Weathering, Erosion and Sediment Transport
Breaking Rocks Down: Weathering
Before sediment can travel anywhere, rock must first be broken apart. Weathering is the in-place breakdown of rock. It happens at or near the Earth’s surface where rock is exposed to the atmosphere, water and living organisms. Once weathered material is loosened from the parent rock, erosion picks it up and transport carries it away. Understanding these three linked stages — weathering, erosion, transport — is essential for interpreting the sedimentary rock record.
Physical weathering breaks rock into smaller fragments without changing its chemistry. Freeze-thaw action is the most important process in cold climates: water in a crack expands by about 9% when it freezes, exerting pressure on the crack walls until the rock fragments. Repeated freeze-thaw produces angular boulders and scree slopes directly below cliffs.
Chemical weathering involves reactions between minerals and water, oxygen or acids. Feldspar is particularly susceptible to hydrolysis — it reacts with weakly acidic rainwater to produce clay minerals and dissolved ions. Calcite dissolves readily in acidic water (carbonation), which is why limestone landscapes develop karst features such as caves and sinkholes. Iron-bearing minerals oxidise to produce the red-brown iron oxides (haematite, goethite) that colour many red sedimentary sequences.
Grain Size: The Archive of Transport Energy
Grain size is the most directly readable record of the energy conditions in the transport system. High-energy environments — fast rivers, storm beaches, strong winds — move large particles; low-energy environments — deep water, slow rivers, quiet lagoons — deposit only fine particles. Grain size classifications run from boulders and gravel (coarsest) through sand to silt and clay (finest). These categories correspond to specific environments and are used to reconstruct depositional settings from ancient rocks.
Coarse grain size = high-energy transport (fast river, beach, wind). Fine grain size (silt/clay) = low-energy, quiet conditions (deep sea, lake, lagoon). Grain size directly records the energy of the depositional environment and is a primary tool for palaeo-environment reconstruction.
Grain Shape: Recording Transport Distance
Fresh sediment produced by weathering is angular — the sharp corners and edges that broke from the parent rock are still intact. As grains travel, they collide with each other and with the channel bed, gradually knocking off corners and edges. The result is progressive rounding. Rounding therefore indicates transport distance: angular grains have not travelled far; well-rounded grains have been transported over long distances or have been recycled through multiple sedimentary cycles.
The environment also matters: wind transport is much more effective at rounding grains than water transport because air provides less cushioning between grains. Wind-blown (aeolian) sand grains are typically frosted and well-rounded even over shorter distances; river sand grains take longer to round because they are partly cushioned by water. Scree deposits directly below a cliff contain angular fragments — they have barely moved.
Sorting: Reading Energy Consistency
Sorting describes how uniform grain sizes are within a deposit. Well-sorted sediment has a narrow range of grain sizes; poorly sorted sediment mixes coarse and fine together. Sorting reflects the consistency of the energy in the transport system. A beach exposed to regular wave action is highly sorted because the waves repeatedly winnow out fine grains, leaving only grains of similar size. A glacial till is extremely poorly sorted because ice carries everything from clay to boulders with equal indifference and dumps it all together when the ice melts.
Well-sorted = consistent energy environment (e.g. aeolian dune, wave-worked beach). Poorly sorted = variable or powerful depositional event (e.g. glacial till, debris flow). Sorting is a powerful clue to environment in sedimentary rock identification questions.
Screes: Angular, poorly sorted coarse fragments. Short transport, formed directly below cliffs by freeze-thaw. No rounding has occurred.
Rivers: Mixed grain sizes in active channels; gravel and sand bars in braided rivers; finer sands and silts on floodplains. Downstream, grain size decreases and rounding increases as transport distance grows.
Shallow seas: Sand-sized grains, well-sorted by wave and tidal action. Often shows ripple marks and cross-bedding.
Deep seas: Very fine silt and clay that has settled through the water column out of suspension. Also receives turbidite deposits — graded beds of sand to mud triggered by underwater avalanches.
Wind (aeolian) dunes: Well-sorted, fine to medium sand. Frosted, well-rounded grains; large-scale cross-bedding with a characteristic dune geometry.
Porosity is the proportion of empty space in a rock. Permeability is how well fluid can move through those spaces. These two properties are easily confused in exams but they measure different things. A rock can be porous without being permeable if the pores are not connected (for example, pumice has many pores but they are isolated bubbles). Conversely, a fractured rock may be permeable despite having low porosity if the fractures form a connected network.
Well-sorted coarse sandstone tends to have both high porosity (large connected pores) and high permeability (fluid passes easily). Clay-rich shale has very small, poorly connected pores — high porosity can exist but permeability is very low because the tiny pore spaces impede fluid flow. This distinction is critical when students answer questions about groundwater aquifers or reservoir rocks for oil and gas — mixing the two terms is a common and costly error.
The sediment properties covered in this lesson — grain size, shape and sorting, plus porosity and permeability — directly determine which sedimentary rock types form and what environments they record. The next lesson uses this foundation to classify the spec sedimentary rocks and to show how sedimentary structures like cross-bedding and graded bedding add further detail to palaeo-environment reconstruction.