural at all. Charcoal in a dry wash of eastern Montana means something different than here. A lens of dark organic material means that something happened: outwash, riverine, splashed, dropped, pushed, or space ship launching pads (just kidding.