By Lambert Strether of Corremte.
Everybody know what mud is, right? Here’s a picture of mud in season from the Bangor Daily News:
The reporter writes:
Mud season in Maine, sometimes called the state’s “fifth season,” generally occurs between March and late April or early May. It happens when the snow and ice start melting. All that extra water leads to a lot of mud…. The mud is deep, the texture often seems more like than hardpan and the damage it can do to your car or your peace of mind is real.
Sadly, I see I have given way to my tendency to be too optimistic about the approach of Spring — I learned to hard way never to plant before Memorial Day when two days of snowish rain rotted all the seeds I’d planted two weeks early — because Mud Season begins in towards the end of March, not the beginning. In fact, it’s still winter, at least in the Northern Hemisphere. Winter, it is said, derives from either from the Proto-Indo-European (PIE) *wed, meaning “wet,” or the the PIE *wind-, meaning “white.” Spring, the 14th century “springing time,” refers to plants “springing” from the ground. Summer comes from the PIE *sam-, meaning… summer, which seems to be a variant of the Proto-Indo-European *sem- meaning “together / one,” as at barbecues, baseball games and so forth. Fall comes from falling leaves (and perhaps, I would speculate failing light). But only Mud Season comes from the soil!
Soil and water do mix. But not always completely! From the Farmer’s Almanac:
Those living in Maine and other New England states, as well as places like Colorado and Montana, know this weather better than most. That’s because mud season occurs in places like these, where the ground freezes in winter and allows large amounts of snow to accumulate and cover it all winter long. As winter wanes and air temperatures warm above freezing (32º F), the ground thaws from the surface down, triggering the snow on top of it to melt. But because the ground’s lower layers deep underground don’t warm as quickly and remain frozen, water from melting snow and chilly spring rains aren’t able to seep down very far. Instead, this water “sits” near the surface where it waterlogs the top layer of soil and creates a sea of mud—sometimes up to several inches thick!
Here, however, is a second picture of mud. From the United States Geological Service (USGS):
And the caption:
from the Clarion-Clipperton Zone, an expanse of the deep Pacific seafloor rich in manganese nodules. Amy Gartman (USGS) and Phoebe Lam (University of California, Santa Cruz) will study chemical interactions between the mud and metals in seawater.
I’m not seeing a Jello texture, here. Much less here. From Space.com, “Mud Cracks on Mars Suggest a Watery Ancient Past,” a photo from the Curiosity Rover:
And the caption:
A Martian rock slab called “Old Soaker” on Mount Sharp has a network of cracks that might have formed from more than 3 billion years ago. This image was taken by the Mast Camera (Mastcam) on NASA’s Curiosity rover on Dec. 20, 2016. (Image credit: NASA/JPL-Caltech/MSSS)
At this point, baffled by the notion of a Jello-like substance that was self-contained yet might, at some point, dry out, I started looking for a definition of mud. From my Webster’s app:
Wet, sticky, soft earth, as on the banks of a river.
Hmm. I see river banks as being muddy, but not mud; they have structure (see “Sediment, Big Spring Run, and River Restoration to Pre-Colonial Times” at NC). From my OED app:
1. Soft wet soil, sand, dust, or other earthy matter; mire, sludge. Also, hard ground produced by the drying of an area of this; colloq. soil. lme.
‣c spec. in Geology. A semi-liquid or soft and plastic mixture of finely comminuted rock particles with water; a kind of this. l19.
The OED’s sense 1 at least conveys the notion that mud by dry out and still be mud, as (possibly) on Mars; but not sense 1(c), the geologists’ definition! With a sigh, we turn to WikiPedia:
Mud is soil, loam, silt or clay mixed with water. It usually forms after rainfall or near water sources. Ancient mud deposits harden over geological time to form sedimentary rock…
Here as with the OED, we have the notion of time introduced, but according to Wikipedia’s definition, whatever Maine has in Mud Season is not in fact mud, since its water portion comes from snow- or ice-melt, not rain.
