Tuesday, December 20, 2016


Like most readers of this Blog, I often (always) peruse rock and mineral shops when visiting towns and cities across the nation.  This fall while camping in western South Dakota I paid a call to about all shops in the Black Hills.  Two of them had interesting and similar specimens with labels stating: 1) moss rock; and 2) coral rock.  Both rocks were labeled as collected from Cascade Springs.
Now that name brought back a flood of memories from days gone past.  I first visited the spring, located south of the city of Hot Springs, back in the mid-1960s while a student at the University of South Dakota.  I had several friends from the nearby small town of Edgemont and could tag along on their trips home.  I was first introduced to Cascade Springs in the form of lounging in the warm sun and sharing a few bottles of a cold adult beverage.  Later in life I visited the Springs on a field trip and even at a later time camped in a small tent in the area and watched the stars twinkle in a very dark sky.

At any rate, I might accept moss rock as an identification that could result in a sale to an unwary tourist, but coral rock is just basically an untruth.  All it takes is a couple of clicks on a computer to receive information that fresh water corals really do not exist in South Dakota.  Although I knew the answer to the rock identification question, I wanted relive some memories and so off we went to Cascade Springs (six miles to Cascade Springs; eight miles to Cascade Falls).

Although my mind may be used and a bit rusty, it certainly indicated “things” have changed since my last visit about three decades ago.  What I first noticed was the increase in vegetation around the Springs and the resulting stream outlet, especially the rather prominent displays of poison ivy.  I remember, at least my mind thinks it remembers, walking along the stream below the Springs without getting tangled in a mess of vegetation.  Today that is an impossible task.  Oh well, maybe that thought is true, maybe not!
Cascade Creek below the Springs. Note the massive vegetation along the edge.
Black Hills National Forest (BHNF) manages Cascade Springs as a natural area and picnic ground and per the Agency (see References) there are several rare plants growing near the springs: These species include “tulip gentian (Eustoma grandiflorum), beaked spikerush (Eleocharis rostellata), southern maidenhair fern (Adiantum capillus-veneris), and stream orchid (Epipactis gigantea).”  The plants like the availability of open water during all four seasons as the discharge temperature is a constant 67ºF---not a hot spring, as most people would testify, but not a cold one either.  However, most articles I read would characterize 67º as “warm water” and above the ambient air temperature.  The BHNF pegs its discharge rate as ~22.5 cubic feet per second, the largest spring(s) in the Black Hills.  The Springs emerge from six different outlets, now covered with rock debris and gravel, and water is captured in a concrete pool before wandering downstream in the newly formed Cascade Creek. Ultimately Cascade Creek reaches the local base level, the Cheyenne River above Angostura Reservoir. The Springs release water from the Paleozoic Madison Limestone (aka Pahasapa Limestone of Mississippian age) and the Minnelusa Formation (limestone of Pennsylvanian-Permian age), both common aquifers (collectively known as the carbonate aquifer) in the region and a source of springs, both hot and cool/ambient, in South Dakota, Wyoming and Montana.  Or, the Springs could issue from the contact of the Minnekahta Limestone [Permian age] and the Spearfish Formation [an aquitard shale of Permian-Triassic age], or from the contact of the Minnelusa and Opeche formations (a possible aquitard between the Minnekahta and Minnelusa formations).  The Minnekahta is sometimes included in the term “carbonate aquifer” noted above. 
A stratigraphic section showing aquifer units around the southern Black Hills.  Section courtesy of Gries (2009).
Ford and others (1996) opinion is that rainwater passing through surface soil horizons picks up calcium carbonate that mixes with the aquifer water and travels through the karstic solution cavities to emerge at springs or streams.  The biogenic activities in the soil horizons have high levels of calcium bicarbonate.
Tufa collected from Cascade Falls during a personal outing decades ago!  Note longitudinal views of plant debris (tubes) weathered out of specimen.  Note porous nature of tufa as compared to travertine pictured below.  Width of specimen ~13 cm.
Collecting pool for Cascade Springs.

