diff --git a/cmd/gendump/main.go b/cmd/gendump/main.go
index 8b78e99..dd6b205 100644
--- a/cmd/gendump/main.go
+++ b/cmd/gendump/main.go
@@ -111,6 +111,20 @@ func main() {
 	countAt(16, 40, "y=16..40")
 	countAt(1, 7, "y=1..7 (transition)")
 	countAt(-64, -1, "y<0 (deepslate)")
+	// The deepslate rule is a vertical_gradient over absolute anchors 0..8:
+	// everything solid below y=0 is deepslate, everything above y=8 is stone,
+	// and the band between them is a scatter. A zero here means the rule is
+	// firing but its block is being dropped, or its anchors are misread.
+	switch {
+	case deepStone["y<0 (deepslate)"] == 0:
+		fmt.Println("  FAIL: no deepslate below y=0")
+	case deepStone["y=16..40"] != 0:
+		fmt.Println("  FAIL: deepslate above the transition band")
+	case deepStone["y=1..7 (transition)"] == 0:
+		fmt.Println("  FAIL: the stone/deepslate transition band is empty")
+	default:
+		fmt.Println("  OK: deepslate below y=0, scattered through y=1..7, none above")
+	}
 
 	// Bedrock floor: y=-64 must be solid bedrock everywhere, y=-63..-59 a
 	// thinning scatter of bedrock over stone/deepslate, and NOTHING in that band
diff --git a/internal/world/deepslate_verify_test.go b/internal/world/deepslate_verify_test.go
new file mode 100644
index 0000000..66e1255
--- /dev/null
+++ b/internal/world/deepslate_verify_test.go
@@ -0,0 +1,52 @@
+package world
+
+import "testing"
+
+// StateDeepslate is minecraft:deepslate with axis=y, the upright default the
+// surface rule places.
+const StateDeepslate uint16 = 27924
+
+// TestDeepslateLayer guards the stone/deepslate boundary. The rule that draws
+// it is a vertical_gradient over absolute anchors 0 and 8; the anchor decoder
+// only read above_bottom, so both collapsed onto y=-64 and the whole world was
+// stone from bedrock to sky. The block name was missing from the ID table too,
+// so even a firing rule resolved to 0 and was dropped.
+func TestDeepslateLayer(t *testing.T) {
+	gen := NewVanillaGenerator(12345)
+	deepBelow, stoneBelow := 0, 0
+	deepAbove := 0
+	transition := 0
+	for _, p := range [][2]int32{{0, 0}, {5, -7}, {-13, 21}} {
+		ch := gen(p[0], p[1])
+		for lx := 0; lx < 16; lx++ {
+			for lz := 0; lz < 16; lz++ {
+				for wy := MinY; wy <= 40; wy++ {
+					switch b := ch.GetBlock(lx, wy, lz); {
+					case b == StateDeepslate && wy < 0:
+						deepBelow++
+					case b == StateDeepslate && wy >= 0 && wy < 8:
+						transition++
+					case b == StateDeepslate && wy >= 8:
+						deepAbove++
+					case b == StateStone && wy < 0:
+						stoneBelow++
+					}
+				}
+			}
+		}
+	}
+	t.Logf("deepslate below y=0: %d (stone there: %d), in y=0..7: %d, above y=8: %d",
+		deepBelow, stoneBelow, transition, deepAbove)
+	if deepBelow == 0 {
+		t.Error("no deepslate below y=0")
+	}
+	if stoneBelow != 0 {
+		t.Errorf("%d plain stone blocks survive below y=0; the gradient is not reaching them", stoneBelow)
+	}
+	if deepAbove != 0 {
+		t.Errorf("%d deepslate blocks above y=8; the upper anchor is not holding", deepAbove)
+	}
+	if transition == 0 {
+		t.Error("the y=0..7 stone/deepslate scatter is empty")
+	}
+}
diff --git a/internal/world/store.go b/internal/world/store.go
index c1496eb..b0a3e77 100644
--- a/internal/world/store.go
+++ b/internal/world/store.go
@@ -32,7 +32,7 @@ const dataVersion26 = 4790
 // first time it ran: chunkAt prefers the store over the generator, so the
 // already-explored area around spawn keeps its old terrain and every later fix
 // looks like it did nothing in exactly the place you are standing.
-const generatorVersion = 4
+const generatorVersion = 5
 
 // generatorVersionTag is the NBT key holding generatorVersion. It is namespaced
 // because it is ours, not part of the vanilla chunk format.
diff --git a/internal/world/vanilla.go b/internal/world/vanilla.go
index a4bf8d7..23ed4a8 100644
--- a/internal/world/vanilla.go
+++ b/internal/world/vanilla.go
@@ -79,8 +79,9 @@ func generateVanilla(od *worldgen.OverworldDensity, fluidPicker worldgen.FluidPi
 	}
 	wg.Wait()
 
-	// The surface rule tree is seed-independent; load once (cached). If it fails
-	// to parse, surface fill falls back to the biome-blind heuristics.
+	// The surface rule set is compiled against the world seed at load time. If
+	// it failed to parse, the surface pass falls back to biome-blind heuristics
+	// rather than leaving the terrain bare.
 	surfaceRule, ruleErr := od.SurfaceRule()
 
 	// The aquifer decides fluid per position while the column is laid down. Its
@@ -92,19 +93,43 @@ func generateVanilla(od *worldgen.OverworldDensity, fluidPicker worldgen.FluidPi
 	}
 
