The Geology of Finding Caves

By Andy Porter, NSS 26258FE

(Outline for TAG Caving Lecture Series)



  1. INTRODUCTION
    1. Understanding geology to find caves
    2. Just Cumberland Plateau


  2. ROCKS
    1. Cumberland Plateau
      1. Alabama to Kentucky
      2. Largest concentration of caves in US
      3. Jackson Co. Al, >1500 caves
      4. Plateau forms ideal setting to form caves, Why?
    2. Four Geologic Layers (Draw Cross Section)
      1. Top 1/4, Sandstone, Shale, Coal
        1. 310-320 million years old
        2. Formed during Pennsylvanian age
        3. Swamps, lagoons, beach environment
        4. Sea levels rose and fell
      2. Bottom 3/4, Mississippian Carbonates
        1. 320-340 million year old
        2. Shale, dolomite, near top/ Rest Limestone
        3. Formed in shallow ocean
          1. Like carbonate platforms in Florida today
    3. Pennsylvanian, Break into two units
      1. Warren Point Sandstone
        1. Forms cliffs around edge of Plateau
      2. Racoon Formation, Shale
    4. Mississippian, Break into three units
      1. Pennington Formation
        1. Various layers of dolomite, shale, limestone, some sandstone
        2. Same age layer can be different in different areas of TAG
      2. Bangor Limestone
        1. Massively Bedded, Very Pure, 150-250 feet thick
      3. Monteagle Limestone
        1. Massively Bedded, Very Pure, 150-250 feet thick
      4. Hartselle
        1. Between Bangor and Monteagle
        2. In Tennessee, 5-50 feet thick Sandstone
        3. In Alabama, 0-15 foot thick Shale
          1. Rare as sandstone
          2. Found ½ way up trail to Neversink
        4. Although thin, important, Allows large caves to form.


  3. EROSION AND HYDROLOGY
    1. Pennsylvanian Caprock
      1. Big Sponge, stores groundwater
      2. When groundwater-table full, runoff
      3. Runoff carves huge canyons, Gulfs, Valleys, and Coves
      4. Groundwater discharges at plateau edge as springs
      5. Runoff responds to rain. Flashy and violent
      6. Groundwater constant, steady. Affected by Drought
    2. Caprock Resists Erosion, Hillside Retreat
      1. Limestone rapidly removed by dissolution
      2. If not for Caprock, Cumberland Plateau dissolved away by now
        1. Several 10s of 1000s of years, hillside retreats inch or two
        2. Several 100s of 1000s of years, hillside retreats 10s of feet
        3. Few million years, hillside retreats hundreds of feet to a mile
        4. In a few tens of Millions of years Cumberland Plateau will be gone
        5. Imagine block of ice melting in Sun
    3. Rebound
      1. Rocks on top of one another create pressure
      2. Hillside retreat removes rock, lower layers experience pressure relief.
        1. Known as rebound
        2. The rocks will expand by tiny fraction of inch
        3. Happens in large and deep open pit mines.
          1. Floor of surface pit warps upward
      3. Flank of Mountain rebounds while core of mountain under compression
        1. Joins or weaknesses open and expand to relieve stresses between rebounding flank and compressed core
        2. Joints open a tiny fraction of an inch, allows water in
        3. Water finds path to preexisting conduit or new path to valley floor
        4. Will take 1000s of years to form new cave from new path
    4. Hydrology
      1. Discharge or runoff from plateau sinks into upper Pennington carbonates
      2. Shale or other impermeable layer forces water to surface
        1. known as "Pennington spring"
        2. Caves form at these springs
      3. Water reaches Pennington-Bangor Contact
        1. Commonly as large surface waterfall
    5. Pennington Contact
      1. Contact usually Dolomite (CaMgCO3) and chert
      2. Less soluble than limestone, Forms bench
      3. Bench covered in thick layer of soil.
        1. Collects and concentrates rainwater, becomes acidic, runs off to edge of contact
      4. Focused stream or drip finds joint, same spot for 1000s of years, forms pit
        1. I call this concentrated drip over time