Casts (the tubes) of plant debris in tufa from Cascade Falls. See longitudinal view in photo above. Diameter of tubes ~1-2 mm. 
Cascade Falls September 2016.

Cascade Falls ca 1930s.  Original postcard owned by, and courtesy of, www.neplains.com.
Cartoon showing tufa formation at Cascade Falls.  The water drops off a hard ledge of sandstone and then scours out a basin in the Skull Creek Shale.  Personal observation plus information derived from Ray and Rahn (1997).

Downstream two miles from Cascade Springs is Cascade Falls, one of the more famous “swimming holes“ in South Dakota, and the site of  several tufa layers.  It was always my impression that tufa was a calcium carbonate (CaCO3) deposited in cool water situations as opposed to travertine (also a calcium carbonate) that forms in a warm to hot water environment.  Tufa generally is found attached to plants or plant debris, is usually quite porous and forms a carbonate layer over the plant debris that later “rots away” leaving behind tufa casts of the plants (and sometimes insects, vertebrates and mollusks).

This description of tufa and travertine has resided in my mind for decades but now was prodding my senses with a question—what is the temperature that distinguishes the formation of tufa from the formation of travertine? So, off I go to try and find the answer.

Travertine "terraces" produced by hot water springs at Hot Springs State Park, Wyoming.  See Blog posting May 13, 2011.
Ford and Pedley (1996) and Capezzuoli and others (2013) noted that travertine and tufa were often used indiscriminately as alternative names for fresh water limestone.  One person’s tufa seemed to be another person’s travertine.  This seemed especially true as I delved into the literature and found both names used for the same stratigraphic outcrops.  Capezzuoli and others (2013) defined the term travertine: “continental carbonates mainly composed of calcium carbonate deposits produced from non-marine, supersaturated calcium bicarbonate-rich waters, typically hydrothermal in origin. Travertine deposits are characterized chiefly by high depositional rates, regular bedding and fine lamination, low porosity, low permeability and an inorganic crystalline fabric. Bacteria and cyanophytes [photosynthetic bacteria] typically are the only associated organic constituents, due to the presence of unsuitable factors (for example, high temperature, high rates of deposition, pH and sulphur) for plant and tree growth (macrophytes). Aragonite rather than calcite may also be present… Such deposits are typical of tectonically active areas where geothermal heat flux (endogenic or volcanic) is high [generally higher than ~86º].”
Banded travertine collected from the Mayer "onyx" quarry in northern Arizona. Note compact nature of travertine and compare with photo of tufa above. See Blog posting March 20, 2015.

Tufa refers to “continental carbonates, composed dominantly of calcite and typical of karstic areas. These are typically produced from ambient temperature [generally less than ~68º], calcium bicarbonate-rich waters which are characterized by relatively low depositional rates producing highly porous bodies with poor bedding and lenticular profiles, but containing abundant remains of microphytes and macrophytes, invertebrates and bacteria. Secondary carbonate deposits (cements and speleothems) may also be associated. Aragonite is usually absent (except from peculiar high Mg/Ca ratio spring waters.”

It appears, then, that travertine does form in warm to hot waters heated by geothermal mechanisms, has a high rate of deposition, has low porosity and permeability, and does not contain the plant debris common in tufa.  Tufa forms in areas where ground water has traveled through rocks rich in calcium bicarbonate via fractures and caves [karstic], has poor bedding features, and contains plant and bacterial (“algal”) debris.  In addition, travertine often forms as mounds and terraces while tufa is often found in stream cascades and dams.
The water from Cascade Springs has its original source percolating through soil horizons over a wide area in the southern Black Hills and then traveling through the karstic cavities of the carbonate aquifer and therefore has a dissolved CO2 content much higher than the local atmosphere. The turbulence created by CO2-rich water flowing over the Falls degasses the dissolved CO2, the water chemistry equilibrium is messed up (CO2 level drops), and precipitation of tufa (CaCO3) takes place. I have not seen studies on Cascade Creek but in a “normal” situation after degassing and precipitation, the pH of the stream water decreases and the acidity increases.  I presumed since the water at Cascade Springs was saturated with CO2 that the pH would strongly basic; however, Lund (2016) noted the pH was neutral at 7.0.  But again, I am far from a water chemist.

