Sediment · transport · landscapes
Follow one grain from the mountain that made it to the seafloor that buries it — and learn the whole surface system on the way.
Everything you weather off a mountain has to go somewhere. This is the machine that moves it: water and wind pick up grains, carry them by size and energy, and drop them in places that leave a signature you can read back. Master this and you've got the core of both Sedimentary Geology and Geomorphology in one system.
The source-to-sink profile, mountain to deep sea. Tap any station to see what happens there — the process, the grain size that settles, the structure it leaves, and the landform it builds. No pressure, just wander downstream.
Whether a grain erodes, travels, or settles comes down to two things: how big it is and how fast the water moves. This is the Hjulström diagram — set the current and watch which zone your grain falls into. Then take the challenges.
Challenge
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A deposit is a confession. Grain size records the energy, sorting and roundness record the journey, and structures record the flow. Read the clues and name the environment that made this rock.
“Well sorted” is a useful thing to say and a hopeless thing to compare. Textural analysis turns the same observation into numbers — and once a distribution is numbers, two samples from opposite sides of the world can be put on the same axis.
Grain size spans seven orders of magnitude, so it is worked in logarithms. The phi scale is a negative base-2 log of the diameter in millimetres, which makes each whole phi step a halving of grain size — exactly the Wentworth boundaries — and turns most natural distributions into something close to a normal curve you can do statistics on.
Seven sedimentary environments, each a realistic grain-size distribution. Pick one and read what the statistics say about it — then compare the numbers with the environment they came from.
Verbal classification—
Grading—
No single technique spans the whole range, so any full distribution is stitched together from two or more — and the joins are where errors live.
| Material | Size range | Method | What it actually measures |
|---|---|---|---|
| Unconsolidated | Boulders – pebbles | Direct measurement with calipers | The three axes, one clast at a time |
| Pebbles – sand | Sieve stack, shaken | The intermediate axis — whatever squeezes through a square aperture. A blade passes a sieve its length never would | |
| Silt – clay | Settling: pipette, hydrometer, Sedigraph | Settling velocity, converted to an equivalent spherical diameter through Stokes’ Law | |
| Consolidated | Conglomerate, breccia | Direct measurement on outcrop or slab | Axes of exposed clasts |
| Sandstone, siltstone | Thin section, traverse method | Apparent diameters in a 2-D cut — a random slice rarely passes through a grain’s centre, so thin-section sizes are systematically too small and need a correction | |
| Laser diffraction | Whole range, small sample, fast | Diffraction pattern converted to an equivalent spherical diameter. Fast and reproducible, but it does not measure the same thing a sieve does — results from the two are not interchangeable | |
Settling methods work because a small sphere falling through a viscous fluid reaches a terminal velocity that depends on the square of its diameter. Halve the grain size and it settles four times slower.
Where it stops working
Stokes’ Law assumes laminar flow around the grain, which holds only while the Reynolds number stays below about 1. For quartz in water that fails at roughly 100 µm — above it, turbulent wake drag takes over and Stokes overestimates the settling velocity badly.
Which is a convenient accident: sieving becomes impractical below about 63 µm, and Stokes becomes valid at about 100 µm. The two methods hand over almost exactly at the sand–silt boundary — the overlap is small, but it exists.
It also assumes spheres. Real platy clays settle more slowly than their mass implies, so a settling method reports them as finer than they are — one more reason sieve, settling and laser results on the same sample do not agree.
Graphical measures, computed from percentiles read off the cumulative curve. All values in phi.
| Statistic | Formula | What it tells you |
|---|---|---|
| Mean | Mz = (φ16 + φ50 + φ84) / 3 | Average size. Uses three percentiles rather than the median alone, so a skewed tail still registers |
| Sorting | σI = (φ84−φ16)/4 + (φ95−φ5)/6.6 | The energy indicator. Spread of the distribution — low means one process worked the sediment hard and repeatedly |
| Skewness | SkI = (φ16+φ84−2φ50) / 2(φ84−φ16) + (φ5+φ95−2φ50) / 2(φ95−φ5) | Which tail is longer. Positive = a fine tail (fines that settled out); negative = a coarse tail (a lag the current could not lift) |
| Kurtosis | KG = (φ95−φ5) / 2.44(φ75−φ25) | Peakedness. A sharply peaked curve with wide tails often means sediment sorted elsewhere then dumped somewhere new |
| σI — sorting | SkI — skewness | KG — kurtosis |
|---|---|---|
| < 0.35 very well sorted | +0.3 to +1.0 very fine skewed | < 0.67 very platykurtic |
| 0.35–0.50 well sorted | +0.1 to +0.3 fine skewed | 0.67–0.90 platykurtic |
| 0.50–0.70 moderately well sorted | +0.1 to −0.1 symmetrical | 0.90–1.11 mesokurtic |
| 0.70–1.00 moderately sorted | −0.1 to −0.3 coarse skewed | 1.11–1.50 leptokurtic |
| 1.00–2.00 poorly sorted | −0.3 to −1.0 very coarse skewed | 1.50–3.00 very leptokurtic |
| 2.00–4.00 very poorly sorted | > 3.00 extremely leptokurtic | |
| > 4.00 extremely poorly sorted |
Size tells you about energy. Shape tells you about history — how far a grain travelled, by what, and how many times. And fabric, the way grains sit against one another, records the moment of deposition and everything that happened to the deposit afterwards.