 	var columns [16][16][WorldHeight]uint16
-	var surfTop [16][16]int // top solid index, -1 if none
-	var grass [16][16]bool  // grassy land surface (tree-plantable)
+	var surfTop [16][16]int      // top solid index, -1 if none
+	var worldSurface [16][16]int // topmost non-air Y, the WORLD_SURFACE_WG heightmap
+	var grass [16][16]bool       // grassy land surface (tree-plantable)
+
+	// Terrain and fluids first, for the whole chunk. The surface pass has to
+	// wait for all of it: the "steep" condition reads the heights of the
+	// column's neighbours, which vanilla takes from the heightmap that doFill
+	// finishes before buildSurface starts.
 	for lx := 0; lx < 16; lx++ {
 		wg.Add(1)
 		go func(lx int) {
 			defer wg.Done()
 			interp := make([]float64, len(od.Interpolated))
 			for lz := 0; lz < 16; lz++ {
-				var rule worldgen.SurfaceRule
+				surfTop[lx][lz], worldSurface[lx][lz], grass[lx][lz] =
+					fillVanillaColumn(od, aq, fluidPicker, grids, interp, &columns[lx][lz], baseX+lx, baseZ+lz, lx, lz)
+			}
+		}(lx)
+	}
+	wg.Wait()
+
+	for lx := 0; lx < 16; lx++ {
+		wg.Add(1)
+		go func(lx int) {
+			defer wg.Done()
+			var sctx *worldgen.SurfaceContext
+			if ruleErr == nil {
+				sctx = surfaceRule.NewContext()
+			}
+			for lz := 0; lz < 16; lz++ {
+				rng := newColumnRand(baseX+lx, baseZ+lz, int(seed))
 				if ruleErr == nil {
-					rule = surfaceRule
+					applySurfaceRule(od, surfaceRule, sctx, &columns[lx][lz],
+						baseX+lx, baseZ+lz, lx, lz, &worldSurface, biomeName[lx][lz], rng)
+				} else {
+					fillLegacySurface(&columns[lx][lz], surfTop[lx][lz], rng)
 				}
-				surfTop[lx][lz], grass[lx][lz] = fillVanillaColumn(od, aq, fluidPicker, grids, interp, &columns[lx][lz], baseX+lx, baseZ+lz, lx, lz, seed, rule, biomeName[lx][lz])
 			}
 		}(lx)
 	}
@@ -165,13 +190,13 @@ func fillBiomes3D(c *Chunk, od *worldgen.OverworldDensity, s2D [16][16]worldgen.
 // then does the surface rule tree walk the finished column. Doing it the other
 // way round is what forced the old unconditional "flood everything under sea
 // level" pass, which left every cave below y=63 underwater.
-func fillVanillaColumn(od *worldgen.OverworldDensity, aq *worldgen.Aquifer, fluidPicker worldgen.FluidPicker, grids []cornerGrid, interp []float64, out *[WorldHeight]uint16, wx, wz, lx, lz int, seed int64, rule worldgen.SurfaceRule, biomeName string) (int, bool) {
+func fillVanillaColumn(od *worldgen.OverworldDensity, aq *worldgen.Aquifer, fluidPicker worldgen.FluidPicker, grids []cornerGrid, interp []float64, out *[WorldHeight]uint16, wx, wz, lx, lz int) (top, worldSurface int, grass bool) {
 	cx0 := lx / cellWidth
 	cz0 := lz / cellWidth
 	fx := float64(lx%cellWidth) / cellWidth
 	fz := float64(lz%cellWidth) / cellWidth
 
-	top := -1
+	top, worldSurface = -1, MinY-1
 	for i := 0; i < WorldHeight; i++ {
 		cy0 := i / cellHeight
 		fy := float64(i%cellHeight) / cellHeight
@@ -186,6 +211,9 @@ func fillVanillaColumn(od *worldgen.OverworldDensity, aq *worldgen.Aquifer, flui
 		if isDefaultBlock {
 			top = i
 		}
+		if state != StateAir {
+			worldSurface = y
+		}
 	}
 
 	topY := MinY + top
@@ -194,16 +222,22 @@ func fillVanillaColumn(od *worldgen.OverworldDensity, aq *worldgen.Aquifer, flui
 	const beachBand = 3
 	beach := top >= 0 && topY >= SeaLevel-beachBand && topY <= SeaLevel+1
 	deepWater := top >= 0 && topY < SeaLevel-beachBand
+	return top, worldSurface, top >= 0 && !beach && !deepWater && topY >= SeaLevel
+}
 
-	// Per-column RNG for the bedrock floor and the bandlands/gradient rules.
-	rng := newColumnRand(wx, wz, int(seed))
-
-	if rule != nil {
-		applySurfaceRule(od, out, wx, wz, SeaLevel, MinY, biomeName, rule, rng)
-	} else {
-		fillLegacySurface(out, top, beach, deepWater, rng)
+// steepAt is SurfaceRules.SteepMaterialCondition: true where the column's
+// neighbours inside the chunk differ in height by four blocks or more. The
+// neighbour indices are clamped to the chunk, as vanilla's are — the condition
+// deliberately does not look at the chunk next door.
+func steepAt(worldSurface *[16][16]int, lx, lz int) bool {
+	north := max(lz-1, 0)
+	south := min(lz+1, 15)
+	if worldSurface[lx][south] >= worldSurface[lx][north]+4 {
+		return true
 	}
-	return top, top >= 0 && !beach && !deepWater && topY >= SeaLevel
+	west := max(lx-1, 0)
+	east := min(lx+1, 15)
+	return worldSurface[west][lz] >= worldSurface[east][lz]+4
 }
 
 // substance resolves one position to the block the terrain pass leaves behind:
@@ -241,7 +275,7 @@ func substance(aq *worldgen.Aquifer, fluidPicker worldgen.FluidPicker, x, y, z i
 // One *rand.Rand is created per column (not per block) — bandlands/gradient
 // consume from it sequentially, which is correct because vanilla seeds those
 // per-column too. This avoids ~98k rand.New allocations per chunk.
-func applySurfaceRule(od *worldgen.OverworldDensity, out *[WorldHeight]uint16, wx, wz, seaLevel, minY int, biomeName string, rule worldgen.SurfaceRule, rng chunkRand) {
+func applySurfaceRule(od *worldgen.OverworldDensity, rules *worldgen.SurfaceRuleSet, sctx *worldgen.SurfaceContext, out *[WorldHeight]uint16, wx, wz, lx, lz int, worldSurface *[16][16]int, biomeName string, rng chunkRand) {
 	top := -1
 	for i := WorldHeight - 1; i >= 0; i-- {
 		if out[i] != StateAir {
@@ -252,26 +286,21 @@ func applySurfaceRule(od *worldgen.OverworldDensity, out *[WorldHeight]uint16, w
 	if top < 0 {
 		return
 	}
-	// One per-column RNG for all surface rules in this column.
-	colRng := rng.toRand()
-	// Column-constant surface quantities, computed once per column exactly as
-	// SurfaceRules.Context.updateXZ does.
+	// Column-constant surface quantities, refreshed once per column exactly as
+	// SurfaceRules.Context.updateXZ does. The context itself is reused across
+	// the whole 16-column strip to avoid ~98k allocations per chunk; the fields
+	// that vary per block are set inside the loop below.
+	rules.BeginColumn(sctx, wx, wz)
 	surfaceDepth := od.Surface.SurfaceDepth(wx, wz)
-	// Reuse one context across the column (mutated per block) to avoid ~98k
-	// heap allocations per chunk; the fields that vary per block are set inside
-	// the loop, the rest are column-constant.
-	sctx := &worldgen.SurfaceContext{
-		X:                wx,
-		Z:                wz,
-		SeaLevel:         seaLevel,
-		BiomeName:        biomeName,
-		MinY:             minY,
-		SurfaceNoise:     od.Surface.Noise(wx, wz),
-		SurfaceSecondary: od.Surface.SurfaceSecondary(wx, wz),
-		SurfaceDepth:     surfaceDepth,
-		MinSurfaceLevel:  od.MinSurfaceLevelAt(wx, wz, surfaceDepth),
-		Rng:              colRng,
-	}
+	sctx.SeaLevel = SeaLevel
+	sctx.BiomeName = biomeName
+	sctx.MinY = MinY
+	sctx.SurfaceSecondary = od.Surface.SurfaceSecondary(wx, wz)
+	sctx.SurfaceDepth = surfaceDepth
+	sctx.MinSurfaceLevel = od.MinSurfaceLevelAt(wx, wz, surfaceDepth)
+	sctx.Steep = steepAt(worldSurface, lx, lz)
+	sctx.Rng = rng.toRand()
+	minY := MinY
 	stoneDepthAbove := 0
 	waterHeight := worldgen.NoWaterAbove
 	nextCeilingStoneY := math.MaxInt
@@ -309,7 +338,10 @@ func applySurfaceRule(od *worldgen.OverworldDensity, out *[WorldHeight]uint16, w
 		if old != StateStone {
 			continue
 		}
-		if state, ok := rule.Apply(sctx); ok && state != 0 {
+		// A matched rule places its block even when that block is air: the
+		// frozen-ocean surface deliberately carves one away. Only "no rule
+		// matched" leaves the default block alone.
+		if state, ok := rules.Apply(sctx); ok {
 			out[i] = state
 		}
 	}
@@ -323,10 +355,14 @@ func isFluidState(s uint16) bool { return s == StateWater || s == StateLava }
 // isStoneState is SurfaceSystem.isStone: solid, non-fluid, non-air.
 func isStoneState(s uint16) bool { return s != StateAir && !isFluidState(s) }
 
