Evaluation of the Carroll Cave Area Hydrogeologic Data Base


By M.D. Hall    August 2003




The purpose of this report is to describe the Carroll area hydrogeologic database and the potential controls on cave development in the area.  Exploration potential within the Carroll area will be discussed.  The database is a collection of 19 different map types that can be overlaid and compared to find relationships between different data sets and aerial photo, Landsat and topographic maps.



These maps will be discussed in greater detail below.




Topographic Analysis



The area being evaluated is the Montreal Digital Elevation Model (DEM), which represents the same area as covered by the Montreal 7.5 Minute Quadrangle (raster image). There are no sinkholes on the raster topographic map near or over the known cave, although many small sinkholes are known to exist in the area.  A method of identifying these sinkholes was sought out and the digital  (DEM) and Landsat images were used to find sinks in the area.  The sinkholes are key to understanding the potential extensions of the cave system and where undiscovered portions of the cave may exist.



Part of Montreal 7.5 Minute Quadrangle (USGS) over Carroll Cave


Part of the database includes Landsat 5 images over the Montreal area.  The images below are at a bandwidth that would be near the infrared portion of the electromagnetic spectrum (non-visible light).  Many sinkhole and sinkhole ponds show up on the second Landsat image as dark blue circles on the image.  The third Landsat image is a close-up of the Carroll Cave area. The fourth Landsat image is a reverse color near- infrared composite image that shows the sinkholes and sinkhole ponds as light colored circles.


It is interesting that very few sinkholes appear over the known cave -see previous reports at   (GEOLOGY).  The Landsat images confirm the lack of sinkholes over the known cave, although many sinks exist west of Carroll.  The procedure for locating sinkholes is therefore: (1) check raster topo (2) look at Landsat infrared band (3) filter the DEM for sinkholes and (4) field check and verify the location and type of sinkhole present.  Aerial photos were also used to confirm some of the Landsat sinkhole positions and were found to be useful. One thing I would like to try is to use stereoscopic images of the aerial photos to better identify sinkhole locations in the area.





Landsat Images



Landsat 5 image from Ozark Fisheries to Toronto Springs Area







Landsat 5 image over Carroll Cave area.  Note sinkholes and sinkhole ponds as blue circles

Landsat 5 image over Carroll Cave


Landsat 5 image over Carroll Cave – Reverse color showing possible sinkhole locations near Carroll


Aerial Photograph over Carroll Cave Area


Accuracy of Maps


It is important to evaluate the accuracy of these maps.  Based on comparing of several known points, it appears that the database maps are accurate to within 100’.

As an example, the Carroll Cave entrance along Mill Creek is compared with the Raster topo available from TOPOZONE.  The difference between the raster and digital versions of each maps (based on UTM coordinates) is 9 m north-south and 29 m east-west for the cave location.  This is consistent with the accuracy of the DEM which is based on a 30 m grid cell spacing.  In other words, the accuracy of the maps will be generally about 100’ in relationship to the true location on the earth.

Raster Image of Carroll Cave Area with Mill Creek entrance located


Digital Elevation Model (DEM) around Carroll Cave with Mill Creek Entrance Location shown in lower left corner of map in UTM coordinates (meters)



3-D Topography


A three dimensional image of the Montreal area DEM shows surface drainage runs off north and east of the topographic ridge that runs N-S from the town of Montreal, MO. The elevation high is in the extreme SW corner of the area and the elevation low is in the NE part of the area near Toronto Springs.




3-Dimensional View of Montreal Area Topography showing primary surface drainage towards east-north-east.

Shaded Relief Map of Montreal Area Topography







Topography and Sinkholes

Topographic Elevations with contours and Carroll Survey

5’ sinkholes and shaded relief topography and Carroll Survey

Map showing 5’deep sinkholes and Carroll Survey

Map showing 5’deep sinkholes and Carroll Survey (zoomed)

Detailed Shaded Relief Map, 5’ sinkholes, and Carroll Survey (zoomed)



As previously mentioned, sinkholes were identified using several methods.  The above maps show in sinkholes greater than 5’ in depth and greater than .2 acres in area in the vicinity of the cave.  Note that most sinks are located either in Barnett Hollow to the northwest or Traw Hollow to the south: not over the known cave.


Likely candidates for sinkholes over known Carroll Passageways













Surface Hydrology of DEM area showing Toronto Spring and Toronto Cave Spring


Surface flow, as indicated by the topography, runs towards the north and east.  Toronto Springs, the discharge outlet for the Carroll Cave system, is the spring located in the extreme northeast corner of the Montreal quadrangle, while Toronto Cave Spring outlet is located immediately north and west of Toronto Springs (see above map).

Water-table elevation of Gasconade aquifer


Groundwater flows northeast towards Toronto Springs (see above map).


