Loess Particles

Loess is, basically, dust. However, it's dust of a certain kind.

During the 1960s the much-mentioned team from Millsaps College, headed by J.O. Snowden, Jr. and Richard Priddy (Snowden & Priddy, 1968), conducted a remarkable battery of tests on Mississippi loess. Therefore, today we have a good picture of exactly what our loess is.

Loess particles have a particular size. Average "dirt" is composed of particles of many sizes, ranging from relatively large grains of sand (0.05-2.0 mm diameter) to particles of silt (0.002- 0.05mm), down to particles of clay (smaller than 0.002 mm). Diameters of most Mississippi loess particles are amazingly uniform, centering in the "medium silt" range. This results from wind sorting. Larger particles were too large for the wind to carry, smaller ones were carried beyond the loess-bluff zone.

Loess particles are more or less spherical, in an angular sort of way -- as if someone had tried to carve a ball in a tremendous hurry and with only a few glancing blows.

Loess particles make soil that drains well. When subspherical loess particles pack together, air spaces remain between them. It's exactly the case as with a jar filled to the brim with marbles, where the jar also contains plenty of air. These air spaces enable water to pass through loess much easier than regular cotton-field silt, where tiny clay particles clog open spaces between larger silt and sand grains. One consequence of this is that often gardeners in the loess zone find that water drains away faster than they'd expect so that they must water their gardens frequently.

Corps of Engineers bluff work at NatchezBut loess becomes gravy when saturated with water. When the ample air spaces mentioned above become filled with water, the roundish loess particles stop adhering to one another. If you place your wet hand in the sand, the sand adheres to it. Then place your wet hand with sand sticking to it into water, and the sand grains fall off. It's the same with a soil's loess particles when they are surrounded by nothing but water and other particles. Just the opposite to partly dried-out loess, water-logged loess becomes a slush. The picture at the right shows the loess bluff at Natchez where the U.S. Army Corps of Engineers had to do some very fancy and expensive work to keep the loess bluff from collapsing whenever it became saturated with water.

Loess particles are composed of certain minerals. Like sand at the beach, loess is mostly (about three-quarters) quartz, SiO2, even though loess particles are very much smaller than sand particles. The Millsaps College team found that of the remaining one-quarter non-quartz minerals, loess is about 20% carbonate, mainly in the dolomite form, CaMg(CO3)2. The next largest component, at about 6.5% of content, is potash feldspar, KAlSi3O8. By the way, the 20% carbonate has meant a lot to farmers in the loess zone.  Early farmers in the Natchez area used unleached loess to neutralize their soil when it became too acidic

About 2% of loess content is composed of a hodgepodge of minerals which look very interesting through a microscope. There are green and brown varieties of biotite, clear muscovite, pale-green to brownish green epidote, red and yellow rutile, pale yellow staurolite, brown and green forms of tourmaline, almost spherical, colorless grains of zircon, green hornblende, black ilmenite, magnetized grains of magnetite, reddish hematite, and even pale pink and colorless varieties of garnet, and more.

sand and gravel

There's "little loess kids" in loess. If you hike the bottom of our area's loess-zone ravines or bayous you're bound to find strewn in the channels' gravel and sand something looking like the picture above, which is about a foot long and taken from a deep bayou on Laurel Hill Plantation 12 miles south of Natchez. This is a loess concretion, composed of the mineral calcite, which is a form of carbonate. These concretions are sometimes called by their German name Loesskindchen, which means something like "little loess kid."

Now, there are different kinds of loess-embedded concretions, and some would say that the term Loesskindchen should only refer to a particular type of carbonate concretion not seen in the Lower Mississippi Valley. Real Loesskindschen are small and rounded, often form around snail shells rather than around roots as they do down here, and are found in the Upper Mississippi Valley, as in Iowa, instead of our Lower Mississippi Valley.

I have seen other geologists referring to our cylindrical form of concretion as  Loesskindchen and I really am tickled by that name, so despite others' preference for reserving the name Loesskindchen for Yankee loess-concretions, I'm continuing to use it for our "root-form" kind, too. Besides, if we don't call them Loesskindchen, then what do we call them? By the way, some geologists theorize that certain cylindrical  Loesskindchen are formed away from tree roots, in down-draining soil fissures.

Loess face in bayou near Natchez Loess particles erode steep faces, as shown in the picture at the left.  There you can see this site's author, Jim Conrad, standing next to a bluff wall inside a bayou near Natchez, Mississippi.Why does loess erode such steep walls?

One theory points out that loess is not completely free of tiny clay particles. However, instead of these clay particles clogging air channels as in average soils, there are so few of them that they cling to the surface of much larger individual loess particles, forming a kind of thin clay husk around each grain. Studies by E.L. Krinitzsky and W.J. Turnbull lead them to believe that loess's steep-slope property is largely or mostly due to these clay husks bonding to one another.

Other workers suggest the water coating of each loess particle causes it to stick to other particles.Earl Manning in Tulane's Geology Department seems satisfied with the simple observation that loess produces steep walls "just because the very homogeneous silt particles pack together so well (with or without any added clay). Most sediment is reduced by slumping to the standard angle-of-repose because there are planes of weakness where different sized particles are adjacent. Loess defeats the normal angle-of-repose."

Whatever the cause, it's for certain that ravines, or bayous, eroded into loess soil have very steep sides, and that's proved to be very important for the Loess Hills' ecology and history.

Loess comes in two different colors: Earl Manning also point this out to us: "Fresh, unweathered loess, which has not yet oxidized the iron from ferrous to ferric, is bluegray in color, and often contains carbonized plant remains. As it's exposed, it turns to the yellowish tan most people think of. Often, in a fresh-cut exposure, there's a sharp line dividing the gray part below and the tan part above. This is an important point to a geologist, as people confuse the two types as two different beds."