Rock formations of the Olympic Peninsula, and how games build them

Question: Day Hike, a co-op hiking game in Babylon.js, is set on a fictional stretch of the Olympic Peninsula's coast and foothills. Its steep ground reads flat: a pale cobbled hillside, a rock band that should look jagged and three-dimensional, and cliff models that stand in rows rather than making a wall. How do rock formations actually occur in the Pacific Northwest, what do they look like at the 10 to 100 m a walker sees them from, and how do AAA games build convincing ones?

Short answer: the Olympics are sea floor stood on end. Sandstone, shale and slate scraped off the Juan de Fuca plate were folded until their beds stand steep or vertical, and a horseshoe of pillow basalt, estimated at up to 7.6 km thick on the northern Peninsula, wraps them [1], [2], [3]. Ice cut cirques into them, bowls whose walls run to about 200 m in comparative data; frost breaks the walls into talus that rests at roughly 28 to 36 degrees; and rivers and waves cut bluffs of 15 to 90 m and leave stacks of the harder rock standing in the surf [4], [5], [6], [1]. What a walker reads is structure at three scales: blocks of 0.1 to 2 m, ledges and beds of a few metres, and walls of tens to hundreds of metres. Games get there the same way almost everywhere: the terrain carries the large shape, and meshes carry the rock. Cliffs and rock assemblies are authored as kits, placed by hand or by rules read from the terrain, and blended into the ground at the seam [7], [8], [9], [10], [11]. A height field alone cannot make an overhang or a sharp facet at the spacing a game can afford [12], [13]. Browser engines have instancing, triplanar materials and compute, but no tessellation stage and no production virtual geometry, so a browser game has the cliff kit and the shaped height field to work with, not the tessellated ground of the last console generation [14], [15], [16].

The game is public at game-dayhike; this note pairs with the lakes note [17], whose cirques are the same ones.

Part one: how rock formations occur in the Pacific Northwest

The Olympic Mountains are basalt and sedimentary rock laid down offshore between about 57 and 18 million years ago, then uplifted, bent, folded and eroded as the Juan de Fuca plate was forced under North America [1]. The sedimentary core began as turbidites: undersea landslides of mud, sand and gravel that hardened into shale, sandstone and conglomerate, and under more pressure into slate. The subducting plate scraped them off and wedged them beneath older rock, so that, in the Park Service's words, rocks laid down on the ocean floor "now stand vertically" [2]. The mapped structure is a core of folded, faulted and overturned beds, wrapped by the basalt horseshoe [18]. The core units run from intact interbedded sandstone and slate to "broken formation", sandstone pulled apart into lenses in a matrix of slate or phyllite [18].

The horseshoe is the Crescent Formation: mostly oceanic basalt of the early and middle Eocene, erupted on the sea floor as pillows and flows, and the oldest rock in the range with a few exceptions [2], [18]. It is pillow basalt, flow breccia, amygdaloidal basalt and tuff; it thins westward from an estimated 7.6 km (25,000 ft) in the southeast to 2.0 km (6,500 ft) near Bear Creek [3]. It forms the ridges and peaks along the north, east and south margins of the Olympics, Hurricane Ridge among them [2].

The range still rises. Uplift and erosion are in balance at about 1 mm a year [2]; thermochronology fits an erosion rate of 0.9 to 1.0 mm a year, and a wedge in steady state since about 14 million years ago [19]. So Mount Olympus, 2,432 m (7,980 ft), is not getting taller [2]. For a game the consequence is that rock everywhere is being broken down as fast as it rises: fresh faces, steep gullies, and debris below them.

Two ice systems worked the range. The Cordilleran ice sheet advanced about 16,900 years ago and stood until about 14,000 years ago, about 1,100 m thick [1]. It split against the Olympics, scoured their north and east edges, and dropped granite carried from British Columbia on their slopes: boulders of rock types from the North Cascades and the Coast Ranges lie up to about 1,070 m (3,500 ft) around the north and northeast end of the range, though "there is no bedrock of granite anywhere in the Olympics" [20], [2]. The Park's handbook records granite erratics 40 km (25 miles) up the Elwha and at about 900 m (3,000 ft) on Klahhane Ridge [21].