At Cascade Falls the actual waterfall started as Cascade Creek flowed over a ledge of an indurated Cretaceous sandstone, the Newcastle Sandstone.  With this turbulence of the CO2-rich water, tufa began to form on the Newcastle and actually raised the height of the Falls.  Cascade Falls has been around for a long time since the terrace levels above the stream are composed of tufa.  I suppose this signals that Cascade Creek meandered over the valley in the geological past and CO2-rich water degassed and forced calcium carbonate out of solution as solid CaCO3.  Visitors can easily see the tufa at the Falls while an observant eye can locate tufa on the stream terraces. I presume this is the collecting area for the moss rock and coral rock displayed in the shops.
Tufa forming at Cascade Falls.
Why is travertine absent at Cascade Springs? Evidently, there does not seem to be a source in the immediate area that would supply heat to the aquifers in the Paleozoic limestones.  However, a few miles away the city of Hot Springs was named for their warm spring water (something like 8-9 warm/hot springs) and became an early soaking spa and advertised “disease-curing” resort.  The Mammoth Hotel and Bath House was built in the late 1880s and their spring water was ~90ºF (Lund, 2016).  For many years one of the top attractions in Hot Springs has been a large constructed swimming pool known as Evans Plunge that is fed by 87ºF springs that release about 11+ cubic feet per second.  So, why the presence of hot/warm springs at Hot Springs?  Rahn and Gries (1973), in their extensive study of large springs in the Black Hills, could not answer that question with certainty.  Their first “supposition” was heat supplied by the earth’s normal geothermal gradient; however, their studies concluded that “the unusually warm springs near the town of Hot Springs are too warm to be explained by the normal geothermal gradient.”  So, what about their other possibilities: 1) magma or some intrusive body may lie at a shallow depth under Hot Springs; however, there is little evidence for that possibility; 2) the ground water could be warmed by chemical weathering reactions of the water flowing through the rocks.  This mechanism seemed a good possibility; 3) the ground water may have been heated by radioactive decay in nearby rocks.  Locally the community of Provo had a flowing artesian well where water was about 139ºF and was evidently heated by decay of radioactive minerals.  However, the thermal waters at Hot Springs are not very radioactive; 4) Precambrian rocks under the town may have created a higher geothermal gradient.  This may be possible but not probable.  In studying their publication, I really don’t believe Rahn and Gries found a reasonable (at least one they believed in) mechanism to answer the heat question and I have been unable to locate in the literature other possible heat sources.  The best that I could come up with was to note that several deep wells in the Madison (across South Dakota, North Dakota and Wyoming) have elevated water temperatures!
One of the early resorts in Hot Springs, the Hotel Minnekahta.  Photo is from the Library of Congress collection and was taken by John Grabill ca. 1890.  
One can find travertine, or tufa, in the city of Hot Springs since it serves as a cement for the prominent beds of conglomerate along the Fall River.  In reference to the conglomerate, Gries (2009) stated that “at some time in the past, probably in late Pleistocene time, clay and gravel partly choked the [Fall River] canyon.  Then calcium carbonate, precipitated from the warm spring water, cemented them into solid rock.”  Is this travertine or tufa?   I was unable to identify with visual examination; however, the 87º water at Evans Plunge would suggest travertine.  Whatever the case, Rahn and Gries (1973) map of spring temperatures in the Black Hills has a nice anomalous, hot/warm, birdseye perched right on Hot Springs.
It seems as everyone in Hot Springs calls this "The Waterfall."  I am uncertain of the name or the source of the warm water.  Perhaps it comes from a spring above? Travertine is forming on rocks behind the falls. The cemented conglomerate is what caught my eye.
In contrast to the southern hot/warm springs the remainder of springs in the Black Hills are in the 40ºs-50ºs F range (Rahn and Gries, 1973).  One particular spring of interest is found about three miles above the junction (near community of Savoy) of Little Spearfish Creek and Spearfish Creek.  Here a spring in the Madison Limestone (aka Pahasapa) releases about 13 cubic feet per second of CO2-rich water into Little Spearfish Creek. Little Spearfish Creek rapidly flows east toward its merger with Spearfish Creek but soon encounters a hard dolomite bed of the Ordovician Whitewood Dolomite, and thus, there is massive turbulence in the Creek and the saturated CO2-rich water is degassed and tufa forms.  This feature is known as Roughlock Falls and is considered one of the more scenic spots in the Black Hills.  Again, the water is considered karstic in nature and flows through the quite porous Madison (Pahasapa) Limestone of Mississippian age. The water is “cold” but I could not locate an exact temperature but a nearby lodge owner told me the Falls freeze up in the winter.  It is interesting, at least to me, that the springs in the northern Hills seem cold water springs.  The northern Hills have numerous relatively young, ~50 Ma, igneous intrusions that would seem to be a good source of heat.  But then again, I am not a hard rock person or an aqueous geochemist or a hydrologist!
The upper falls at Roughlock Falls in Little Spearfish Canyon.  Note the heavy vegetation at the Falls and the formation of tufa where degassing of saturated water takes place.
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Old Postcard ca. ?? of upper and lower Roughlock Falls.  Even in the "olden days" the Falls attracted a massive amount of vegetation.

