Chapter 3 – Coasts, Beaches, Cliffs, Islands, Arches

Chapter 3: Coasts, Beaches, Cliffs, Islands, Arches

Created by Sarah Choi (prompt writer using ChatGPT)

Coasts, Beaches, Cliffs, Islands, Arches — Formation Logic for Environment Concept Artists

Why shoreline geology matters for believable scenes

Coasts are where planetary energy concentrates: waves, tides, currents, storms, river sediment, and sea‑level change all collide along a narrow strip. When you understand how those forces shape rock and sand, your coastlines will compose themselves—bays nest inside headlands, beaches migrate with seasons, barrier islands roll landward, and sea arches collapse into stacks on a schedule you can visualize. This article turns coastal geomorphology into practical design language for both concept and production artists.

The controlling variables: energy, sediment, rock, sea level, and planform

Wave/tide energy: Swell height, period, and tidal range set the power available to move sediment and chew rock. High‑energy coasts are stripped to bedrock or coarse shingle; low‑energy coasts accumulate fine sands and muds.
Sediment budget: Sources (rivers, cliff erosion, offshore banks, bio‑producers like coral) must offset sinks (submarine canyons, longshore export, dune trapping). Where supply exceeds removal, beaches prograde; where it falls short, they retreat.
Rock type & structure: Resistant headlands (basalt, quartzite) hold capes and cliffs; weak rocks (shale, marl) recess into bays. Joints and bedding dictate notch lines, caves, arches, and stacks.
Sea‑level and subsidence/uplift: Rising seas drown valleys into estuaries and carve new shorelines higher on cliffs; uplift strands wave‑cut platforms as terraces.
Planform feedback: Waves refract around headlands, focusing energy on points and protecting bays—driving coasts toward smooth, logarithmic spirals with pocket beaches.

Beaches: dynamic conveyors, not static strips of sand

A beach is a mobile wedge of sediment shaped by alternating seasons and storms.

Cross‑shore profile: From offshore bars to breaker zone, foreshore (swash zone), berm crest, and back‑beach dunes or bluffs. Storms flatten berms and move sand offshore into bars; calm periods rebuild steep berms as waves gently return sand.
Longshore drift: Oblique waves drive along‑coast transport; spits, baymouth bars, and tombolos (sand ties to islands) grow down‑drift. Interruptions (headlands, jetties) create up‑drift accretion and down‑drift erosion.
Grain size: Coarse cobble/shingle beaches form steep, reflective profiles with loud surf; fine sand produces wide, flat dissipative beaches with spilling breakers. Color cues come from source rock (basaltic black, coral white, garnet red).
Dunes: Wind lifts dry sand landward to build foredunes stabilized by grasses; dune fields march inland where vegetation is sparse.

Concept cues: Place storm wrack lines, berm scarps after heavy weather, and crescent bars just seaward of the breaker line. Let rip currents cut temporary gaps in bars. Vary beach width by season or exposure: wider in sheltered coves, pinched around headlands.

Production tips: Drive wetness/darkening masks by swash elevation; add micro‑ripples aligned with recent wave direction; scatter driftwood, kelp, and shell lag at the highest wrack line; carve vehicle/foot tracks only in damp, cohesive sand near the swash zone. Ensure longshore continuity—if a jetty exists, show up‑drift sand build‑up and down‑drift deficit.

Rocky coasts and sea cliffs: headlands, notches, caves, arches, stacks

Wave attack concentrates at headlands. Mechanical abrasion (sand/pebble “sandblasting”) and hydraulic action exploit joints and bedding planes.