The second system was the range's own glaciers. They flowed from high cirques down the valleys, as much as 40 to 64 km (25 to 40 miles) down the Hoh, Queets and Quinault, and a terminal moraine still dams Lake Quinault [21]. A cirque is a bowl with walls on three sides and an open lip. In a comparative dataset of 1,593 cirques the medians are 625 m long, 656 m wide, with walls 210 m high and a steepest headwall gradient of 57 degrees [4]. Most Olympic mountain lakes lie in such bowls between about 900 and 2,000 m [17].

Frost and talus. Water freezing in joints expands by about 9 %, enough to open cracks or wedge blocks loose, though how much force ice in open joints can apply is debated [22]. What falls gathers at the foot of the wall as talus (scree). Its slope is the angle of repose of its debris: the average slope of talus cones at four sites in the Alps and on La Réunion was 28 to 36 degrees [5], and natural repose on talus in Lanzarote and the South Shetlands 28.0 to 33.4 degrees [23]. The textbook account is that a talus slope is sorted, larger blocks running out to its foot [22]. A measured study of runout found no clear relation between block size and how far a block travels, and could neither confirm nor reject that sorting [5]. A game should not lean hard on it. By definition scree is "a collection of broken rock fragments at the base of a cliff or other steep rocky mass that has accumulated through periodic rockfall" [22]: a deposit with a source above it, not a surface that any slope of the right angle carries.

Rivers. The larger western rivers have cut through 30 to 90 m (100 to 300 ft) of Pleistocene deposits into older bedrock, and the Hoh and Queets valleys carry at least two prominent terraces [6]. Cliffs of 15 to 90 m (50 to 300 ft) are common "along the coast and at many places along the principal streams" [6].

Waves. The coast is cut in the Hoh rock assemblage, a mélange: hard volcanic rock and sandstone held in softer mudstone. Waves remove the mudstone and leave the hard rock as sea stacks, as at Ruby Beach and Third Beach [1]. At Ruby Beach, massive, badly fractured sandstone forms the lower 15 m (50 ft) of the cliffs and the stacks on and off shore, its fractures filled largely with calcite; from Abbey Island north to the Hoh the cliffs in many places rise over 46 m (150 ft) [24]. Stacks fail as well as form: a 15 m (50 ft) stack at Rialto Beach collapsed entirely in a winter of storms around 2015 to 2016 [25]. James Island at La Push, a former stack, is 49 m (160 ft) high [26].

Part two: what they typically look like

At a walker's distances the three scales that matter are these. Blocks of 0.1 to 2 m make talus and the surface of a crag; a pillow is one. Ledges and beds of 1 to 20 m set the profile of a wall. Walls of 20 to 200 m set the skyline: a cirque headwall, a stack, a bluff. A slope that shows only one of these reads as a texture. A slope that shows two reads as rock.

Rock colour. The core sandstone is light grey with visible grains, the shale dark grey to black, and the slate splits along smooth fractures [20]. In the photographs below, the crags of the northern and eastern peaks are dark grey to brown and the pillow basalt grey under pale lichen. The coastal sandstone's fractures are filled largely with calcite [24]. A walker on the north side sees white granite boulders that match nothing around them [20].

Lichen and moss. Crustose lichens grow so tightly on rock that they cannot be removed without damaging it. In the neighbouring Cascades they include the map lichens (Rhizocarpon), common in the alpine zone, and the bright orange sunburst lichen [28]. In the photographs the rock is patched, not uniform: grey-green crusts on the open faces, moss and grass on every ledge that holds soil, and on the coast trees on the stack tops.

What the photographs show. All are on Wikimedia Commons under the licences given in the sources.

