Causes for 1 meter of clay over meters of sand

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In summary, the yards in the 88005 area code in the USA have about 1 meter of light brown clay over meters of sand. The clay is likely caused by floods of the Rio Grande river, while the sand may be part of the old sea or lake floor from when the region was under water. There is evidence of constant sedimentation and periodic flooding in the area. The depth of the sand is likely to vary and can be determined by consulting a geological map or speaking to a local geologist.
  • #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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  • #2
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 don't cost much. The map will contain stratigraphy diagrams throughout the area and probably a crosssection or 2 showing depth etc info

Dave
 
  • #3
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.
 
  • #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.
 
  • #5
Here's a hundred year old report by the USDA on the soils of the Mesilla Valley: http://soils.usda.gov/survey/online_surveys/new_mexico/mesillavalleyNMTX1912/mesillavalleyNMTX1912.pdf [Broken]. 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.
 
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  • #6
Stephen Tashi said:
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?

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.

hmmm...

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

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

mount_st_helens_early_morn_fishers_landing_2005_med.jpg

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.
 
  • #7
OmCheeto said:
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.
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.
 
  • #8
D H said:
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.

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.

pf.rr.msh.2013.06.16.1422.jpg

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:
 
  • #9
  • #10
Ophiolite said:
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.

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.


D H said:
Here's a hundred year old report by the USDA on the soils of the Mesilla Valley: http://soils.usda.gov/survey/online_surveys/new_mexico/mesillavalleyNMTX1912/mesillavalleyNMTX1912.pdf [Broken].

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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1. What factors contribute to the formation of 1 meter of clay over meters of sand?

The formation of clay over sand is influenced by several factors such as the type and composition of the parent rock, the climate, and the geological processes that occurred over time. Clay is formed from the weathering and erosion of rocks rich in minerals such as feldspar, while sand is formed from the weathering of quartz-rich rocks. The amount of rainfall and temperature also play a role in the formation of clay over sand, as they affect the rate of weathering and erosion.

2. How does the thickness of the clay layer compare to the sand layer?

The thickness of the clay layer can vary greatly compared to the sand layer, depending on the location and geological history. In some areas, the clay layer may be only a few centimeters thick, while in others it can reach up to several meters in thickness. The sand layer, on the other hand, is typically thicker and can range from a few meters to tens or even hundreds of meters.

3. What is the significance of having a thick layer of clay over sand?

The presence of a thick layer of clay over sand can have various implications. It can affect the permeability of the soil, making it less suitable for agriculture or construction. The clay layer can also act as a barrier, preventing water and nutrients from reaching the underlying sand layer, which can impact plant growth and biodiversity. Additionally, the thickness of the clay layer can indicate the past environmental conditions and geological processes that occurred in the area.

4. Are there any environmental factors that can cause the clay layer to be thicker than the sand layer?

Yes, there are several environmental factors that can contribute to the thickness of the clay layer over sand. One of the main factors is the type of rock that is being weathered and eroded to form the clay and sand particles. Different types of rocks have varying rates of weathering, which can affect the thickness of the resulting layers. Climate also plays a role, as areas with higher rainfall and temperatures tend to have thicker clay layers due to increased weathering and erosion. Human activities such as land use changes and pollution can also affect the thickness of the clay layer over sand.

5. How can studying the clay and sand layers help in understanding past environmental changes?

The layers of clay and sand can provide valuable information about past environmental changes. By analyzing the composition and properties of these layers, scientists can determine the type of rock that was weathered to form them and the environmental conditions that were present at the time. For example, a thick clay layer may indicate a wetter and warmer climate, while a thinner layer may suggest a drier and cooler climate. Studying these layers can also help in understanding the processes that shaped the landscape over time.

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