-// fillLegacySurface is the biome-blind heuristic used when no surface rule is
-// available (parse failure). It dresses the stone the terrain and aquifer
+// fillLegacySurface is the biome-blind heuristic used when no surface rule set
+// is available (parse failure). It dresses the stone the terrain and aquifer
 // passes already laid down, leaving their air and fluids alone.
-func fillLegacySurface(out *[WorldHeight]uint16, top int, beach, deepWater bool, rng chunkRand) {
+func fillLegacySurface(out *[WorldHeight]uint16, top int, rng chunkRand) {
+	const beachBand = 3
+	topY := MinY + top
+	beach := top >= 0 && topY >= SeaLevel-beachBand && topY <= SeaLevel+1
+	deepWater := top >= 0 && topY < SeaLevel-beachBand
 	for i := 0; i < WorldHeight; i++ {
 		y := MinY + i
 		if !isStoneState(out[i]) {
diff --git a/internal/worldgen/blockids.go b/internal/worldgen/blockids.go
index 9043744..557f865 100644
--- a/internal/worldgen/blockids.go
+++ b/internal/worldgen/blockids.go
@@ -20,80 +20,97 @@ package worldgen
 
 // surfaceBlockID resolves a surface-rule result_state (Name + optional
 // Properties) to its network block-state ID. It handles the snowy property on
-// snowable blocks and the layers property on snow; unknown blocks return 0
-// (air) so a missing entry is visually obvious rather than crashing.
-func surfaceBlockID(name string, props map[string]string) uint16 {
+// snowable blocks and the layers property on snow.
+//
+// An unknown name is an error, not a fallback. It used to return 0, which the
+// rule application then read as "no block" and skipped — so a name missing from
+// this table silently left stone behind. That is exactly how deepslate went
+// missing from the entire world: the rule fired, resolved to 0, and was dropped.
+func surfaceBlockID(name string, props map[string]string) (uint16, bool) {
 	switch name {
+	case "minecraft:air":
+		return 0, true
+	case "minecraft:deepslate":
+		// A pillar block; the surface rule always asks for the upright axis.
+		return 27924, true
+	case "minecraft:mud":
+		return 27922, true
+	case "minecraft:brown_terracotta":
+		return 11456, true
+	case "minecraft:red_terracotta":
+		return 11458, true
+	case "minecraft:light_gray_terracotta":
+		return 11452, true
 	case "minecraft:stone":
-		return 1
+		return 1, true
 	case "minecraft:granite":
-		return 2
+		return 2, true
 	case "minecraft:diorite":
-		return 4
+		return 4, true
 	case "minecraft:andesite":
-		return 6
+		return 6, true
 	case "minecraft:grass_block":
 		if props["snowy"] == "true" {
-			return 8
+			return 8, true
 		}
-		return 9
+		return 9, true
 	case "minecraft:dirt":
-		return 10
+		return 10, true
 	case "minecraft:coarse_dirt":
-		return 11
+		return 11, true
 	case "minecraft:podzol":
 		if props["snowy"] == "true" {
-			return 12
+			return 12, true
 		}
-		return 13
+		return 13, true
 	case "minecraft:bedrock":
-		return 85
+		return 85, true
 	case "minecraft:water":
-		return 86
+		return 86, true
 	case "minecraft:sand":
-		return 118
+		return 118, true
 	case "minecraft:red_sand":
-		return 123
+		return 123, true
 	case "minecraft:gravel":
-		return 124
+		return 124, true
 	case "minecraft:sandstone":
-		return 578
+		return 578, true
 	case "minecraft:red_sandstone":
-		return 13247
+		return 13247, true
 	case "minecraft:snow_block":
-		return 6928
+		return 6928, true
 	case "minecraft:snow":
 		// snow has a "layers" property 1..8; default layer 1 = 6919.
-		return 6919
+		return 6919, true
 	case "minecraft:ice":
-		return 6927
+		return 6927, true
 	case "minecraft:packed_ice":
-		return 12914
+		return 12914, true
 	case "minecraft:powder_snow":
-		return 24689
+		return 24689, true
 	case "minecraft:mycelium":
 		if props["snowy"] == "true" {
-			return 8918
+			return 8918, true
 		}
-		return 8919
+		return 8919, true
 	case "minecraft:terracotta":
-		return 12912
+		return 12912, true
 	case "minecraft:white_terracotta":
-		return 11444
+		return 11444, true
 	case "minecraft:orange_terracotta":
-		return 11445
+		return 11445, true
 	case "minecraft:yellow_terracotta":
-		return 11448
+		return 11448, true
 	case "minecraft:calcite":
-		return 24687
+		return 24687, true
 	case "minecraft:tuff":
-		return 23452
+		return 23452, true
 	case "minecraft:dripstone_block":
-		return 27755
+		return 27755, true
 	case "minecraft:moss_block":
-		return 27862
+		return 27862, true
 	case "minecraft:smooth_stone":
-		return 13480
+		return 13480, true
 	}
-	return 0
+	return 0, false
 }
diff --git a/internal/worldgen/loader.go b/internal/worldgen/loader.go
index ddc6ece..061ca29 100644
--- a/internal/worldgen/loader.go
+++ b/internal/worldgen/loader.go
@@ -56,14 +56,17 @@ type OverworldDensity struct {
 	// rule tree runs.
 	Surface *SurfaceSampler
 