• Sea stack at Ruby Beach [29]. A single stack perhaps 15 to 20 m high, judged by the surf (an estimate), dark brown-grey, its outline stepped in blocks, and a near-vertical seaward face. Vegetation is confined to a few tufts on the top.
• Point of the Arches [30]. A large pyramidal stack capped by conifers and grass, its lower half bare grey rock streaked with lichen. Beside it stand slender pinnacles 2 to 4 times as tall as they are wide, also an estimate from the image. The beach between is studded with dark, weed-covered reef rocks a metre or two across.
• Second Beach [31]. A flat-topped, sheer-sided stack with a forest on its top. The black-green lower walls are cut by vertical joints. Lower, squared-off stacks stand farther out.
• Rialto Beach, Hole-in-the-Wall [32]. The inside of an arch, wet and nearly black, framing a tall stack. The stack's faces are planar facets meeting at sharp edges, with trees on the summit.
• Rocks off Beach 4, Kalaloch [33]. Seen from a forested bluff top, low dark rocks scattered in the surf, a few metres across, with Destruction Island on the horizon.
• Pillow basalt, eastern Olympics, at 47.86 N, 123.05 W by the file's coordinates [34]. A crag of rounded, bulbous grey lumps about the size of pillows, each outlined by dark cracks. Pale lichen crusts cover the tops, and grass and wildflowers fill every gap.
• Steeple Rock on Hurricane Ridge [35]. A lone fin of pale grey-tan rock jutting from a forested slope. Its faces are split into slabs, and small pale scree runs from its foot into the meadow.
• Mount Angeles from Eagle Point [36]. From several kilometres the rock is only at the summit: a dark, reddish-grey crag with gullies, the forest climbing to its foot. Pale scree chutes run down the meadows on its right.
• Sundial from Royal Basin [37]. A dark crag of many small towers, its ledges and gullies picked out by snow. The ledges are short and broken and step up the face at irregular intervals. They do not run continuously across it.
• Royal Basin from Royal Lake [38]. The most useful single image for a game. Dark crags above, then smooth grey talus cones and aprons fanning from every gully, uniform in slope and paler than the crags. A lobe of large dark blocks runs to the lake shore. A lone boulder a few metres across sits on the meadow in front, and grass reaches the foot of the talus.
• Seven Lakes Basin [39]. A basin floor of pale, smoothed bedrock slabs and boulder litter holding small tarns. The rock is lighter and smoother than any wall above it.
• Talus near Mount Angeles [40]. A close view of a talus surface: angular fragments of 2 to 15 cm (judged by the flowers on it, an estimate), grey, red-brown and buff, packed with no soil showing.

Three things recur across them. Walls are darker than the talus beneath them: on crops of the Royal Basin image the talus cones' median linear luminance is 2.2 to 4.5 times the crags', and the crags' coefficient of variation 1.1 to 1.6 against the talus's 0.6 to 0.9 [38]. That is one photograph under one sun, so it is a direction, not a constant. Every ledge that can hold soil is green. And the outline of rock, at any distance, is made of straight segments meeting at angles, never a smooth curve.

Three consequences for a walker's view follow from the sections above.

• The rock class of a slope is set by its angle. Loose debris rests at up to about 28 to 36 degrees, the talus slope's angle of repose [5], [23], [22]; ground standing steeper than that is bedrock. A game that paints its steepest band as loose scree and the gentler band as solid rock has the order backwards.
• Talus needs a wall above it, and covers the ground. It accumulates from rockfall at the base of a cliff [22]; in the photographs it is a packed surface of fragments, not a scatter [38], [40].
• The silhouette is made of facets and steps. Bedding, shearing and jointing break walls into ledges and blocks [18]: short, broken ledges on the Sundial face, planar facets on the Rialto stack [37], [32].

Part three: how AAA games create realistic rock formations

Every documented open-world pipeline splits the job. The terrain, a height field, carries the large landform; meshes carry the rock that a height field cannot.

Horizon Zero Dawn's world data shows the split in its formats. A terrain height at 0.5 m sits beside a separate object height layer at 0.5 m. There are 1 m maps of rock colour variance and of lichen density, and 0.5 m maps of erosion wear, flow, deposition and terrain cavity. Together that is about 4 MB per square kilometre, all generated and all paintable, read by a GPU placement system that "assembles fully-fledged environments while the player walks through them" [10]. For Horizon Forbidden West, Guerrilla first hand-annotated "lots of the rocks, the cliffs, the mountain sides" for climbing. It then switched to a system that detects a handhold in the geometry itself [41], which only works if the cliffs are meshes. On Ghost of Tsushima the world team sculpted the terrain by hand and authored custom cliffs and rock arrangements, for which one artist sculpted more than 100 rocks, weighing "shader versatility" and "low visual noise" [7]. The GPU placement language and runtime systems behind that world are in Sucker Punch's GDC talk [42].

