Chapter 2: Erosion & Stratification as Visual Cues

Created by Sarah Choi (prompt writer using ChatGPT)

Erosion & Stratification — Visual Cues for Believable Terrain

Why erosion and layers are your best guides

Environments feel real when surfaces tell a cause-and-effect story. Erosion sculpts the mass; stratification (the layers within the rock) reveals how that mass formed. When you let processes (water, ice, wind, gravity, and chemical weathering) interact with layered materials (sandstone, shale, limestone, basalt flows, ash/tuff, metamorphic foliation), silhouettes and surface details line up. This article translates the geologic logic into design language for environment concept artists and production artists so your terrain looks inevitable, not arbitrary.

The core idea: processes act on materials over time

Before detailing a scene, decide these four: 1) What is the rock/soil made of? 2) How is the climate delivering energy (rain, floods, frost, wind, waves)? 3) Where does water want to go (base level and drainage)? 4) How long has this been happening? These choices predict shape, texture, fracture spacing, and debris size.

Erosion toolset: how each process draws on the landscape

Running water (fluvial)

Water downcuts and laterally migrates. In bedrock canyons it carves slots and potholes; on floodplains it swings in meanders, undercutting outside banks and building point bars inside bends. Where a hard layer overlies soft rock, waterfalls form and migrate upstream as knickpoints. Debris sorts from coarse near steep headwaters to finer sands and silts downstream.

Ice (glacial and periglacial)

Glaciers abrade and pluck, turning V‑valleys into U‑troughs with over‑steepened headwalls and hanging tributaries. Bedrock shows polish and grooves aligned with flow. Periglacial frost‑shattering builds talus cones, rock glaciers, and gelifluction lobes. Post‑glacial rivers inherit oversized valleys and leave ribbon waterfalls where hanging valleys meet the trunk trough.

Wind (aeolian)

Wind deflates fine particles and sculpts dunes whose slipfaces point leeward. Sand‑blasting rounds corners, carves yardangs parallel to prevailing wind, and leaves desert pavements of armored gravel. Loess blankets downwind plains in fine silt, softening relief.

Gravity (mass wasting)

Rockfalls, slides, and debris flows move material downslope when support fails. Talus accumulates at the angle of repose (≈30–37° for loose rock). Warmer, wetter climates host soil creep and slumps that round hills; arid, tectonically active settings favor fresh scarps and blocky aprons.

Chemical weathering and solution

Water dissolves and alters minerals. Limestone and marble dissolve into karst—sinkholes, disappearing streams, caves, and tower karst in humid tropics. Iron‑bearing rocks weather to red/brown skins; desert varnish darkens sun‑baked faces. Hydrothermal alteration weakens volcanic terrains, predisposing them to landslides.

Stratification: reading the rock’s memory

Primary bedding

Sedimentary rocks preserve depositional layers. Beds maintain consistent thickness over distance, unless pinching out into lenses. Color and grain changes mark shifts in environment (river to floodplain, shoreface to offshore). Keep beds parallel unless tectonically tilted or folded.

Cross‑bedding and foresets

Dunes (sand or subaqueous) migrate, leaving angled internal laminae that dip in the paleoflow direction. On cliff faces, these appear as diagonal “chevrons.” Use cross‑beds to imply wind/wave directions and to add scale—sets commonly range from tens of centimeters (river ripples) to several meters (desert dunes).

Graded bedding

Turbidity currents deposit coarse at the base and fine upward. A single bed fining upward reads as an event layer—great for cliffs in submarine fan sandstones. Stack multiples to imply recurrent flows.

Ripple marks, mud cracks, raindrop imprints

Small‑scale textures speak to environment: ripples for shallow water, polygonal mud cracks for drying flats, and tiny cratered pits for intense rain. These belong on bedding planes (top surfaces), not vertical faces.

Unconformities (missing time)

Erosion surfaces cut older rocks before new layers are deposited. In outcrop, an unconformity is a sharp plane truncating tilted beds beneath horizontal ones—instant drama and time jump. Angular unconformities are especially readable in wide shots.

Interbeds and marker horizons

Thin, persistent layers (ash beds, coal seams, chert) act as visual rulers through a scene. Align them consistently around bends and across modules so the world feels geologically coherent.

Volcanic layering

Basalt plateaus build from stacked flows; cooling joints make vertical columns. Welded ash (ignimbrite/tuff) forms soft, bulbous cliffs with caprock‑crowned hoodoos. Scoria cones exhibit bedded cinders dipping outward from the vent.

Metamorphic banding and foliation

Gneissic banding and schistosity impose planar structure that rock and slopes respect. Expect blade‑like ridges, slabs peeling along foliation, and landslides along those planes.

Process × layer interactions: signature landforms

  • Caprock cliff–bench relief: Hard sandstone/limestone atop soft shale yields sheer faces over stepped slopes. Canyons inherit repeating ledge rhythms; alcoves enlarge where weak layers recess.
  • Slot canyons: Cohesive sandstone + rare, high‑energy floods + joint networks = narrow, sinuous corridors with smoothed, overhung walls and chockstones.
  • Karst windows: Streams vanish into limestone, reappear as springs. Expect sinkhole chains, natural bridges, and tower karst in humid climates.
  • Badlands: Interbedded silt/clay with sparse vegetation and intense storm runoff incise tight rills, razor divides, and pedestal hoodoos under resistant ledges.
  • Tepuis/mesas: Horizontal strata uplifted, then edge‑retreated. Flat tops, sheer margins, talus aprons organized by layer strength.