+	surfaceRule    *SurfaceRuleSet
+	surfaceRuleErr error
+
 	prelim *levelCache
 }
 
-// SurfaceRule returns the overworld surface rule tree, loading it on first use.
-// It does not depend on the seed. A nil rule (on error) is non-fatal: the
-// generator falls back to its default surface heuristics.
-func (od *OverworldDensity) SurfaceRule() (SurfaceRule, error) {
-	return LoadOverworldSurfaceRule()
+// SurfaceRule returns the overworld surface rule set, compiled against this
+// world's seed. A nil rule set (on error) is non-fatal: the generator falls
+// back to its biome-blind surface heuristics.
+func (od *OverworldDensity) SurfaceRule() (*SurfaceRuleSet, error) {
+	return od.surfaceRule, od.surfaceRuleErr
 }
 
 // LoadOverworldFinalDensity builds the overworld final_density function for the
@@ -168,6 +171,12 @@ func LoadOverworldFinalDensity(seed int64) (*OverworldDensity, error) {
 		secondaryNoise: secondaryNoise,
 		positionalRand: l.rs.Positional(),
 	}
+
+	// The rule tree is seed-bound: its noise_threshold conditions sample seeded
+	// noises and its vertical_gradient rolls against a seeded positional
+	// factory. A failure here is reported but not fatal — the generator keeps
+	// going on the fallback heuristics rather than refusing to start.
+	od.surfaceRule, od.surfaceRuleErr = l.loadSurfaceRuleSet(od.MinY, od.Height)
 	return od, nil
 }
 
diff --git a/internal/worldgen/structure.go b/internal/worldgen/structure.go
index e15d061..75eaa91 100644
--- a/internal/worldgen/structure.go
+++ b/internal/worldgen/structure.go
@@ -33,7 +33,7 @@ func LoadTemplate(path string) (*Template, error) {
 	if err != nil {
 		return nil, err
 	}
-	
+
 	// NBT files are usually gzipped.
 	var r io.Reader = bytes.NewReader(b)
 	if len(b) >= 2 && b[0] == 0x1f && b[1] == 0x8b {
@@ -81,7 +81,7 @@ func LoadTemplate(path string) (*Template, error) {
 				nameTag, _ := stateComp.Get("Name")
 				name := string(nameTag.(nbt.String))
 				props := make(map[string]string)
-				
+
 				if propTag, ok := stateComp.Get("Properties"); ok {
 					if propComp, ok := propTag.(*nbt.Compound); ok {
 						for _, k := range propComp.Keys() {
@@ -92,9 +92,11 @@ func LoadTemplate(path string) (*Template, error) {
 						}
 					}
 				}
-				id := surfaceBlockID(name, props)
-				if id == 0 && name != "minecraft:air" {
-					// Fallback to default block ID for the name
+				id, ok := surfaceBlockID(name, props)
+				if !ok {
+					// Structure templates name far more blocks than the
+					// surface rules do; fall back to the broader
+					// default-state table.
 					id = defaultBlockIDs[name]
 				}
 				tmpl.Palette = append(tmpl.Palette, id)
@@ -112,7 +114,7 @@ func LoadTemplate(path string) (*Template, error) {
 				}
 				stateTag, _ := blockComp.Get("state")
 				stateIdx := int(stateTag.(nbt.Int))
-				
+
 				var pos [3]int
 				if posTag, ok := blockComp.Get("pos"); ok {
 					if posList, ok := posTag.(nbt.List); ok && len(posList.Elems) == 3 {
diff --git a/internal/worldgen/surface.go b/internal/worldgen/surface.go
index 96fb841..03bf43d 100644
--- a/internal/worldgen/surface.go
+++ b/internal/worldgen/surface.go
@@ -5,7 +5,6 @@ import (
 	"fmt"
 	"math"
 	"math/rand"
-	"sync"
 )
 
 // surface.go implements the vanilla SurfaceRules interpreter: a rule tree that
@@ -45,11 +44,8 @@ type SurfaceContext struct {
 	BiomeName string
 	// MinY is the world bottom for relative-anchor resolution.
 	MinY int
-	// SurfaceNoise is the "minecraft:surface" noise sample at (X,Z); the
-	// noise_threshold condition ranges over it.
-	SurfaceNoise float64
-	// Steep is true when the local slope exceeds the vanilla steep threshold
-	// (~1.0 surface-depth delta between neighbours).
+	// Steep is true when the column's neighbours in the chunk differ in height
+	// by four or more blocks (SurfaceRules.SteepMaterialCondition).
 	Steep bool
 	// SurfaceDepth is how thick the biome's surface layers are at this column
 	// (SurfaceSystem.getSurfaceDepth): usually 3, sometimes 0 or less, which is
@@ -63,9 +59,14 @@ type SurfaceContext struct {
 	// the interpolated preliminary surface level plus SurfaceDepth less 8.
 	// above_preliminary_surface tests Y against it.
 	MinSurfaceLevel int
-	// Rng is a per-column deterministic source for vertical_gradient and
-	// bandlands. It is seeded by the column so results are stable across runs.
+	// Rng is a per-column deterministic source for the bandlands rule. It is
+	// seeded by the column so results are stable across runs.
 	Rng *rand.Rand
+
+	// noiseValues holds one sample per noise the rule tree's noise_threshold
+	// conditions reference, refreshed once per column by BeginColumn. Vanilla
+	// caches these the same way, through LazyXZCondition.
+	noiseValues []float64
 }
 