Generated from the terrain. Far Cry 5 built Houdini tools that, among biomes and rivers, "generate cliff rocks" over 100 square kilometres whose terrain changed daily [8]. Ghost Recon Wildlands scattered rocks with Houdini by rules read from the terrain: material, curvature, alignment to slope, cliff and road detection. Its rock materials used detail maps with masks "for better sharp blending" and per-ecosystem dirt, snow and moss layers [43].

Assembled by hand, reused by rule. Unreal's Electric Dreams sample builds its large cliff from "Assemblies", arrangements of rock meshes saved as data and dropped into the world. One assembly can be laid along a procedurally generated path, and a flat-area detector places "hero rock assemblies" without floating them [9], [44]. A cliff is designed once as a group of rock pieces and then placed as a unit.

Captured from the real thing. Kojima Productions scouted Iceland and converted its photographs to 3D data for "rocks, cliffs, lava, or terrain" in Death Stranding [45].

Kept small. On Days Gone, a senior environment artist recommended "weighted normals and tiling materials with vert blends for as much content as you can". The aim was a small, flexible library varied by material and colour swaps and by placement rather than by unique meshes [46].

What these share: the rock a player looks at is a mesh, placed by hand or by rules read from the terrain, in pieces designed to be combined, and its shading is made to agree with the ground's (§10).

Terracing is a standard height-field operation. Houdini's terrace node steps the terrain to a maximum step height and fades between terrace and source, and its documentation says plainly that out of the box "terraces often look artificial, because they have hard edges", and that they need masks, smoothing and erosion to look natural [47]. Gaea's Stratify makes "broken strata": layers created in confined local zones, such as between two broken plates, each independent of the rest, with a second level of substrata between them [48]. Research systems go further: implicit blocks generated from fracture patterns build cliffs, crags and promontories whose blocks follow strata and terrain [49], and layered material stacks with implicit surfaces produce overhangs, arches and rock piles [50].

A height field's limit is its sample spacing. A feature shorter than about two samples cannot be represented and aliases or blurs when it is drawn [13]. Ghost Recon Wildlands kept its height map at 50 cm because "too much precision is just overkill", and put hardware tessellation on top for the foreground [51]. At the 0.5 to 1 m spacing games use, ledges of several metres can live in a height field; blocks of a metre, and anything overhanging, cannot.

A rock mesh that meets the ground on a hard line reads as placed. The documented fixes are these.

• Pixel depth offset. The material pushes its own depth, so the seam is dithered between rock and ground rather than cut [52].
• Runtime virtual texturing. The terrain's shading is cached in a virtual texture that other meshes sample, so a rock's base can wear exactly the ground it sits in. The cache suits static objects only and is not updated every frame [11].
• Height-lerp blending. Two layers blend by their height maps, not their weights alone, so "tops of cobble-stones remain pure whereas sand lies in cracks between them" [53].
• Shared world-space materials. Both rock and ground take the same texture by world position (triplanar), so the pattern runs across the seam [54], [12].
• Deforming the terrain around the mesh. Unreal's Landscape Patch lets a mesh carry a patch that edits the heightmap and weightmaps around it, in the editor only [55]. Horizon's object height layer lets placement read where objects already stand [10].

Triplanar projection textures steep surfaces without UV stretch by projecting along three axes and blending; a blend range of about 10 to 20 degrees works well [12]. It is also often done wrong for normal maps, which need their own reorientation per projection [54].

Macro variation. Tiled rock repeats. Horizon paints rock colour variance and lichen density as world maps at 1 m [10], and Wildlands layers detail maps, masks, dirt, snow and moss per ecosystem [43]. Days Gone varies a small kit by vertex blends and colour swaps [46].

Parallax. Red Dead Redemption 2 ships parallax occlusion mapping as a setting, with ultra 4 % slower than the rest. Its tessellation setting governs trees, mud and other deformable surfaces, and turning tree tessellation off gains up to about 10 % [56]. Parallax is a near-field treatment of small relief. It cannot change a silhouette.

