Agents of Transport, Limestone Caves, Karst Landscapes & Mass Wasting

Agents of Transport (Post-Weathering Pathways)

• Once rock is broken down (weathering) and physically removed from its source (erosion), the next step is transportation.

• Generic term for any loose, in-transit material: colluvium.

• Gravity is the universal driving force; it usually works in tandem with a second agent.

Principal Agents & Where They Re-appear in the Course

• Water & Mud on Slopes
– Manifest as mass movement / mass wasting (details in Chapter 12).
– Dominated by gravity; water simply lubricates or adds mass.

• Glaciers (Chapter 14, Unit 4)
– Ice acts as a conveyer belt embedded with rock debris.
– Gravity pulls the glacier downhill; ice abrades, plucks, and transports.

• Running Water in Streams (Chapter 13, first in Unit 4)
– Flowing water carries a load ranging from clay-sized particles to boulders.
– Transport capacity depends on discharge $Q$ , velocity $v$ and channel slope $S$ .

• Waves Along Shorelines (Chapter 15)
– Move sediment both laterally (longshore drift) and on/offshore.
– Shape beaches, spits, barrier islands.

• Biomes & Biogeography is the only Unit 4 chapter not governed by a transport agent.

Limestone Caves (Subsurface Dissolution Landscapes)

• Form mainly in carbonate rocks (limestone, dolostone) through dissolution ⇒ a chemical weathering process.

Step-wise Cave Creation (Fig. 12-6-A1)

Original Bedding: horizontally layered limestone.
Water Infiltration via joints/bedding planes. Slightly acidic water (H₂CO₃) starts dissolving CaCO₃.
Positive Feedback: widening fracture ⇒ more flow ⇒ faster dissolution ⇒ enlarged voids.
Underground Streams & Pools develop; entire network water-filled.

How “Dry” Caves Form

• Water Table Drop – climatic shift or regional drainage lowers groundwater.
• Tectonic Uplift – cave body is physically raised above saturated zone.
• Result: air-filled passage, but still damp; slow drip continues.

Roof Collapse & Surface Connection

• Progressive thinning + loss of buoyant water support ⇒ ceiling failure.
• Collapse provides human entry & accelerates drying.

Cave Features & Deposits

• Sinkhole – surface depression formed as underlying roof subsides/dissolves. First stage of a roof collapse.
• Flowstones / Draperies
– Water rich in $ext{Ca}^{2+}+2 ext{HCO}3^-$ flows over walls/floors; CO₂ degasses ⇒ $ext{CaCO}3$ precipitates.
– Rock name: travertine. Looks like a frozen waterfall.

• Stalactites
– Hang tight to the ceiling (t for top).
– Form drip-by-drip; each droplet leaves a CaCO₃ ring.

• Stalagmites
– M on the ground; shape resembles an $M$ (wide base, pointy top).
– Grow upward from falling droplets.

• Column – stalactite + stalagmite merge into a single pillar.

Karst Topography (Surface Expression of Dissolution)

• Develops where thick, water-soluble bedrock (limestone, gypsum, halite) is coupled with ample water.

Diagnostic Surface Traits

Pitted / Uneven Terrain – dense field of closed depressions (sinkholes).
Poor Surface Drainage – few rivers; precipitation quickly percolates underground.
Disappearing Streams – surface stream enters karst zone, vanishes into a swallow hole, continues in cave conduits.
Sinkholes & Collapsed Sinkholes – latter often become circular lakes if the water table intersects.
Karst Towers – residual steep pinnacles left after vertical joints widen and surrounding limestone is removed; common in warm, humid settings (e.g.
– Hạ Long Bay, Vietnam
– Southern China, Guilin region).

Real-World Examples

• Central Florida – Lake-studded landscape near Orlando/Disney owes its lakes to collapsed sinkholes.
• Hạ Long Bay – dramatic karst towers rising from the sea; once a contiguous limestone block.

Global Distribution

• Concentrated where past/present climate provided water + carbonate bedrock.
• Cold high-latitude karst (Greenland, N. Canada) implies earlier low-latitude positions (continental drift evidence).