We also note, in Wikipedia’s definition, a distinct lack of footnotes; apparently there is no authoritative or science-based definition of mud to which they can cite. Can this really be true? Yes, dear reader, it can be — and this has turned out to be the point of this post, it did have one — as an examination of the work produced by two standards bodies will show.
But before quoting great slabs of material on soil science from the United States Department of Agriculture (USDA) and the UN’s Food and Agriculture Organization (FAO), let me justify a focus on mud as a topic to those not already fascinated by it. From Nature in 2015:
Against such enormous societal problems, the Geological Society’s decision to focus on mud might at first elicit a snigger: mud is a problem to face with wellies, not with a global research agenda. But take a moment to consider how mud influences the world of science.
For starters, there is the entire record of the history of life. Fossils preserved in mudstone, such as the exquisite Burgess Shale high in the Canadian Rockies, reveal the story of vertebrate evolution. Without small creatures drowning in and being encased by mud, we would have a much harder time unravelling the relationship between organisms past and present.
Then there is the economic importance of mud-based rocks. Petroleum engineers have been exploring shale as a future source of both oil and natural gas. Although controversy rages about how much shale gas might ultimately be available (see Nature 516, 28–30; 2014), extraction rates have soared in the United States, driven by big reserves such as the Marcellus Shale underlying much of Pennsylvania and neighbouring states. In March, industry and academic experts will gather in London to assess the numbers behind a possible ramp-up in shale-resource production in the United Kingdom.
Finally, consider how soil and mud combine to underpin many of the globe’s natural disasters. Assessing flood risk requires knowing what soils are where, and how likely they are to turn to mud in times of heavy downpours.
The Geological Society’s “decision” mentioned by Nature was to proclaim 2015 The Year of Mud; there’s a kick-off lecture in the Appendix at the end of this post. (It describes one type of mud as having “a cauliflower-like structure erupting on the surface,” which is the sort of thing for which any definition of mud, were there to be one, would have to give an account.). Mental Floss introduced the Year, adding to Nature’s list of why mud is important and interesting:
There’s more to mud than most people realize. According to the Geological Society’s website:
Mud represents both an end and a beginning—the end of the cycle of erosion and transport, and the beginning of the generation (through burial and transformation) of new materials of great value to society.
Mud may begin as wet dirt, but it ends up in all kinds of useful places. Its texture and malleability make it a favorite building material of people all over the world. Liquid mud, or slip, is an essential component of pottery. Sedimentary rocks like shale are actually made of mud. Tourists and spa visitors pay good money to be plastered with therapeutic mud. And millions of creatures, great and small, make their homes in mud puddles, banks, and riverbeds.
So, given the importance of mud as a substance, geologically, ecologically, and economically, I’d would have expected Science (the social organism, not the magazine) to proffer a definition. In fact, I’d expect a taxonomy, as with soil (see NC here). Wet vs. dry is one obvious axis of comparison. Chemical and mineral composition would be another. Water source would be a third. Duration would be a fourth. But such was not to be!
I promised I would look at two authoritative books. Here, I will quote a great slab of material from the USDA’s wonderful (PDF) “Soil Taxonomy: A Basic System of Soil Classification for Making and Interpreting Soil Surveys.”
Sidebar: I’m quoting so much partly because I just love the subject matter; but also as a mental exercise for any citizen scientists and/or activists out there. If, for example, you are fighting a permitting battle against a landfill, or a pipeline, or some other atrocity, texts like these, very much including their taxonomies and classification systems, inform the lingua franca of the science behind the permitting process as understood by the powers that be. So it’s important to master them (even, I would urge, in cases of direct action, since communication with the media and public relations generally is much enhanced by an obvious mastery of the science. Ideally you want the press calling you because you’re a subject matter expert). So becoming literate in documents like this is important. End sidebar.