Flat-shaded facets. Rock reads as hard when its faces catch light differently from their neighbours. Day Hike's own rock props were cut at load by planar fractures with flat-shaded faces, after a probe found their normal maps contributed nothing visible on a smooth shape [57].

Voxels and signed distance fields. A density function meshed by marching cubes gives caves, overhangs and arches that "the simple height fields that the CPU can process do not offer" [12]. Its costs are generation and level of detail. The GPU Gems 3 implementation built blocks of 32³ voxels at 6.6 to 260 per second on a GeForce 8800, depending on method [12], and stitching neighbouring resolutions needs a dedicated method such as Transvoxel [58]. A 2026 browser series building voxel terrain on WebGPU reached the Transvoxel seam stage as test rigs, with no frame figures [59].

Virtualized geometry. Nanite splits meshes into a hierarchy of triangle clusters and swaps them by screen error, so it suits meshes with many or very small triangles and many instances, rock and cliff kits in particular [60]. Its designers' aim is to draw about the same number of clusters every frame whatever the scene holds, and to handle "a million instances easily" [61]. Nanite tessellation adds runtime displacement by map or material [60]. A WebGPU reimplementation in TypeScript runs in Chrome with meshlet levels of detail, culling and a software rasterizer, on demo scenes of 640 million and 1.7 billion triangles [16].

• Stages. WebGPU's shading language has vertex, fragment and compute stages, and no tessellation or geometry stage [14]. WebGL 2 follows OpenGL ES 3.0 [62], and tessellation and geometry shaders "don't arrive until ES 3.2" [63]. Displacement must come from geometry that already exists or from compute.
• Instancing. Babylon's thin instances draw large numbers of one mesh from a matrix buffer with no per-instance JavaScript objects, but all or none of them are drawn, a single bounding box covers them all, and instances whose matrices have mixed positive and negative determinants do not render correctly in one mesh [15]. Mirrored rock variants therefore need a mesh of their own. Three.js's BatchedMesh draws different geometries with one material in one multi-draw call, with per-object frustum culling [64].
• Materials. Babylon ships a triplanar material that needs no UVs, with per-axis diffuse and normal maps [65]; custom terrain shaders can do the same in plugins.
• What is missing. No engine-level runtime virtual texturing, no Nanite-class geometry outside demos, no tessellation. The tools left are the ones the 2010s consoles used: a well-shaped height field, instanced kits with levels of detail, triplanar and height-blended materials, and careful seams.

Day Hike's simulation owns its height field, and every peer must agree on it. Its cliff bands are a terrace remap on a 26 m period whose risers take 80 % of each band's rise, with walkable benches between [66]. Its rock ground is a slope class in a tiled triplanar texture [67]. Three attempts so far, and what the research says of each:

• Deeper parallax on steep rock lifted the cobbles at 10 m, stepped at grazing angles and left the crest smooth [68]. Confirmed: parallax is near-field relief and cannot change a silhouette (§11).
• Displacing the drawn terrain with folds of 1 to 2 m on a 0.5 to 1 m vertex lattice broke the crest head-on, but a wind-streaked dune from the side and dimples from above, and the same flat sheet from 80 m [68]. Confirmed: under two samples per wavelength the fold aliases (§9). The same section qualifies the diagnosis that a scarp needs 4 to 20 m ledges. Such ledges are within a height field's reach, and in a game whose simulation owns the height field they belong there, where collision agrees with sight.
• Cliff modules: two granite cliff models, first one per 12 m cell, then laid in runs along the contour with overlap and made solid [68]. They read as bands of wall from 80 m and along the face, but as a jumble of boulders along the crest at 30 m, at +1.68 ms at four times native pixels [69]. Confirmed as the industry's answer (§7, §8). The research adds three things the runs lack: rock of the Peninsula's own kind, bedded and steep (§1, §5); a shared material at the seam (§10); and talus at the foot (§3).

No studio publishes a frame cost for its cliff kits, or a rule for how dense talus must be before it reads as talus. The sources on block sorting disagree. The photographs give shapes and colours, not dimensions; the sizes taken from them above are marked as estimates. And no browser project has shown voxel or virtual-geometry terrain at open-world scale with frame times. Every cost in a browser proposal built on this note has to come from the game's own paired measurements.

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