Undercutting & notches: A wave‑cut notch at high‑energy tide levels destabilizes the cliff above; collapses retreat the cliff, leaving angular talus that storms quickly remove in very energetic settings.
Sea caves → arches → stacks: Where a jointed headland is attacked on both sides, caves meet to form an arch. Collapse of the roof isolates a stack. Eventual truncation yields stumps and skerries.
Wave‑cut platforms: At the foot of retreating cliffs, a bench is planed at roughly wave base. If sea level falls or land uplifts, old platforms become marine terraces stepping up the slope.
Differential erosion: Interbedded hard/soft layers create tiered cliffs and pocketed notches; cross‑bedded sandstones display sweeping laminae visible in cliff faces; columnar basalt shows hexagonal prisms and tabular talus.

Concept cues: Align arches and stacks with structural lines—arches span along pre‑existing weaknesses, not random holes. Repeat bed thickness and color through every isolated stack. Add collapse blocks at arch toes and fresh bright scars where rock recently parted.

Production tips: Use slope/curvature masks to keep bedrock exposed on vertical faces; place spray‑wet decals, sea foam streaks, and orange iron/rust films in splash zones; add bio‑bands (dark algae in lower intertidal, barnacle/limpet roughness, then lighter lichen above). Spawn loose blocks just landward of the platform edge only in lower energy bays; high‑energy points should look “swept clean.”

Islands: how to read their ancestry

Islands are identities written in planform, profile, and rim detail.

Volcanic arcs and hotspots: Cones and shields with radial drainages, lava benches, black sand beaches, cinder slopes, and sea caves at flow contacts. Coral atolls may rim subsiding volcanoes: outer reef, lagoon, and motu islets.
Tectonic fragments: Fault‑bounded slices with linear cliffs and triangular facets; uplifted blocks may carry marine terraces and tilted shorelines.
Carbonate platforms: Low, bright, sandy islands with surrounding reefs, blue holes, and karst sinkholes inland. Expect blowholes along exposed rims and beachrock ledges at swash level.
Glacially carved islands: Fjard and skerry coasts: smoothed roches moutonnées, striations, and countless islets aligned with ice flow.
Barrier islands: Long, narrow, low‑relief sand bodies parallel to mainland. Inlets migrate; overwash fans spread landward during storms; the whole island “rolls” toward the mainland over time.

Concept cues: Give each island a consistent origin story and let that choose the palette—basalt darks and columnar joints vs. coral whites and cyan shallows vs. sandstone golds and green pocket forests.

Production tips: For barriers, keep elevation modest and allow overwash “saddles”; add dune ridges with blowouts. For volcanic rims, implement stacked flow benches and column talus at capes. For carbonate keys, blend sub‑tidal reef materials to bright sand with seagrass patches on the lagoon floor.

Arches and natural bridges: failure with reasons

Arches grow where stress concentrates around voids.

Marine arches: Form along sea cliffs where waves exploit joints. Expect smooth, fluted inner walls, a debris cone beneath the landward abutment, and spray‑wet algae streaks.
Eolian/stream arches in deserts: Result from differential cementation and cross‑bed weathering; not coastal but can appear inland on uplifted former shore facies.
Longevity & collapse: Thick, strong caprock can hold wide spans; thinly bedded or highly jointed stone collapses quickly. After failure, stacks or buttresses remain—keep broken blocks aligned with former bed planes.

Concept cues: Avoid placing arches randomly. Tie them to headland tips or joints that also appear in adjacent cliffs. Echo their bed layering through the arch and into detached stacks.

Estuaries, deltas, lagoons: mixing zones

Where rivers meet the sea, sediment and salinity gradients build complex mosaics.

Deltas: Form where river supply exceeds marine removal. Planform reflects wave, tide, or river dominance—smooth lobate shorelines (wave), branching tidal channels (tide), or protruding “bird‑foot” distributaries (river). Subsidence and compaction create natural levees and backswamps.
Estuaries: Drowned river valleys with brackish stratification. Tidal flats ripple with mud and sand; salt‑marsh platforms step up to levees and higher ground.
Lagoons & barrier systems: Behind barriers, quieter waters settle fine sediments and host seagrass; tidal inlets focus strong currents and sandy shoals.

Concept cues: Place industries and towns along natural levees and stable Pleistocene terraces, not in backswamps. Let channels migrate; show abandoned meander scars and oxbow lagoons inland.