Roundness runs left to right, sphericity bottom to top — the two vary independently, which is the point the grid makes better than any sentence. Click a grain.
Sorting and roundness improve together as sediment is worked, so they can be combined into a single ladder describing how much reworking a deposit has had. It is the clearest single link between a hand specimen and a depositional environment.
Stage 1
Poorly sorted, angular grains, and clay-grade matrix still present. Deposited close to source with little reworking — alluvial fans, till, debris flows.
Stage 2
Matrix largely winnowed out, but sorting is still poor to moderate and grains remain angular to subangular. Some current work has happened.
Stage 3
No matrix, moderately to well sorted, grains subangular to subrounded. Sustained transport or repeated reworking — most river and shallow-marine sands.
Stage 4
Very well sorted, narrow unimodal distribution, grains rounded to well rounded. Beaches and dunes, or sediment recycled through several cycles.
Two cautions. Maturity is not age — a supermature sand can be young if the environment worked it hard, and an immature one can be ancient. And durability confounds it: a quartz-only sand may look supermature simply because everything else dissolved or broke down, not because it travelled further. Compositional and textural maturity usually track each other, but not always.
Your digital grain-size card and cheat sheet — the Wentworth scale, what sorting and roundness look like, and the structures that give an environment away.
Coarsest at the top. Each step is a doubling (or halving) of diameter — the same φ (phi) steps you'll measure in lab.
How uniform the grains are. Well-sorted = a long, selective journey (wind, waves); poorly sorted = dumped fast (glaciers, debris flows).
How worn the corners are. Angular = close to source; well-rounded = carried a long way, especially by water.
Hydrolysis strips cations out of feldspar and leaves clay behind. The Chemical Index of Alteration turns that into a number you can compare between rocks and climates.
The order minerals crystallise out of a melt is very nearly the order they break down at the surface, reversed. A mineral that formed at 1200 °C and 30 km depth is furthest from equilibrium in cool, wet, oxygenated air — so it goes first. Quartz, last to crystallise and closest to surface conditions, survives to become the beach.
Click least stable first. The series is branched, not a single line — the ferromagnesian chain and the plagioclase chain weather in parallel and only merge at potassium feldspar. Either branch may be advanced at any point.
The CIA becomes far more useful plotted than tabulated. Cast a rock as three molar quantities — A = Al2O3, CN = CaO* + Na2O, K = K2O — and its position in the triangle is its CIA: the value equals the point’s height above the base. Fresh igneous rocks cluster along the bottom near 50; weathering drives samples upward toward the clays.
Molar proportions, any consistent scale — only the ratios matter.
You do not need the balanced equations. You need the four slots: what you started with, what attacked it, what was left behind as a solid, and what was carried away dissolved. Get those and the rest is bookkeeping — because those two outputs become two entirely different rocks.
The distinction that organises all of it:
A mineral that dissolves congruently contributes nothing to the detrital sediment pile — it leaves as ions and comes back, somewhere else, as limestone or evaporite. That is why karst terrain has so little soil on it.
The point of tracking solid versus dissolved is that they build different rocks in different places. Solid residues travel as particles and stack up as siliciclastics; dissolved loads travel as ions and precipitate as chemical sediments, often an ocean away. After Boggs, Table 1.3.
| Weathering process | Product type | Example | Ultimate depositional product |
|---|---|---|---|
| Physical | Particulate residues | Resistant silicate grains — quartz, feldspar — and rock fragments of every kind | Sandstones, conglomerates, mudrocks |
| Chemical — hydrolysis | Soluble constituents | Silicic acid; K⁺, Na⁺, Mg²⁺, Ca²⁺ | Cherts, limestones |
| Secondary minerals | Clay minerals — kaolinite, smectite, illite | Mudrocks and shales | |
| Chemical — simple solution | Soluble constituents | Silicic acid; K⁺, Na⁺, Mg²⁺, Ca²⁺, HCO₃⁻, SO₄²⁻ | Limestones, evaporites, chert |
| Chemical — oxidation | Secondary minerals | Ferric oxides and oxyhydroxides; manganese oxides | Minor constituent in siliciclastic rocks — the red in red beds |
| Soluble constituents | Silicic acid; SO₄²⁻ | Chert, evaporites |
USDA Soil Taxonomy. Soil is the weathering interface between rock and sediment — the order tells you the climate and how long it has been forming.
The organising question of process geomorphology: does the landscape get shaped by the everyday event, or the rare catastrophe?
Domain D territory: what ice and gravity leave behind.
The flow signatures — each one narrows down the energy and the environment.