A very interesting field observation is that a number of sinkholes occur in alluvial fill: sands, gravels and hillside debris, which has filled in the bottoms of Barnett Hollow, Traw and Davis Hollow.  The alluvial fill is extensive and tends to obscure the sinkholes that have formed in the Gasconade below the fill.

Location of Alluvium- Red line-approximate location of perennial surface flow


























Geologic Structure Top of Lower Gasconade and Carroll Survey


Carroll Cave follows the dip of geologic structure in the Upper Thunder River-Carroll River passages, but follows strike of geologic structure in the Lower Thunder River sections.



Residual Topography


Residual Topographic analysis refers to the attempt to reconstruct the pre-structural terrain of the area.  The present day topography reflects both structural deformation of the geologic strata and surface erosion/ karst development of that structured surface. Residual topography is the theoretical topography that would have existed had there not been bending or folding of the rocks.  This allows one to see the effects of simple erosion and karst development on the terrain easier.


Closed residual topographic lows in Traw, Barnett and Garwin Hollows suggest that these areas were more intensely karsted than other nearby areas.




Residual Contour Map and Carroll Survey

Residual Contour Map around Carroll Cave (zoomed)





Carroll Cave is generally between 50 and 200’ below the surface.  Overburden of 118 feet exists at the new entrance borehole.  The collapse end has about 80’ of overburden while Traw Hollow has 20-40’.


Overburden and Sinkhole Map


Overburden, Sinkhole Map, and Carroll Survey (zoomed) (20’ contour interval-red= 0 ‘)


The most favorable areas to dig into the Carroll system are in the Traw Hollow, Barnett Hollow and Davis Hollow areas.  Sinkholes south of Montreal are in an area where the overburden is thicker and therefore not a favorable.


Detail of Sinkhole Locations, Carroll Survey and Overburden Map


Surface Geology


The surface geology of the area is dominated by the Roubidoux and Gasconade formations. The Roubidoux is a dolomite and sandstone formation while the Gasconade is a dolomite with extensive chert lens in the Upper Gasconade.  The Lower Gasconade formation is less chert prone and is the formation that Carroll is formed in.


Sinkholes formed in the Lower Gasconade would therefore have higher probability of connecting with Carroll since they would have a common strata of development.

Surface Geologic Map-Red Upper Roubidoux, Green Lower Roubidoux, Brown Upper Gasconade, Yellow Lower Gasconade

Geologic Map, 5’ Sinkholes, and Carroll Survey

The preceding map shows that most of the sinkholes in the immediate vicinity of Carroll are developed in the Lower Gasconade formation although a number of sinkholes are located in the Upper Gasconade.


Fracture Detection and Relationship to Karst


Another geologic question is the orientation of fractures in the Gasconade.  The surface topography was mathematically processed and evaluated using a narrow band color scale.   This procedure helps delineate the sharp, linear features, which could be joint (fracture) or fault patterns.


Map showing possible joint (fractures) patterns



Map Showing possible joint (fractures) patterns and Carroll Survey


The fracture patterns that seem to be showing up are NE-SW and N-S with a third set NW-SE.  This is consistent with observation of joint patterns in Carroll and my fieldwork in the area. 




Map Showing possible joint (fractures) patterns and Carroll Survey (zoomed)

Map Showing possible joint (fracture) patterns and sinkholes

Fractures and 5’ sinkholes


It is interesting that many of the sinkholes lie along the fractures as highlighted on the preceding map.




Map Calculus


Several different calculus operations were performed on the topography to highlight various features. 


First Directional Derivative of Topography


I have found derivative maps will occasionally help differentiate geologic formation boundaries. In this case, there doesn’t appear to be high correlation with the geologic boundaries and the derivative of topography.



First Directional Derivative of Topography

Profile Curvature of Topography


Profile curvature is another map calculus technique, which can sometimes be used to locate displaced fractures: faults.  The curvature map of the Montreal area shows no well-defined faults, except for a possible fault near Carroll (see map below).


Profile Curvature of Topography

Karst Features


Karst Features Map


Perhaps the most important aspect of the database is to keep track of the current cave survey and the current sinkhole survey.


Detailed Map of Karst Features that have been field surveyed


Karst Features Map and Topography




The karst features of the area are scattered somewhat randomly.  Most sinkholes that have been field surveyed are located in the Gasconade formation.  Many sinkholes are located west of the cave, south of the town of Montreal.  It is likely that Carroll extends northwest and south towards sinkholes in Barnett Hollow and Traw Hollow.





-Accuracy of the database maps is +/- 100’

-Sinkholes are detectable on Landsat, aerial photos and the DEM

-Closed residual topographic lows in Traw, Barnett and Garwin Hollows suggest      that these areas were more intensely karsted than other nearby areas

-Overburden varies from 0- 170’ over the cave

-Most sinkholes occur in the Gasconade formation around Carroll

-The fracture patterns that seem to be showing up are NE-SW and N-S with a third set NW-SE.

-Only one small fault is detected close to Carroll