 // SurfaceRule decides the block at a context. Apply returns ok=false when the
@@ -113,19 +114,21 @@ func (r conditionRule) Apply(ctx *SurfaceContext) (uint16, bool) {
 type bandlandsRule struct{}
 
 func (bandlandsRule) Apply(ctx *SurfaceContext) (uint16, bool) {
+	orange, _ := surfaceBlockID("minecraft:orange_terracotta", nil)
 	if ctx.Rng == nil {
-		return surfaceBlockID("minecraft:orange_terracotta", nil), true
+		return orange, true
 	}
 	// Vanilla chooses band by Y + a per-column random offset; the rotation
 	// cycles white/orange/yellow/orange terracotta. Pick from the cycle by Y.
-	band := (ctx.Y + ctx.Rng.Intn(7)) % 4
-	switch band {
+	white, _ := surfaceBlockID("minecraft:white_terracotta", nil)
+	yellow, _ := surfaceBlockID("minecraft:yellow_terracotta", nil)
+	switch (ctx.Y + ctx.Rng.Intn(7)) % 4 {
 	case 0:
-		return surfaceBlockID("minecraft:white_terracotta", nil), true
+		return white, true
 	case 1, 3:
-		return surfaceBlockID("minecraft:orange_terracotta", nil), true
+		return orange, true
 	default:
-		return surfaceBlockID("minecraft:yellow_terracotta", nil), true
+		return yellow, true
 	}
 }
 
@@ -153,12 +156,12 @@ type steepTest struct{}
 
 func (steepTest) Test(ctx *SurfaceContext) bool { return ctx.Steep }
 
-// holeTest passes in surface "holes" below the surrounding terrain — we
-// approximate as "below sea level and not the top" since true hole detection
-// needs a neighbourhood. Conservative: false (rare rule, low visual cost).
+// holeTest passes where the surface depth noise came out at or below zero — a
+// bare patch with no surface layer at all, which is how coarse dirt and gravel
+// scars appear in the middle of grass.
 type holeTest struct{}
 
-func (holeTest) Test(ctx *SurfaceContext) bool { return false }
+func (holeTest) Test(ctx *SurfaceContext) bool { return ctx.SurfaceDepth <= 0 }
 
 // waterTest passes when the block is clear of the water above it — either there
 // is none, or it sits far enough below the water's underside
@@ -208,34 +211,21 @@ func isColdBiome(name string) bool {
 	return false
 }
 
-// yAboveTest passes when Y is above an anchor (absolute, above_bottom, or
-// below_top), with optional surface-depth and stone-depth offsets.
+// yAboveTest passes when Y clears an anchor, with optional surface-depth and
+// stone-depth offsets. The anchor is resolved against the world's height bounds
+// at parse time.
 type yAboveTest struct {
-	absolute           int
-	hasAbsolute        bool
-	aboveBottom        int
-	hasAboveBottom     bool
-	belowTop           int
-	hasBelowTop        bool
-	addStoneDepth      bool
-	surfaceDepthMul    int
+	anchorY         int
+	addStoneDepth   bool
+	surfaceDepthMul int
 }
 
 func (t yAboveTest) Test(ctx *SurfaceContext) bool {
-	var anchor int
-	switch {
-	case t.hasAbsolute:
-		anchor = t.absolute
-	case t.hasAboveBottom:
-		anchor = ctx.MinY + t.aboveBottom
-	case t.hasBelowTop:
-		anchor = (ctx.MinY + 384) - 1 - t.belowTop
-	}
-	threshold := anchor + ctx.SurfaceDepth*t.surfaceDepthMul
+	y := ctx.Y
 	if t.addStoneDepth {
-		threshold += ctx.StoneDepthAbove
+		y += ctx.StoneDepthAbove
 	}
-	return ctx.Y >= threshold
+	return y >= t.anchorY+ctx.SurfaceDepth*t.surfaceDepthMul
 }
 
 // stoneDepthTest passes when the block is within `offset` of the surface it
@@ -266,19 +256,22 @@ func (t stoneDepthTest) Test(ctx *SurfaceContext) bool {
 	return depth <= 1+t.offset+surfaceDepth+secondary
 }
 
-// noiseThresholdTest passes when the named surface noise is within [min,max].
+// noiseThresholdTest passes when its noise, sampled once per column at y=0, is
+// within [min,max]. slot indexes SurfaceContext.noiseValues, which the rule set
+// refreshes per column.
+//
+// Six of the seven noises the overworld tree uses were unsupported and fell
+// through as false, so calcite on stony peaks, ice and packed ice on frozen
+// peaks, powder snow, swamp water windows and gravel patches on stony shores
+// never appeared at all.
 type noiseThresholdTest struct {
 	min, max float64
-	noise    string
+	slot     int
 }
 
 func (t noiseThresholdTest) Test(ctx *SurfaceContext) bool {
-	// Only "minecraft:surface" is sampled in SurfaceContext; other noises fall
-	// through as false (conservative).
-	if t.noise != "minecraft:surface" {
-		return false
-	}
-	return ctx.SurfaceNoise >= t.min && ctx.SurfaceNoise <= t.max
+	v := ctx.noiseValues[t.slot]
+	return v >= t.min && v <= t.max
 }
 
 // notTest inverts its inner test.
@@ -286,33 +279,30 @@ type notTest struct{ inner ConditionTest }
 
 func (t notTest) Test(ctx *SurfaceContext) bool { return !t.inner.Test(ctx) }
 
-// verticalGradientTest reproduces the bedrock-floor gradient: a deterministic
-// band from true_at_and_below to false_at_and_above where membership tapers via
-// the column RNG. Anchors are above_bottom offsets from the world floor.
+// verticalGradientTest is the scattered transition between two layers: true
+// below one anchor, false above another, and in between a per-position coin
+// flip whose bias falls linearly with height. It draws the bedrock floor and
+// the stone-to-deepslate boundary.
+//
+// The anchors are resolved once at parse time, so this needs the world's height
+// bounds; the random factory is named by the rule (bedrock_floor, deepslate)
+// and forked from the world seed, so the same y gets the same answer every
+// time the chunk regenerates.
 type verticalGradientTest struct {
-	randomName      string
-	trueAtAndBelow  int // above_bottom
-	falseAtAndAbove int // above_bottom
+	trueAtAndBelow  int
+	falseAtAndAbove int
+	random          PositionalRandomFactory
 }
 
 func (t verticalGradientTest) Test(ctx *SurfaceContext) bool {
-	loY := ctx.MinY + t.trueAtAndBelow
-	hiY := ctx.MinY + t.falseAtAndAbove
-	switch {
-	case ctx.Y <= loY:
+	if ctx.Y <= t.trueAtAndBelow {
 		return true
-	case ctx.Y >= hiY:
+	}
+	if ctx.Y >= t.falseAtAndAbove {
 		return false
 	}
-	// Taper band: probability decreases linearly. Use the per-column RNG once
-	// per Y so the floor is stable but noisy. We approximate vanilla's
-	// random-based interpolation.
-	if ctx.Rng == nil {
-		return false
-	}
-	band := hiY - loY
-	pos := ctx.Y - loY
-	return ctx.Rng.Float64() > float64(pos)/float64(band)
+	probability := mapRange(float64(ctx.Y), float64(t.trueAtAndBelow), float64(t.falseAtAndAbove), 1.0, 0.0)
+	return float64(t.random.At(ctx.X, ctx.Y, ctx.Z).NextFloat()) < probability
 }
 
