Caves and Karst Geology Field Trip

The cliffs rising above the village of Monarch located in the Little Belt Mountains in Central Montana (bottom center in the photo above) are made of limestone.  The limestone is composed of shells of marine animals deposited in a shallow sea.  The limestone is about 1700 feet thick and is known as the Madison Limestone.  It looks like a simple story--just layer on layer of limestone, all nicely parallel to one another.  However, there are complications!

Limestone is slowly soluble in water.  Over millions of years large volumes of limestone were dissolved.  Perhaps 25% of the Madison limestone has been removed by dissolution.  Madison limestone and Swiss cheese have one thing in common.  They are both full of holes.  Lick Creek Cave in the Little Belt Mountains has a room over 500 feet in diameter with an 80 foot ceiling--the largest room in Montana.  It is part of a network of fissures, cracks, crawlways, tunnels, caves, and caverns in the Madison Limestone.

On the time scale of hundreds to tens of thousands of years large openings underground are unstable.  Pieces break off of the ceiling of a large room and accumulate on the floor.  The cave gradually migrates upwards.  When it reaches the surface a "sinkhole" is formed.  The south wall of the "Monarch Sink" shown on the right is part of one such sinkhole.  Rod Benson, Helena High School Earth Science teacher, is standing on the snow at the base of the 84 foot high cliff.  There is a cave underneath.  The floor of this sinkhole is very slowly, but actively sinking as shown by the "fresh" and unweathered appearance of the cliff.

The caves that are currently open in the Madison Limestone are from the latest of three major periods of cave formation.  Madison Limestone was deposited as sea shells on the sea floor about 330 million years ago.  By about 320 million years ago this area was uplifted so that it was above sea level.  Red mud was deposited in many freshwater lakes and eventually became the bright red Kibbey shale.  The mud was red because at that time Central Montana was located very close to the equator.  (Red tropical soil is called laterite.)  Due to sea floor spreading and plate tectonics, Montana has moved northwards to its present position.  Groundwater percolated down through the cracks in the limestone and dissolved out an extensive cave system.  Some of the red clay washed down into the cave system creating a bright red "dye."  Then gradually the land subsided and the limestone was deeply buried.  The weight of the overburden was so great that all caves collapsed and all open holes were filled.  This was the first cycle of "karst" or cave formation.  By 290 million years ago, the area had again been uplifted and a second very extensive system of caves was formed by groundwater percolating down through the limestone.  Subsequent burial caused the collapse of all of the second generation of caves.  Due to uplift during the last 50 million years a third generation of caves has developed.  Many of these caves are open.

The gray limestone on the left of the photo consists of angular fragments of limestone.  Solid limestone in the walls and ceiling of a cave were "shattered" when the cave collapsed due to the weight of the overburden during subsidence and burial.  The fragments were cemented together so that this is now a cohesive rock.  The orange/light red/tan/light brown rocks on the right are "breakdown" that accumulated in an formerly open cave--similar to what is seen on the floor of the Dome Room of Lick Creek Cave today.  The red color comes from red clay in the overlying Kibbey shale that was washed down through the cave by groundwater.  During burial the angular fragments and clay matrix have been cemented together to make cohesive rock.

In the photo on the right four open caves are visible as black holes in the red-stained cliff.  However, the entire red-stained middle part of the photo is a former cave that was filled with limestone blocks and red clay (washed down from the formation above--the red-colored Kibbey shale).  In other words, third generation caves have formed in a filled-in former cave from the first or second generation.  The cliff on the left side of the photo above consists of "breakdown breccia" or cave rubble created by the collapse of a former cave.

There is evidence of a large former cave in the cliff of Madison Limestone at Albright, located in Sluice Boxes State Park. The cliff in the photo on the left is nearly 300 feet high.  The orange-stained upper half to two thirds of the cliff consists of cave collapse breccia--angular broken fragments of limestone formed by the collapse of a former cave.  This former cave was over 150 feet high and more than 500 feet in diameter.

Some caves were huge.  There is a collapsed-filled in cave on the northeast side of Belt Butte that is half a mile wide and three-quarters of a mile long.  The ceiling dropped 600 feet. [See the discussion by Berg (1991).]

The photo on the right shows the north end of Sluices Boxes State Park where Belt Creek flows out from the narrow canyon and across flat, bottom land at Riceville.  The parking loop for the Park is on the left of the photo.  Beyond the flat bottom land one can see near-horizontal bedding in the steep slope above Belt Creek--the faint white layer in the shadow in the upper right of the photo.  In contrast, the sunlit limestone beds on the right side of the photo are tilted so much that they go down to the creek and extend under the bottom land.  Why did the limestone "sag downwards" here, but is nearly horizontal (with only a 7° dip) in other places?  It is because there is a large collapsed cave underneath.  The cave may extend under much of the bottom land seen in the photo.

The bright red Kibbey shale shown in the photo on the left is across the highway from the Scenic Overlook on U.S. 89 above the north end of Sluice Boxes State Park.  The Scenic Overlook is located just to the right of the right edge of the photo above.  The Kibbey shale overlies the Madison Limestone.  Note how the red shale is missing from the left half of the photo.  A large slump block of the shale broke off and slid downhill into the depression below--perhaps when there was an open sinkhole in what is now the bottom land of the photo above.  The details in the roadcut tell us quite a bit about how and when the slumping occurred.  The tan-colored material on the left side of the photo is gravel with large boulders, known as conglomerate.  The conglomerate was deposited by Belt Creek during a large flood when the floor of the valley was at this elevation, about 150 feet higher than the current floor of the valley--during the Ice Ages, tens of thousands to hundreds of thousands of years ago.  You can see in the lower left corner of the photo how conglomerate was deposited against the wall of red shale and then a piece of red shale and sandstone slid down on the conglomerate.  More conglomerate was deposited, followed by another piece of shale and sandstone, and then more conglomerate on top.

Much of the rainwater falling on ground where Madison Limestone is exposed on the surface simply disappears underground into the Madison Limestone Aquifer--the largest artesian aquifer in the United States.  As Belt Creek flows across the Madison Limestone through the Sluice Boxes State Park, it loses water to the aquifer.  Most of this groundwater in the Madison Limestone Aquifer comes to the surface at Giant Springs.  However, some of the water stays in the aquifer and flows underground to Manitoba.

Terms defined and illustrated on the field trip include: karst, phraetic, vadose, speleothem, breakdown, lappies or solution domes, piezometric surface, hornfels, sill, laccolith, marble.

This field trip was held 4 times in July and August of 1999 and a number of times since then. A guidebook was written for the field trip participants.

References:

Berg, R.B., 1991, Field Guide to Belt Butte and Tiger Butte, in Baker, D.W. and Berg, R.B., Guidebook of the Central Montana Alkalic Province, Montana Bureau of Mines and Geology Special Publication 100, p. 163-174.

Campbell, N.P., 1978, Caves of Montana. Montana Bureau of Mines and Geology Bulletin 105.

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