From the beginning of Chapter One, “The Soils that We Classify”:
The word “soil,” like many common words, has several meanings. In its traditional meaning, soil is the natural medium for the growth of land plants, whether or not it has discernible soil horizons. This meaning is still the common understanding of the word, and the greatest interest in soil is centered on this meaning. People consider soil important because it supports plants that supply food, fibers, drugs, and other wants of humans and because it filters water and recycles wastes. Soil covers the earth’s surface as a continuum, except on bare rock, in areas of perpetual frost or deep water, or on the bare ice of glaciers. In this sense, soil has a thickness that is determined by the rooting depth of plants.
About 1870, a new concept of soil was introduced by the Russian school led by Dokuchaiev (Glinka, 1927). Soils were conceived to be independent natural bodies, each with a unique morphology resulting from a unique combination of climate, living matter, earthy parent materials, relief, and age of landforms. The morphology of each soil, as expressed by a vertical section through the differing horizons, reflects the combined effects of the particular set of genetic factors responsible for its development.
This was a revolutionary concept. … The Russian view of soils as independent natural bodies that have genetic horizons led to a concept of soil as the part of the earth’s crust that has properties reflecting the effects of local and regional soil-forming agents. Soil in this text is a natural body comprised of solids (minerals and organic matter), liquid, and gases that occurs on the land surface, occupies space, and is characterized by one or both of the following: horizons, or layers, that are distinguishable from the initial material as a result of additions, losses, transfers, and transformations of energy and matter or the ability to support rooted plants in a natural environment….
The upper limit of soil is the boundary between soil and air, shallow water, live plants, or plant materials that have not begun to decompose. Areas are not considered to have soil if the surface is permanently covered by water too deep (typically more than 2.5 m) for the growth of rooted plants. The horizontal boundaries of soil are areas where the soil grades to deep water, barren areas, rock, or ice. In some places the separation between soil and nonsoil is so gradual that clear distinctions cannot be made.
The document has no glossary, so it is perhaps not fair to ask that “mud” be defined; however, mud, as such, is not even used (we have muddy water, mudflow material, and mudflows). One would think that mud is “a natural body comprised of solids (minerals and organic matter), liquid, and gases that occurs on the land surface [and] occupies space,” but perhaps, being muddy, it lacks horizons/layers? (Can this be true when it dries?) In any case, mud is firmly refused a place in soil canon by USDA.
We turn now to the FAO’s (PDF) “World Reference Base for Soil Resources” (WRB). Again, the Introduction is magisterial. (It’s a real testimony to the power of state capacity still that such documents are available, for free, on the Internet, and not paywalled.) From the first chapter, the section titled “The object classified in the WRB”:
Like many common words, ‘soil’ has several meanings. In its traditional meaning, soil is the natural medium for the growth of plants, whether or not it has discernible soil horizons (Soil Survey Staff, 1999).
In the 1998 WRB, soil was defined as:
“… a continuous natural body which has three spatial and one temporal dimension. The three main features governing soil are:
• It is formed by mineral and organic constituents and includes solid, liquid and gaseous phases.
• The constituents are organized in structures, specific for the pedological medium. These structures form the morphological aspect of the soil cover, equivalent to the anatomy of a living being. They result from the history of the soil cover and from its actual dynamics and properties. Study of the structures of the soil cover facilitates perception of the physical, chemical and biological properties; it permits understanding the past and present of the soil, and predicting its future.
• The soil is in constant evolution, thus giving the soil its fourth dimension, time.”
Although there are good arguments to limit soil survey and mapping to identifiable stable soil areas with a certain thickness, the WRB has taken the more comprehensive approach to name (Sokolov, 1997; Nachtergaele, 2005). This approach has a number of advantages; notably that it allows for the tackling environmental problems in a systematic and holistic way, and avoids sterile discussion on a universally agreed definition of soil and its required thickness and stability. Therefore, the object classified in the WRB is: . If explicitly stated, the object classified in the WRB includes layers deeper than 2 m.