 // abovePreliminarySurfaceTest gates the whole biome surface subtree: below the
@@ -325,8 +315,77 @@ func (abovePreliminarySurfaceTest) Test(ctx *SurfaceContext) bool {
 
 // ---- Parser ------------------------------------------------------------
 
-// ParseSurfaceRule parses a surface_rule JSON node into a rule tree.
-func ParseSurfaceRule(raw json.RawMessage) (SurfaceRule, error) {
+// SurfaceRuleSet is a compiled surface rule tree together with the seeded
+// noises and random factories its conditions reference.
+//
+// The tree used to be parsed once, globally, and shared by every world: the
+// conditions that need the seed simply did not work. Binding it to a
+// RandomState is what lets noise_threshold sample a real noise and
+// vertical_gradient roll a real per-position coin.
+type SurfaceRuleSet struct {
+	root   SurfaceRule
+	noises []*NormalNoise
+}
+
+// NewContext returns a SurfaceContext sized for this rule set's per-column
+// noise cache. Reuse one per goroutine; BeginColumn refreshes it.
+func (s *SurfaceRuleSet) NewContext() *SurfaceContext {
+	return &SurfaceContext{noiseValues: make([]float64, len(s.noises))}
+}
+
+// BeginColumn samples every noise the tree references at (x, z) and stores the
+// column coordinates. Vanilla samples these lazily and caches them per column;
+// sampling all of them up front costs a handful of evaluations per column and
+// keeps the tree free of hidden state.
+func (s *SurfaceRuleSet) BeginColumn(ctx *SurfaceContext, x, z int) {
+	ctx.X, ctx.Z = x, z
+	for i, n := range s.noises {
+		ctx.noiseValues[i] = n.GetValue(float64(x), 0, float64(z))
+	}
+}
+
+// Apply runs the tree at the context's current position.
+func (s *SurfaceRuleSet) Apply(ctx *SurfaceContext) (uint16, bool) { return s.root.Apply(ctx) }
+
+// surfaceParser carries the seed-dependent state a rule tree needs while it is
+// being built: where to get noises and random factories, and the world's height
+// bounds for resolving vertical anchors.
+type surfaceParser struct {
+	loader       *Loader
+	minY, height int
+	noises       []*NormalNoise
+	noiseSlots   map[string]int
+}
+
+// noiseSlot returns the per-column cache index for a named noise, loading and
+// seeding it on first use.
+func (p *surfaceParser) noiseSlot(name string) (int, error) {
+	if slot, ok := p.noiseSlots[name]; ok {
+		return slot, nil
+	}
+	n, err := p.loader.noiseField(name)
+	if err != nil {
+		return 0, err
+	}
+	slot := len(p.noises)
+	p.noises = append(p.noises, n)
+	p.noiseSlots[name] = slot
+	return slot, nil
+}
+
+// resolveAnchor is VerticalAnchor.resolveY.
+func (p *surfaceParser) resolveAnchor(a anchorJSON) int {
+	switch a.kind {
+	case anchorAboveBottom:
+		return p.minY + a.value
+	case anchorBelowTop:
+		return p.minY + p.height - 1 - a.value
+	default:
+		return a.value
+	}
+}
+
+func (p *surfaceParser) parseRule(raw json.RawMessage) (SurfaceRule, error) {
 	var obj struct {
 		Type string `json:"type"`
 	}
@@ -344,7 +403,11 @@ func ParseSurfaceRule(raw json.RawMessage) (SurfaceRule, error) {
 		if err := json.Unmarshal(raw, &b); err != nil {
 			return nil, err
 		}
-		return blockRule{state: surfaceBlockID(b.Result.Name, b.Result.Properties)}, nil
+		state, ok := surfaceBlockID(b.Result.Name, b.Result.Properties)
+		if !ok {
+			return nil, fmt.Errorf("surface: no block-state ID for %q %v", b.Result.Name, b.Result.Properties)
+		}
+		return blockRule{state: state}, nil
 
 	case "minecraft:sequence":
 		var s struct {
@@ -355,7 +418,7 @@ func ParseSurfaceRule(raw json.RawMessage) (SurfaceRule, error) {
 		}
 		rules := make([]SurfaceRule, 0, len(s.Sequence))
 		for _, child := range s.Sequence {
-			r, err := ParseSurfaceRule(child)
+			r, err := p.parseRule(child)
 			if err != nil {
 				return nil, err
 			}
@@ -371,11 +434,11 @@ func ParseSurfaceRule(raw json.RawMessage) (SurfaceRule, error) {
 		if err := json.Unmarshal(raw, &c); err != nil {
 			return nil, err
 		}
-		test, err := parseCondition(c.IfTrue)
+		test, err := p.parseCondition(c.IfTrue)
 		if err != nil {
 			return nil, err
 		}
-		then, err := ParseSurfaceRule(c.Then)
+		then, err := p.parseRule(c.Then)
 		if err != nil {
 			return nil, err
 		}
@@ -388,7 +451,7 @@ func ParseSurfaceRule(raw json.RawMessage) (SurfaceRule, error) {
 }
 
 // parseCondition parses an if_true condition node into a ConditionTest.
-func parseCondition(raw json.RawMessage) (ConditionTest, error) {
+func (p *surfaceParser) parseCondition(raw json.RawMessage) (ConditionTest, error) {
 	var obj struct {
 		Type string `json:"type"`
 	}
@@ -413,9 +476,9 @@ func parseCondition(raw json.RawMessage) (ConditionTest, error) {
 
 	case "minecraft:water":
 		var w struct {
-			Offset               int  `json:"offset"`
-			SurfaceDepthMul      int  `json:"surface_depth_multiplier"`
-			AddStoneDepth        bool `json:"add_stone_depth"`
+			Offset          int  `json:"offset"`
+			SurfaceDepthMul int  `json:"surface_depth_multiplier"`
+			AddStoneDepth   bool `json:"add_stone_depth"`
 		}
 		if err := json.Unmarshal(raw, &w); err != nil {
 			return nil, err
@@ -446,32 +509,26 @@ func parseCondition(raw json.RawMessage) (ConditionTest, error) {
 		if err := json.Unmarshal(raw, &n); err != nil {
 			return nil, err
 		}
-		return noiseThresholdTest{min: n.Min, max: n.Max, noise: n.Noise}, nil
+		slot, err := p.noiseSlot(n.Noise)
+		if err != nil {
+			return nil, fmt.Errorf("noise_threshold %q: %w", n.Noise, err)
+		}
+		return noiseThresholdTest{min: n.Min, max: n.Max, slot: slot}, nil
 
