Specialists in sedimentary rocks study their details using precise, often automated measurements in the laboratory that are then analyzed with statistics and indexes. But this list is here to help you observe the more basic set of sedimentary textures that can be seen in the field with the naked eye or a hand lens. If you see enough rocks, these textural features become easy to spot and evaluate, almost automatically—but always look twice anyway.
Clastic or Crystalline Texture
Conglomerate has a really large-scale clastic texture.
This is a very basic feature of sedimentary rocks. Clastic texture
means that the rock is composed of clasts: pieces of rocks or minerals. "Clastic" comes from the Greek word for "broken," and clasts are worn-down things. The general run of sandstones, mudstones and conglomerates are called siliciclastic rocks, composed of clasts of the common silicate rocks and minerals and laid down in bodies of water. Pyroclastic texture
refers to a rock made of clasts of volcanic origin. Crystalline texture
means that the rock consists of things that grew instead of broke. Crystalline sedimentary rocks are composed of particles made by living things (plankton tests, seashells, peat and so on), mineral cements, and chemical precipitates (rock salt, gypsum and so on). The term is not used often—mostly for limestones.
Gravel has the largest grain size, greater than 2 millimeters.
The size of particles is rigorously described with the Wentworth scale
and similar tools. But the four gross divisions of the size scale are easy to use: clay, silt, sand and gravel. Clay is microscopically small, smooth to the touch and teeth
. Silt is gritty but invisible, with particles up to 1/16th millimeter in size. Sand is familiar, and gravel is anything larger than 2 millimeters. The coarse, medium and fine subdivisions of those categories can be learned too with a little practice. Particle size is the primary way of assigning the right name to siliciclastic rocks: shale, siltstone, sandstone and conglomerate.
Clasts in this conglomerate are very similar in size.
Sedimentary rocks are made of grains in a mixture of sizes. Sorting refers to the range of sizes in that mixture (officially, the standard deviation of grain sizes); for instance, a rock that is mostly medium sand
, with a small size range, is well sorted. Sorting is pretty easy to gauge once you're aware of it, but careful observations can tease out subtleties. For instance, this conglomerate
is better sorted than this one
. Sorting can provide information about the history of a sediment before it settled down and became rock.
Beach cobbles quickly become well rounded.
The most obvious thing about the shape of grains is their roundness. The experts go through tedious measurements to get numerical values of roundness, but it's easy to study a rock by eye, especially with the help of simple charts, and distinguish the six degrees of roundness. There are three angular divisions (very angular, angular and sub-angular
) and three rounded ones (sub-rounded, rounded and well rounded
). Roundness can tell us how far away the sediment is from the place it formed, and how it was carried away from there. A more esoteric measurement is grain form, a geometric scheme that ignores roundness and places grain shapes into bladed
categories. And sphericity is a geometric index of how far grains have been worn down toward the ideal of a sphere: a cube has an equant grain form but low sphericity. Form and sphericity were invented by professionals, who are still not sure how useful they are.
Strong winds gave these ventifacts a polished surface.
At the smallest scale, individual sediment grains (typically quartz grains) may show various textures on their surfaces: pitting, frosting, notching, scratching, etching and so on. The experts use microscopes to study these, but a hand lens may show you the typical frosted finish of sand grains carried long distances by the wind. It's always worth looking closely at sand grains, if only because they're beautiful and individual.
Clast packing is important in coarse clastic rocks.
Fabric is the name for features that are larger than single grains, but smaller than sedimentary structures
. There are usually two important fabric features to look for. First is the orientation of grains—how they're lined up (or not). That's an indicator of water currents; for instance, platy grains lie flat and prismoidal ones line up with currents. In particular, look at gravel-size grains for signs of imbrication
. The second fabric feature is packing, how grains fit together. This is primarily important in conglomerates, where you want to notice whether the large grains are clast supported
(the grains are touching each other, a sign of river transport) or matrix supported
(the grains are held apart, a sign of mass wasting
or a dense slurry called a hyperpycnal flow).