Production tips: Use flow maps to organize tidal channels with branching at acute angles; blend mudflat sheen at low tide with wrinkled desiccation polygons higher up; sprinkle shell hash and marsh hummocks by elevation.

Sea‑level change and coastal terraces: time in steps

Pulses of sea‑level rise and fall, plus tectonic uplift, leave stair‑stepped shorelines. Marine terraces with fossil beachrock or notch lines climb hillside flanks—excellent natural highways or settlement benches in level design. Strandlines on lake coasts work similarly.

Climate overlays: temperate, tropical, polar

Temperate storm coasts: Frequent cyclones rework beaches seasonally; pebble/cobble shores are common near glacially sourced catchments. Kelp forests thrive in cold, nutrient‑rich upwelling.
Tropical trade‑wind coasts: Coral reefs, white sands, mangroves in sheltered bays, and intense rainfall gullies on volcanic flanks. Salt spray prunes windward trees into “salt‑shapes.”
Polar/fjord coasts: U‑shaped troughs plunge below sea level; hanging valleys end in waterfalls; sea ice grinds platforms; bergs strand boulders on beaches.

Concept cues: Vegetation belts track salt tolerance and spray exposure: bare splash zone → lichens/algae → succulents/saltwort → grasses/shrubs → coastal forest inland.

Traversal and settlement logic on coasts

Routes follow the backs of dunes, tops of marine terraces, and along tombolo necks at low tide. Ports prefer natural harbors behind headlands, river mouths with stable levees, or lagoons with inlet control. Lighthouses crown points above fog belts. Fishers stash boats in pocket coves sheltered by stacks at fair‑weather angles.

Common pitfalls and fixes

Random arches/stacks: Tie them to joints and adjacent layer geometry; add collapse rubble and notch lines.
Beaches with no sediment source: Introduce up‑drift rivers, eroding cliffs, or offshore bars; otherwise, shrink the beach and expose bedrock.
Uniform beach wetness: Add swash gradient and distinct wrack lines; darken sand beneath recent waves and lighten dry back‑beach.
Headlands without wave focus: Show refracted wave crests bending around points (foam streaks) and calmer water in lee bays.
Barrier islands too tall/rocky: Lower relief, add overwash fans and migrating inlets; keep cores sandy with sparse shrubs, not sheer cliffs.

From thumbnail to level build: a workflow

Pick an energy regime (swell, tide, storminess) and a sediment story (source and sinks).
Sketch the planform: headlands and pocket bays shaped by wave refraction; add longshore drift arrows and sediment traps (points, jetties, canyons).
Choose lithology for rocky segments and set joint/bedding directions. Place potential arches where joint sets intersect cliffs.
Lay in vertical zoning: intertidal bio‑bands, spray zone, back‑beach dunes, terraces.
Add time markers: marine terraces, stranded platforms, abandoned inlets, overwash fans.
In production, drive materials with slope/aspect/wetness; use flow maps for swash and tidal channels; keep longshore continuity in the sand budget across LODs.

Lighting and FX grounded in process

Raking light across ripples and berm scarps sells micro‑relief; specular streaking reads wet swash; volumetric sea spray blooms at headlands during storms; subsurface scattering in clear tropics reveals reef textures beneath waves; muted, milky surf signals suspended silt near river plumes.

Quick reference checklist

Does the pocket bay curve like a refracted spiral with a headland at the focus?
Is there a credible sediment source feeding each beach segment?
Do arches/stacks align with joints and repeat the cliff’s layering?
Are wetness, foam, and bio‑bands correctly zoned by tide/splash height?
Do barrier islands show overwash saddles and migrating inlets rather than rocky cliffs?

Believable coasts emerge when you let waves, rock, and sand negotiate. Give them reasons to be where they are—then follow the logic from planform to pebbles. Your scenes will feel lived‑in by the sea, not just decorated with it.