(Yes, the “like many common words” really does occur in both sources!”) Surely, then, mud forms “part of the epiderm of the earth”? Apparently not. Here is the single reference to mud in WRB:
A terric horizon (from Latin terra, earth) is a mineral surface horizon that develops through addition of, for example, earthy manures, compost, beach sands, loess or . It may contain stones, randomly sorted and distributed. In most cases it is built up gradually over a long period of time. Occasionally, terric horizons are created by single additions of material. Normally the added material is mixed with the original topsoil.
So there you have it; once again, mud is not part of the soil canon. Mud is mentioned, but what is it? Apparently, everyone knows (even if it can form a horizon, contra USDA). So no reason to define or classify it.
Perhaps mud is not defined or classified (at least by the two bodies, USDA and FAO) because it is seen as not amenable to definition or classification. WikiPedia has the following chart at the end of its entry on Mud:
As you can see, mud falls into the “non-systematic” section. And yet non-systematic soil types — described by “common words,” note well — are commercially important (“sand,” “clay,” “loam”), scientifically important (“Lunar soil,” “Martian soil”), and agriculturally critical (“topsoil,” “subsoil”). As we have seen, mud is important, too. It’s very odd. 66% of the rocks exposed on the earth’s surface are sedimentary, i.e., mud can play a key role in their formation. And yet I can find no definition or classification system for the very substance that formed them. It’s very odd. Why?
 I probably should have included videos, besides pictures. Most of the mud videos involving people are weirdly prurient, not suitable for a family blog; but there is also an entire genre of vehicles getting trapped in mud, or pulled out; here is one such. Chacun à son goût….
 For all I know, there’s a standard definition and taxonomy of mud types out there, and Google being what it is, I was not able to find it in the time available to me; readers please weigh in. For example, I would expect a department or institute of “Mud Studies” somewhere; but a search yields no joy. That said, the two sources I cite are at the pinnacles of this subject matter.
 In my travels, I saw definitions for bay mud, mud crabs, mud owls, drilling mud, and MUDs (Multi-User Dungeons (or Dimensions (or Domains))). But nothing for mud as such. See caveats in note .
 I believe this is an example of mud forming a horizon: A high-water mark. From the USGS:
The left picture shows a poison ivy vine with the bottom leaves covered in dried mud. , and, thus, how high Peachtree Creek got during the storm.
And I’m quoting this part not because it is about mud, but because it is wise about methodology:
The right-side picture shows a limb that hangs over Peachtree Creek. During a flood, rapidly-moving water carries leaves, straw, and even whole trees! Wet leaves get stuck on limbs that are partially submerged in the stream. When the stream recedes the leaves remain on the limbs. The top of the leaves and pine straw indicate how high Peachtree Creek was during the storm.
The mud on the poison ivy vine [above] is a much better high-water mark than the tree limb, though. During high water, part of the tree limb will be submerged in the fast-moving water, which will cause it to move up and down. Hydrologists would not use this type of high-water mark to estimate peak stream stage during a flood.
 From the USGS, “What Are Sedimentary Rocks?”:
Clastic sedimentary rocks are the group of rocks most people think of when they think of sedimentary rocks. Clastic sedimentary rocks are made up of pieces (clasts) of pre-existing rocks. Pieces of rock are loosened by weathering, then transported to some basin or depression where sediment is trapped. If the sediment is buried deeply, it becomes compacted and cemented, forming sedimentary rock. Clastic sedimentary rocks may have particles ranging in size from microscopic clay to huge boulders. Their names are based on their clast or grain size. The smallest grains are called clay, then silt, then sand. Grains larger than 2 millimeters are called pebbles. Shale is a rock made mostly of clay, siltstone is made up of silt-sized grains, sandstone is made of sand-sized clasts, and conglomerate is made of pebbles
Here is a lecture given by David Manning, Chair in Soil Science at Newcastle University, given as part of 2015’s Year of Mud:
I was multitasking while I listened to it, but if Manning defines mud, or introduces a classification system, I missed it. It was still extremely interesting, and I think soil fans will enjoy it.