 	case "minecraft:y_above":
 		var y struct {
-			AddStoneDepth   bool `json:"add_stone_depth"`
-			SurfaceDepthMul int  `json:"surface_depth_multiplier"`
-			Anchor          struct {
-				Absolute    *int `json:"absolute"`
-				AboveBottom *int `json:"above_bottom"`
-				BelowTop    *int `json:"below_top"`
-			} `json:"anchor"`
+			AddStoneDepth   bool       `json:"add_stone_depth"`
+			SurfaceDepthMul int        `json:"surface_depth_multiplier"`
+			Anchor          anchorJSON `json:"anchor"`
 		}
 		if err := json.Unmarshal(raw, &y); err != nil {
 			return nil, err
 		}
-		t := yAboveTest{addStoneDepth: y.AddStoneDepth, surfaceDepthMul: y.SurfaceDepthMul}
-		if y.Anchor.Absolute != nil {
-			t.hasAbsolute, t.absolute = true, *y.Anchor.Absolute
-		}
-		if y.Anchor.AboveBottom != nil {
-			t.hasAboveBottom, t.aboveBottom = true, *y.Anchor.AboveBottom
-		}
-		if y.Anchor.BelowTop != nil {
-			t.hasBelowTop, t.belowTop = true, *y.Anchor.BelowTop
-		}
-		return t, nil
+		return yAboveTest{
+			anchorY:         p.resolveAnchor(y.Anchor),
+			addStoneDepth:   y.AddStoneDepth,
+			surfaceDepthMul: y.SurfaceDepthMul,
+		}, nil
 
 	case "minecraft:not":
 		var n struct {
@@ -480,7 +537,7 @@ func parseCondition(raw json.RawMessage) (ConditionTest, error) {
 		if err := json.Unmarshal(raw, &n); err != nil {
 			return nil, err
 		}
-		inner, err := parseCondition(n.Invert)
+		inner, err := p.parseCondition(n.Invert)
 		if err != nil {
 			return nil, err
 		}
@@ -488,15 +545,20 @@ func parseCondition(raw json.RawMessage) (ConditionTest, error) {
 
 	case "minecraft:vertical_gradient":
 		var v struct {
+			RandomName      string     `json:"random_name"`
 			TrueAtAndBelow  anchorJSON `json:"true_at_and_below"`
 			FalseAtAndAbove anchorJSON `json:"false_at_and_above"`
 		}
 		if err := json.Unmarshal(raw, &v); err != nil {
 			return nil, err
 		}
+		if v.RandomName == "" {
+			return nil, fmt.Errorf("vertical_gradient: missing random_name")
+		}
 		return verticalGradientTest{
-			trueAtAndBelow:  v.TrueAtAndBelow.aboveBottom,
-			falseAtAndAbove: v.FalseAtAndAbove.aboveBottom,
+			trueAtAndBelow:  p.resolveAnchor(v.TrueAtAndBelow),
+			falseAtAndAbove: p.resolveAnchor(v.FalseAtAndAbove),
+			random:          p.loader.rs.Positional().FromHashOf(v.RandomName).ForkPositional(),
 		}, nil
 
 	case "minecraft:above_preliminary_surface":
@@ -505,49 +567,56 @@ func parseCondition(raw json.RawMessage) (ConditionTest, error) {
 	return nil, fmt.Errorf("surface: unknown condition type %q", obj.Type)
 }
 
-// anchorJSON decodes a {above_bottom|below_top|absolute: N} surface anchor.
+// anchorJSON decodes a VerticalAnchor: exactly one of absolute, above_bottom or
+// below_top. Which one it was matters — reading the value without the kind made
+// every absolute anchor resolve as an offset from the world floor, which is why
+// the deepslate rule (absolute 0 to 8) collapsed onto y=-64 and never fired.
 type anchorJSON struct {
-	absolute    int
-	aboveBottom int
-	belowTop    int
+	kind  anchorKind
+	value int
 }
 
+type anchorKind int
+
+const (
+	anchorAbsolute anchorKind = iota
+	anchorAboveBottom
+	anchorBelowTop
+)
+
 func (a *anchorJSON) UnmarshalJSON(data []byte) error {
 	var m map[string]int
 	if err := json.Unmarshal(data, &m); err != nil {
 		return err
 	}
-	a.aboveBottom = m["above_bottom"]
-	a.belowTop = m["below_top"]
-	a.absolute = m["absolute"]
-	return nil
+	for key, kind := range map[string]anchorKind{
+		"absolute":     anchorAbsolute,
+		"above_bottom": anchorAboveBottom,
+		"below_top":    anchorBelowTop,
+	} {
+		if v, ok := m[key]; ok {
+			a.kind, a.value = kind, v
+			return nil
+		}
+	}
+	return fmt.Errorf("surface: anchor has none of absolute/above_bottom/below_top")
 }
 
 // ---- Loader ------------------------------------------------------------
 
-var (
-	surfaceRuleOnce sync.Once
-	surfaceRule     SurfaceRule
-	surfaceRuleErr  error
-)
-
-// LoadOverworldSurfaceRule parses and caches the overworld surface_rule tree.
-// The rule tree does not depend on the world seed, so it is loaded once.
-func LoadOverworldSurfaceRule() (SurfaceRule, error) {
-	surfaceRuleOnce.Do(func() {
-		raw, err := dataFS.ReadFile("data/overworld.json")
-		if err != nil {
-			surfaceRuleErr = err
-			return
-		}
-		var doc struct {
-			SurfaceRule json.RawMessage `json:"surface_rule"`
-		}
-		if err := json.Unmarshal(raw, &doc); err != nil {
-			surfaceRuleErr = err
-			return
-		}
-		surfaceRule, surfaceRuleErr = ParseSurfaceRule(doc.SurfaceRule)
-	})
-	return surfaceRule, surfaceRuleErr
+// loadSurfaceRuleSet parses the overworld surface_rule tree, binding its
+// conditions to this loader's seeded RandomState.
+func (l *Loader) loadSurfaceRuleSet(minY, height int) (*SurfaceRuleSet, error) {
+	var doc struct {
+		SurfaceRule json.RawMessage `json:"surface_rule"`
+	}
+	if err := l.readJSON("data/overworld.json", &doc); err != nil {
+		return nil, err
+	}
+	p := &surfaceParser{loader: l, minY: minY, height: height, noiseSlots: map[string]int{}}
+	root, err := p.parseRule(doc.SurfaceRule)
+	if err != nil {
+		return nil, err
+	}
+	return &SurfaceRuleSet{root: root, noises: p.noises}, nil
 }
diff --git a/internal/worldgen/surface_test.go b/internal/worldgen/surface_test.go
index 97c46af..5fc3f4d 100644
--- a/internal/worldgen/surface_test.go
+++ b/internal/worldgen/surface_test.go
@@ -5,16 +5,30 @@ import (
 	"testing"
 )
 