Visual cues and how to use them in concept

  1. Layer continuity: Keep bed thickness and color consistent across the scene, bending smoothly with folds. Breaks only where faults or erosional windows demand.
  2. Dip and strike: Beds dip at an angle; ridgelines and valleys often align along strike-parallel weaknesses. Sketch a few dip arrows early and obey them.
  3. Talus logic: Angular blocks cluster below fresh cliff failures; finer scree feathers downslope. Keep talus angle within 30–37° and thin toward the edges.
  4. Sorted sediments: Coarse gravels at steep gradients; sand bars on inside meander bends; silts/clays on floodplain backswamps. Let textures and vegetation change accordingly.
  5. Knickpoints and falls: Place waterfalls where a resistant bed caps softer rock, at fault steps, at lava flow fronts, or where a hanging glacial valley meets a trunk valley. Add plunge pools and spray‑wet algae staining.
  6. Weathering skins: Use varnish streaks beneath seeps, iron‑oxide halos around rusty layers, salt efflorescence near arid seeps, and lichen patterns on stable, cool faces.
  7. Joint/fracture spacing: Massive rocks (granite, basalt) show wide, orthogonal joints; bedded rocks fracture along bedding plus vertical joints. Repeat spacing to sell scale.

Production translation: materials, masks, and modularity

  • World‑space layering: Map strata with triplanar projection aligned to geologic dip/strike, not UV islands. Use height or scalar masks to blend bed materials at contacts.
  • Slope/aspect masks: Drive material assignment (bedrock vs. talus vs. soil) by slope and curvature; bias snow/ice to shaded and lee aspects.
  • Erosion maps: From heightfields, bake flow/convexity maps to place channels, rills, and wetness decals. Place vegetation along concavities and at fan toes.
  • Module sets by layer: Build a minimal kit: (1) hard caprock cliff, (2) softer ledge with alcove potential, (3) transition bench, (4) talus apron components, (5) collapse blocks. Assemble in long runs to read as a stratigraphic column.
  • Scatter logic: Spawn large blocks near wall bases and along recent slide paths; distribute fines farther downslope. Avoid uniform pebble carpets.
  • Decal storytelling: Add seep streaks, varnish panels, caliche crusts, tufa at springs, tuff welding textures, and travertine terraces where carbonate‑rich waters emerge.

Lighting and color from process

  • Raking light + bedded cliffs: Emphasize the ledge‑shadow rhythm; warm bounce within canyons; cool skylight on upward faces.
  • Wetness cues: Darken rock beneath waterfalls and seeps; add specular micro‑sheen to polished glacial surfaces.
  • Dust and haze: Aeolian plains read with lower contrast, higher sky scatter; canyon interiors hold warm dust veils.

Map‑scale coherence for believable worlds

  • Drainage patterns: Dendritic over uniform rocks; trellis along folded strata; rectangular on jointed crystalline terranes; radial off volcanoes/domes; annular around domes/basins. Ensure tributaries join at acute downstream angles.
  • Terraces: Stepwise benches along rivers indicate pulses of incision (uplift or base‑level fall). Great for paths, farms, or ruins.
  • Alluvial fans and bajadas: At range fronts, individual fans coalesce. Place towns at fan apices where groundwater is accessible; route roads along fan crests.

Quick troubleshooting

  • Layers don’t match around corners: Realign dip/strike; ensure thickness parity before adding micro detail.
  • Waterfalls with no cause: Introduce a resistant bed, a fault step, a lava dam, or a hanging valley. Add corresponding talus/spray.
  • Uniform debris: Re‑grade by slope and source; add block size gradients and fresh scarps.
  • Snow everywhere: Remask by aspect and wind drift; deepen lee pockets and scour windward ridges.

Practice studies for concepting sessions

  • Paint a cliff face with three alternating beds (hard/soft/hard). Carve a canyon and let the soft bed recess into alcoves; add fallen blocks from the upper hard bed and a talus fan at angle of repose.
  • Block out a radial volcanic edifice with stacked basalt flows; cut a river breach that produces a waterfall at a flow front. Place columnar joint talus at the base.
  • Design a karst plateau. Add sinkholes, a disappearing stream, a spring‑fed travertine terrace system, and a cave mouth aligned to dip.

Hand‑off checklist for production

  • Layer directions (dip/strike) drawn on the greybox and provided as world‑space vectors.
  • Material families selected by lithology with notes on fracture spacing and color range.
  • Masks: slope, curvature, flow, aspect, altitude, wetness—exported and named.
  • Prop scatters tied to geomorphology: block fields, scree bands, fan lobes, erratics, tufa.
  • Decal library keyed to processes: varnish, seep stains, caliche, salt crust, travertine, cross‑bed laminae, ripple tops.
  • Rivers validated for continuous down‑grade to base level; knickpoints justified.

Final thought

When you let erosion write the brush strokes and stratification set the sheet music, every rock face, gully, and bench harmonizes. The result is a terrain that feels discovered, not placed—a world where players sense the forces that shaped it, even if they never name them.