Cartoon showing how tufa forms on Roughlock falls.  Personal observation plus information derived from Ray and Rahn (1997).

Falls is the massive amount of vegetation growing along the Falls.  However, the Falls are managed by the South Dakota Parks and Recreation and they have built nice wooden platform viewing areas.  The area attracts numerous visitors wandering over from their drive up Spearfish Canyon.
I have not seen studies on South Dakota tufa and travertine but in other localities paleo-environmental studies have provided important information on climatic conditions at the time of deposition, as well as absolute dates---using carbon dating if the organic materially has not been biogenetically altered and is younger than about 50k.  Isotopic studies can help with absolute dates and often can provide information about climate at the time of deposition.  Since tufa contains plant material, at times vertebrate and arthropods fossils, as well as microfossil such as ostracods, scientists can use these fossils to articulate additional information about past environments.  For example, see Ollivier and others (2012).   But again, I have not observed environmental studies on travertine and tufa deposits in South Dakota.


Capezzuoli, E., A. Gandin, and M. Pedley, 2013, Decoding tufa and travertine (fresh water carbonates) in the sedimentary record: The state of the art; Sedimentology, v. 61, no. 1.

Ford, T.D. and H.M. Pedley, 1996, A review of tufa and travertine deposits of the world: Earth Science Reviews, v. 41.

Gries, J.P., 1996, Roadside Geology of South Dakota: Mountain Press Publishing Company, Missoula.

Lund, J., 1997, Hot Springs, South Dakota: Oregon Institute of Technology Geo-Heat Center Quarterly Bulletin v.18, no. 4.

Ollivier, V., P. Roiron, S. Nahapetyan, S. Joannin, and C. Chataigner, 2012, Tufa and travertine of the Lesser Caucasus: a light on the Quaternary palaeoenvironment of the Circumcaspian regions: Geophysical Research Abstracts v. 14, EGU2012-2124.

Rahn, P. H., and J. P. Gries, 1973, Large springs in the Black Hills, South Dakota and Wyoming: South Dakota Geological Survey, Report of Investigations 107.

Ray, C.M. and P.H. Rahn, 1997, The origin of waterfalls in the Black Hills, South Dakota:  Proceedings of the South Dakota Academy of Science, v. 76.

For a great story about the “old” resort town of Cascade: “Of all the “ghost towns” in South Dakota, the grandest one may have been Cascade, sometimes referred to as Cascade Springs because of the nearby hot springs. Back in 1892, its heyday, the town had about 400 people and 50 businesses, including a hotel, a sanatorium and a bowling alley.”  See http://www.capjournal.com/news/dakota-life-the-life-and-death-of-cascade/article_212d861a-2ebe-11e6-876d-e7d8d36ea850.html