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Causes for 1 meter of clay over meters of sand

  1. Jun 13, 2013 #1

    Stephen Tashi

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    In the yards of my neighborhood there is about 1 meter of a light brown clay over meters of sand. I'm curious what causes this geology. This is in the USA in the 88005 area code. I speculate that clay is caused by floods of the Rio Grande river, but I don't know what explains the sand. There are hardly any rocks in the sand. There aren't even many pebbles in it. I've dug several meters into it without hitting any other type of material. How deep is the sand likely to be?
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  3. Jun 13, 2013 #2


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    Hi Stephen

    I know several of the states the Rio Grande flows through, I have crossed them a few times in my travels. Dunno where your zip code is specifically ?

    maybe the sand is part of the old sea/lake floor when that region of the USA was under water ?
    asking a local geologist would be a great start.... you will find USGS geologists are pretty good guys to talk to

    Also get a geological map of the region ( from the USGS) they dont cost much. The map will contain stratigraphy diagrams throughout the area and probably a crosssection or 2 showing depth etc info

  4. Jun 13, 2013 #3


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    In central Maine much of the substrate is sand and gravel. If you start digging a few miles south of here, you will encounter a top layer of silt and blue (marine) clay due to the ocean's sedimentary deposits during the crustal subsidence during the last glacial period. If you look at a map of Maine, the clay is predominant anywhere south of Skowhegan.

    The clay is not due to periodic flooding, but due to many centuries of constant sedimentation when this land was under the sea.
  5. Jun 16, 2013 #4
    It sounds as if you are on a flood plain. If you look at a typical river in its lower course, it meanders. On the inside of the meander deposition, typically of sand, occurs. On the outside erosion occurs. The meanders, therefore migrate, building up 'wedges' of sand over time. Periodically a flood occurs and finer material is deposited over a wide area, rather than restricted locales.

    Can you see any structure in the sand? If my suggestion is applicable in your case you should be able to pick out cross bedding - layers of sand at an angle.
  6. Jun 16, 2013 #5

    D H

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    Here's a hundred year old report by the USDA on the soils of the Mesilla Valley: http://soils.usda.gov/survey/online...sillavalleyNMTX1912/mesillavalleyNMTX1912.pdf. The pdf apparently is a part of a much larger volume. The first page number is 2011; the discussion on soils starts on page 2025.

    The Rio Grande and irrigation make a mess of the soils in your area. In the flood season the river can carries materials from far away. Apparently the Mesilla Valley is very flat, so that's where the river drops it load. Since the materials suspended in the river waters can come from very far away, the deposited material vary over a short distance and over time. Irrigation adds to the mix, depositing fine materials at a ferocious rate.

    Edit -- How I found this: Your zip code identifies you as being in Las Cruces NM. I pulled up a map, saw that the name for this geographic region is the Mesilla Valley. From there I googled Mesilla Valley soil survey.
    Last edited: Jun 16, 2013
  7. Jun 16, 2013 #6


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    I was doing a similar survey in my area around 1990. It was a sand island near the shore of the Columbia river. The first 6 inches was soil, then 2 feet of sand, then I came upon a foot thick grey clay layer, after which was again all sand.(The order may be a bit off, as that was a long time ago.)

    The sand made sense, and the soil made sense, as the area was densely covered with very young 12 foot tall willow trees. During the summer high water period, the dirt laden river was slowed by the trees, and the sediment settled. But what about the grey clay layer?

    Then I realized, that Mt. Saint Helens had erupted 10 years earlier, and dumped untold amounts of grey ash into the river in a very short period. I theorized that the willows, which couldn't root in plain river sand, with river levels varying by 20 feet through the year, had found a home, in the St. Helens clay.

    My first experience with this location was about 1985, and one could still clearly see the river, 1000 feet away from the shore, over 3 foot tall saplings. Today, I would estimate the trees are nearly 20 ft tall.


    I'm not sure why I'm on a Saint Helens roll this week.

    I suppose it's because she's an interesting lady.

    Toot toot!​

    Ps. I decided to dig my own sewer connection about 15 years ago, and after pick-axing* through 2 feet of the hardest rock laden soil on the planet, I dug through another 10 feet of sand and river rocks. I knew of the Missoula Floods, but it was interesting to see the evidence for real, in my own front yard. :smile:

    * I decided after the first couple of days, that a shovel, was not the correct tool for the job.
    Awesome quality, American made, Hickory Handled, pick-axe back in 1995: $35 (ouch!)
    Cost to have someone else do the job: $3500 (two orders of magnitude ouch!)
    Having flawless plumbing for the last 20 years: priceless.
  8. Jun 16, 2013 #7

    D H

    Staff: Mentor

    Another possibility is the 1700 Cascadia earthquake. That 6 inches topsoil + 2 feet of sand sounds like just about the right depth to have been buried with 400 years of sandy deposits.
  9. Jun 16, 2013 #8


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    Nope. I've watched this happen over the course of half of my lifetime.

    The sandy colored area at the bottom is the main shoreline. The green to the north, is all new landscape, formed over the last 30 years. The yellow line, is ≈1000 feet long.


    But I will ask the oldsters I know, who were there 10 years before I was. I'm sure they will tell me that I'm "just a baby", and too young to know about such things. :tongue2:
  10. Jun 16, 2013 #9

    jim mcnamara

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  11. Jun 17, 2013 #10

    Stephen Tashi

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    I've only excavated places in the yard to about 3 meters (in various home improvement projects). I didn't notice any distinct layers in the sand. I haven't found any place nearby where I can observe a deeper excavation.

    Thak you for that link. The report doesn't reveal what's under my yard but it is very clear description of the local area.

    I think the houses in my neigborhood have footings that don't extend beyond the layer of clay. I'm curious whether this is a weaker or stronger construction than extending the footings to the layer of sand. I've noticed that engineers are usuallyl very happy to design inland structures whose foundations are on sand - contrary to the popular notion that sand is a poor support.
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