Mass Wasting (Chapter 12 Deep-Dive)

• Definition: down-slope transport of weathered material under gravity; may be aided by water, ice, or seismic shaking.

Gravity vs. Slope Angle

• On a flat surface gravity acts normal to ground ⇒ no down-slope component.
• On a slope, gravity has two components:
– Driving force $Gd = G \sin \theta$ (pulls downhill) – Resisting force $Gr = G \cos \theta$ (presses material into slope).
• If $Gd > Gr$ ⇒ slope failure likely.

Angle of Repose (A_r)

• Steepest angle at which unconsolidated material remains stable.

Material	Water Condition	$A_r$ (approx.)
Gravel (angular)	dry	$45^\circ$
Sand (rounded)	dry	$34^\circ$
Sand	water-saturated	$\approx 0{-}15^\circ$ (nearly flat)
Sand	damp / wet	$\approx 45^\circ$ (capillary cohesion)

• Adding more of the same material enlarges the base, preserving $A_r$ .

Talus Slope

• Cone-shaped apron of rock debris at base of a cliff.
• Its surface angle ≈ material’s $A_r$ .

Classifying Slope Failures

Factors considered:

Mechanism – fall, slide, flow, creep.
Material – rock, debris, earth (soil), snow, mixed.
Rate – slow ⇢ extremely rapid.
Water Content – dry ⇢ saturated.

Speed–Moisture Matrix (Simplified)

                D R Y                     W E T
   Fast   ┌ rock fall / debris fall ┐  debris avalanche / debris flow
   |      |      rock slide         |  earth (mud) flow
   v      |  rotational slide       |  debris slide
  Slow    └ creep – solifluction ┘  └—— (increasing H₂O) ——┘

Individual Mass-Wasting Types

Rock Fall / Debris Fall

• Fastest & driest; material drops vertically off a cliff.
• Triggers: frost-wedging, spring melt, earthquakes, road-cut undercutting.
• Diagnostic: fresh talus cones or blocky piles at foot of a steep face.

Rock Slide

• Cohesive block(s) of bedrock sliding down an inclined plane; still relatively dry.
• Leave a planar or slightly stepped scar.

Rotational Slide (Slump)

• Movement along curving concave surface; block rotates outward and downward.
• Water often lubricates a clay layer at depth.
• Generates stair-step scarps.

Creep

• Imperceptibly slow (~mm yr⁻¹).
• Driven by freeze-thaw or wet-dry expansion/contraction cycles.
• Indicators: tilted fence posts/trees, bent bedrock layers, terracettes.

Solifluction

• Arctic/High-alpine equivalent of creep.
• Summer thaw of the active layer slides over impermeable permafrost.
• Produces lobate, tongue-shaped flow structures.

Debris Slide

• Shallow, water-assisted slide of soil + loose regolith.
• Leaves a clear head scarp.
• Speed: moderate; hazards mainly property damage.

Earth (Mud) Flow

• Viscous soupy mass ("wet concrete") of fine-grained sediment.
• Faster than slides but slow enough for people to escape; buildings often destroyed.

Debris Flow / Debris Avalanche

• Fast, high-water, heterogeneous mix (mud, boulders, vegetation).
• Can outrun humans; travel long distances in confined valleys.
• Volcanic variant with ash + water = lahar.

Relative Hazards

• High-risk to life: rock fall, debris avalanche, debris flow.
• High property loss: earth flow, large rotational slide.
• Low immediate danger: creep, solifluction (long-term structural stress).

Ethical & Practical Implications

• Urban Planning – avoid building on talus slopes, old slide scars, or sinkhole-prone karst.
• Infrastructure Design – drainage control, retaining walls, netting, & rock bolts mitigate rockfalls and slides.
• Climate Change – thawing permafrost may accelerate solifluction and destabilize Arctic infrastructure.
• Tourism vs. Conservation – cave speleothems (stalactites/stalagmites) grow mm century⁻¹; breakage or oil from hands halts growth → strict access rules.