-// TestLoadSurfaceRule confirms the embedded overworld surface_rule parses into
-// a rule tree without error. This guards the parser against any rule/condition
-// type the overworld uses.
-func TestLoadSurfaceRule(t *testing.T) {
-	rule, err := LoadOverworldSurfaceRule()
+// loadTestRules compiles the overworld surface rule set at a fixed seed.
+func loadTestRules(t *testing.T) *SurfaceRuleSet {
+	t.Helper()
+	od, err := LoadOverworldFinalDensity(12345)
 	if err != nil {
-		t.Fatalf("LoadOverworldSurfaceRule: %v", err)
+		t.Fatalf("load overworld density: %v", err)
 	}
-	if rule == nil {
-		t.Fatal("nil surface rule")
+	rules, err := od.SurfaceRule()
+	if err != nil {
+		t.Fatalf("compile surface rule: %v", err)
+	}
+	if rules == nil {
+		t.Fatal("nil surface rule set")
+	}
+	return rules
+}
+
+// TestLoadSurfaceRule confirms the embedded overworld surface_rule parses into
+// a rule tree without error, and that every noise its noise_threshold
+// conditions name resolved. Six of the seven used to fall through as false.
+func TestLoadSurfaceRule(t *testing.T) {
+	rules := loadTestRules(t)
+	if len(rules.noises) != 7 {
+		t.Errorf("rule set references %d noises, want 7", len(rules.noises))
 	}
 }
 
@@ -22,25 +36,24 @@ func TestLoadSurfaceRule(t *testing.T) {
 // several biomes to confirm Apply never panics on real-world inputs. A panic
 // during generation would crash the server.
 func TestSurfaceRuleNoPanic(t *testing.T) {
-	rule, err := LoadOverworldSurfaceRule()
-	if err != nil {
-		t.Fatalf("load: %v", err)
-	}
+	rules := loadTestRules(t)
 	biomes := []string{
 		"minecraft:plains", "minecraft:desert", "minecraft:forest",
 		"minecraft:badlands", "minecraft:snowy_plains", "minecraft:ocean",
 		"minecraft:mushroom_fields", "minecraft:wooded_badlands",
 	}
+	ctx := rules.NewContext()
+	rules.BeginColumn(ctx, 100, 100)
+	ctx.SeaLevel, ctx.MinY = 63, -64
+	ctx.MinSurfaceLevel, ctx.WaterHeight = 80, NoWaterAbove
+	ctx.SurfaceDepth = 3
+	ctx.Rng = rand.New(rand.NewSource(1))
 	for _, b := range biomes {
+		ctx.BiomeName = b
 		for y := 0; y < 100; y++ {
-			ctx := &SurfaceContext{
-				X: 100, Y: y, Z: 100,
-				StoneDepthAbove: 100 - y, StoneDepthBelow: y + 1,
-				SeaLevel: 63, BiomeName: b, MinY: -64,
-				MinSurfaceLevel: 80, WaterHeight: NoWaterAbove,
-				Rng: rand.New(rand.NewSource(1)),
-			}
-			rule.Apply(ctx) // must not panic
+			ctx.Y = y
+			ctx.StoneDepthAbove, ctx.StoneDepthBelow = 100-y, y+1
+			rules.Apply(ctx) // must not panic
 		}
 	}
 }
@@ -48,17 +61,17 @@ func TestSurfaceRuleNoPanic(t *testing.T) {
 // TestSurfaceBedrockFloor confirms the bottom of the world resolves to bedrock
 // (the vertical_gradient bedrock_floor rule is the first rule in the tree).
 func TestSurfaceBedrockFloor(t *testing.T) {
-	rule, err := LoadOverworldSurfaceRule()
-	if err != nil {
-		t.Fatalf("load: %v", err)
-	}
-	ctx := &SurfaceContext{
-		X: 0, Y: -64, Z: 0, StoneDepthAbove: 1, StoneDepthBelow: 1,
-		SeaLevel: 63, BiomeName: "minecraft:plains", MinY: -64,
-		MinSurfaceLevel: 62, WaterHeight: NoWaterAbove,
-		Rng: rand.New(rand.NewSource(1)),
-	}
-	state, ok := rule.Apply(ctx)
+	rules := loadTestRules(t)
+	ctx := rules.NewContext()
+	rules.BeginColumn(ctx, 0, 0)
+	ctx.Y = -64
+	ctx.StoneDepthAbove, ctx.StoneDepthBelow = 1, 1
+	ctx.SeaLevel, ctx.MinY = 63, -64
+	ctx.BiomeName = "minecraft:plains"
+	ctx.MinSurfaceLevel, ctx.WaterHeight = 62, NoWaterAbove
+	ctx.SurfaceDepth = 3
+	ctx.Rng = rand.New(rand.NewSource(1))
+	state, ok := rules.Apply(ctx)
 	if !ok {
 		t.Fatal("no rule matched at bedrock floor")
 	}
@@ -84,12 +97,23 @@ func TestSurfaceBlockIDResolution(t *testing.T) {
 		{"minecraft:red_sand", nil, 123},
 		{"minecraft:coarse_dirt", nil, 11},
 		{"minecraft:calcite", nil, 24687},
+		{"minecraft:deepslate", map[string]string{"axis": "y"}, 27924},
+		{"minecraft:mud", nil, 27922},
+		{"minecraft:air", nil, 0},
 	}
 	for _, c := range cases {
-		if got := surfaceBlockID(c.name, c.props); got != c.want {
+		got, ok := surfaceBlockID(c.name, c.props)
+		if !ok {
+			t.Errorf("surfaceBlockID(%q,%v) not in the table", c.name, c.props)
+			continue
+		}
+		if got != c.want {
 			t.Errorf("surfaceBlockID(%q,%v) = %d, want %d", c.name, c.props, got, c.want)
 		}
 	}
+	if _, ok := surfaceBlockID("minecraft:not_a_block", nil); ok {
+		t.Error("surfaceBlockID accepted an unknown name")
+	}
 }
 
 // TestIsColdBiome confirms the snow-cover predicate recognises cold biomes so