      5. Limestone below contact I call Karst Barons,
        1. vast area of karst features, desert like, no pits
        2. Any rain water finds nearest crack, disappears
        3. Each karst feature no more than few 100 ft2 of drainage basin
        4. No concentrated drip over time!
        5. No caprock to protect from hillside retreat, retreat faster than caves have time to form
      6. Pennington contact both provides constant source of water to form caves and protects them from erosion and hillside retreat.
      7. Erosion from mountainside above can fill in cave entrances with
        1. Sandstone float, known as Colluvium
    6. Hartselle Contact
      1. In Tennessee, Hartselle most important
        1. Hartselle is 5-50 foot thick Sandstone layer
        2. Again both focuses water and protects limestone from erosion.
        3. Forms very wide benches, 1000s of feet from mountain side
        4. Easily traced on contour maps
        5. Some benches large and flat enough to farm
        6. Can form sinkhole plains
        7. Forms thousands of caves along contact, mostly nerd holes
        8. Can protect underlying limestone for millions of years
          1. Allows large river caves to reach maturity
          2. Reason why lots of large river caves form in Tennessee
          3. Hartselle in Kentucky allowed largest cave in world to form
            1. Mammoth Cave
            2. Not called the Hartselle there
        9. In Tennessee prevents deep caves from forming
      2. In Alabama Hartselle a shale.
        1. More deep caves, shale easier to breach
        2. Sometimes absent, Bangor limestone on Monteagle limestone
        3. Not worth walking contact, very difficult to spot on surface
    7. Evolution of Large stream Caves
      1. Most large valley floor stream caves follow contour of mountain
        1. Used as evidence for rebound hypothesis
      2. Initial active stream cave deep in mountain flank at edge of rebound
      3. As cave Matures, source of water pirated from valley retreat
        1. Valley retreats into core of plateau from erosion
        2. Insurgence retreats with valley
        3. Spring resurgence retreats also
      4. Hillside retreat intersects cave, begins to fragment, fill with formations
      5. Leaves small pieces of old dry passage, rooms near valley floor
        1. Important to Native Americans for habitat
      6. New active stream cave develops at edge of rebound deep in mountain




  1. Finding Known Caves
    1. Finding known caves really comes down to statistics, here I will prove it
      1. Looking for known cave with 1x1 ft entrance in an area 1000x1000 ft
      2. Jump out of an airplane and land in the middle blindfolded. You peak out of a crack in bottom of your blindfold at the ground.
        1. Your chances to find the cave are 1/1,000,000
      3. You can take off your blindfold and see 20 feet around you
        1. 20x20 feet= 400 ft2 or 1/2500 chance.
      4. You can walk so walk a constant contour all the way through the area
        1. 40x1000=40,000 ft2 or 1/25 chance
      5. You invite four friends along, after all you should not be caving solo. you all contour around staying 40 feet apart.
        1. 40x1000 x 5 people=200,000 ft2 or 1/5 chance.
      6. You can drop to a lower elevation and walk back, do this four more times
        1. 440x1000 x 5 people x 5 times=1,000,000 ft2 or 1/1 chance
      7. QED
    2. What prevents this 1/1 chance?
      1. People don't stay together, they wander off, leave gaps
      2. Don't stick to a contour, Wander all over a mountain in random fashion
        1. End up covering same areas more than once, miss others
        2. Do not know where they have been
        3. If you don't believe me, leave your pack in the woods and wander off, then try to find it again!
      3. Some members lose interest, sit down, take easiest path, not willing to check every 1x1 foot hole they come across.
      4. Brush may obscure hole.
      5. Landowners sometimes cover entrance
      6. Not using a topo, in the totally wrong area, 0/1,000,000 chance
    3. If a cave is located in ravine and you go to that ravine and it is not there, check the next two or three ravines to the left and right of the ravine it supposed to be in
    4. Copy topo points and talk to someone who has been to cave if possible
    5. Have to be able to identify the contacts
      1. Takes practice, sometime hard to find
      2. Stay with contact, even when it is covered by float.
      3. The break in slope gives contact away when covered by float
    6. Use information in survey to help identify type of entrance
    7. Persistence and determination needed for success




  1. Finding New Caves
    1. Push everything you find to the Bitter End, Simple as that
    2. Of 10,000 caves in TAG only 5% significant, 95% are Nerd holes!
    3. Knowing when a lead has potential or is another nerdhole
      1. Based on intuition, how I find lots of cave.
      2. Have to visit a 1000 caves to get a feel for what leads to significant caves and what leads to nerd holes.
      3. Rely on common sense
        1. A strongly blowing hole above a large spring resurgence warrants extreme attention
        2. The same hole miles from the nearest spring may not.
        3. Big air almost always leads to big Cave
        4. Big water almost always leads to big cave
    4. Digging based on ones motivation
      1. Instant gratification
        1. dig takes no longer than an hour or two
        2. Obvious that something opens up on the other side
      2. Project digs
        1. Dig takes many day long trips
        2. labor intensive
        3. Requires specialized equipment
        4. Blasting usually involved
      3. Choosing to dig is a lot like poker, You got to know when to fold, when to bluff (your help), and when to lay it all on the line because you know you have a good hand.
    5. Look for new caves in areas with known caves
      1. Do all known caves in area even if out of your way
      2. Visit known caves that have low State cave survey numbers
        1. The old timers did not push some caves or crawls very hard
        2. Small entrances near low number caves may be unchecked
    6. Look 10 times more closely at ravines than hillsides
    7. Carry some digging tools, only work on instant gratification digs during ridgewalk, save project dig for return trip
    8. Think non-traditional, i.e Take difficult path instead of easy path
    9. Follow your gut feeling, let it guide you to the caves
    10. Stop ridgewalk at large recognizable surface feature so you can continue later
    11. Turn in all your new caves to State cave survey after finish exploring
      1. Don't sit on leads too long, You can get scooped
      2. Turn cave in quickly, someone may name it out from under you
      3. Sometimes you find someone's secret cave
    12. Publish your finds on TAG-NET or local caving magazines for historical record