By: David Dulitz, Forest Manager
Mountain Home Demonstration State Forest
The California Department of Forestry and Fire Protection
Tulare Ranger Unit - P.O. Box 517
Springville, California 93265

August 7, 2000

Introduction

Curious large circular depressions in granite outcroppings in the Southern Sierra Nevada continue to fascinate everyone who sees them. These features, commonly known as rock basins, have also been referred to as Indian bathtubs, Indian basins, Indian washtubs, granite basins, Sierra rock basins, or potholes. The rock basins are more or less circular and bowl shaped. They occur on a variety of sites, but always on granite rock outcrops. They occur as single features or in groups of a dozen or more.  Archaeological evidence of prehistoric occupation is present at most but not all of the sites. Some basins are near water while others are on dry ridgetops. The basins are scattered over a wide area of the Southern Sierra Nevada at mid to high elevations in the conifer belt. Many, but not all of the sites are associated with the giant sequoia forest.

Mt. Home Sites

A total of 20 sites contain rock basins within Mt. Home Demonstration State Forest and in the immediate area. The sites are scattered over an area of approximately 15 square miles. Elevation of the sites averages 6,281 feet and ranges from 5,440 to 7,000 feet. The 20 sites contain a total of 136 rock basins ranging from 1 basin per site to 23 basins per site. Average number of basins per site is 6.8.

The sites at Mt. Home may represent one of the greatest concentrations of sites within the range of the basins. The 136 basins in the Mt. Home area represent over 1/6th of the total recorded basins. Inventory work now in progress will further clarify the distribution of basins throughout their range.

The rock basins within Mt. Home Demonstration State Forest are considered to be a cultural resource and have been recorded as archaeological sites. These sites are protected by law from vandalism, artifact collecting, and excavation of any kind. These sites are also protected from disturbances associated with all management activities undertaken on the State Forest.

Basin Measurements

Diameter measurements have been taken on 78 basins within Mt. Home Demonstration State Forest. Diameter ranged from 55 cm to 153 cm with an average of 115.6 cm. The general basin shape is circular, although several basins have enough difference in diameter measurements that they could be called oval. Depth measurements were taken on 72 basins. Depth ranged from a low of 3 cm to a high of 95 cm with an average of 40.4 cm.

All of the measured basins are larger in diameter than they are in depth. The ratio of depth to diameter averages .34 with a range from .04 to .79.  This average depth to diameter ratio varies by site. One site with exceptionally deep basins has an average depth to diameter ratio of .48. Another site, which contains several shallow or incipient basins, has an average depth to diameter ratio of  .20.

The rock basin depth plotted by diameter shows a rough relationship (R2 = 0.48) of increasing depth with increasing diameter. It should be noted that the shallowest of basins, less than 10 cm deep, still had diameters ranging from 55 cm to 115 cm. If depth can be assumed to be indicative of age or stage of formation, it can be inferred that the basins start development with a large diameter and are not formed by the enlargement of a small diameter hole.

It must be noted that there are limitations to the measurement data presented in this report. Because of the irregular nature of these rock basins it is difficult to accurately measure them to the nearest centimeter. One must assign a modest margin for error or subjectivity in these measurements. These measurements only represent slightly over one-half of the basins in the Mt. Home area. They also represent only a fraction of the total number of basins within the range. It is hoped that in the future, additional measurement data can be accumulated for other sites and included with this data.

Less obvious incipient basins may not be included in this data. For example, within the Sunset Point site, some irregular shaped shallow depressions may not have been measured (Dillion 1992:75).

Formation

Several theories have been reported which account for the formation of these rock basins. These theories can be divided into two basic categories. The first is that the basins are formed by some natural process with little or no help from man. The other is that the basins are a man-made creation. No conclusive evidence exists which has proven any one specific formation theory.

One of the first detailed accounts of rock basins was made by George Stewart and reported in the American Anthropologist. Stewart thought that the basins were man-made and not natural. He hypothesized that the basins were made by building fires on the granite surface and then throwing water on the heated surface. The interior surface of the basins could have been smoothed by pounding or rubbing with rocks (Stewart 1929:429). F. E. Matthes, a geologist, initially concluded that the basins were artificially made but reportedly changed his mind to a natural formation theory later in his life (Plotnicov and Elsasser 1959:25-26).

A few sites show that basins are arranged on the granite outcrop in a systematic way or in a deliberate pattern while other sites seem to display random patterning.  Sites with non-random patterning of basins supports the idea that there was some "thought" to the process. Some formation theories have been put forth, at least verbally, that the basins were made long ago by an unknown race of people or by extraterrestrial beings. Although it may be easy for some to discount these theories, they may have as much validity as those expounded and argued about in print. 

At least two publications indicate the rock basins are glacial in nature, formed when melt water plunges through a glacier and abrades a pothole into the granite base rock (Barnes 1984:3), (Schutt 1962:2).   The extent of the glaciation in the Southern Sierra Nevada is not entirely understood. Some estimates of Pleistocene glaciation indicate a lack of glaciation in the areas where most of the rock basins are found ( Hill 1975:124-125) (Storer and Usinger 1963:20) (Weinberger 1981:3). It has been suggested that rock basins may be one indicator of glacial extension into the Southern Sierra Nevada (Barnes 1984:7). Of course, this is dependent on the theory that these basins are indeed glacial potholes.

The fact that the Mt. Home basins are greater in diameter than they are in depth is an important distinction between these rock basins and streambed or glacier potholes. The potholes formed in streams or by glacial action can be deeper than they are in diameter and may be cylindrical in form (Barnes 1984:3) (Stewart 1929:422).  Other shape and distribution patterns may show differences between rock basins and glacier potholes (Foster et al.1991:54).

Other authors have indicated that the basins may be formed by natural weathering accelerated by the dissolving action of standing water (Plotnicov and Elsasser 1959:26) (Wallace 1993:376). Acidic water in the basins may have an erosionnal effect on some of the granite minerals. There have been conflicting tests as to the acid content of water collected in the basins. Tests at two locations showed that water in basins was high in acid content although another test did not confirm these results (Weinberger 1981:2-3).

Weather pits exist on granite outcrops in the southeastern United States and actually harbor an assemblage of rare plants (Norquist 1997:6-7). Solution pits in granite at Stone Mountain Georgia are said to start as exfoliation scars that serve as catchments for water, granite detritus, and organic remains, which contribute to further weathering of the underlying granite (Size and Khairallah 1989:159-162)

Several questions can be raised concerning the natural weathering formation theory. Weather pits commonly seen on granite rocks are often irregular in shape in contrast to the regular shape of the rock basins. This author has also never seen basins that have coalesced or have weathered past the edge of the granite outcrop. Weather pits also exist on vertical or sloping faces of granite rocks whereas rock basins are restricted to relatively flat surfaces.

Uses

Just as interesting as the formation question is the speculation of the uses for these basins. Since the vast majority of the basin sites are associated with prehistoric occupation, it can be concluded that these basins had some use for the Native Americans (Fig. 6). Many basin sites are associated with seasonal base camps (Wallace 1993:376). Most of these sites contain numerous bedrock mortars, many of which are on the same boulder as the rock basins. One site at Mt. Home has bedrock mortars arranged in a circular pattern around a basin. This type of association with bedrock mortars has led to many theories concerning the use of basins for the processing of food.

Some of the more popular suggested uses include storage bins for holding acorns or pine nuts, fire pits for burning pitch off sugar pine cones, and leaching pits for acorn meal (Wallace 1993:376) (Stewart 1929:428-9) (Weinberger 1981:3-4). Other possible uses include cooking vessels (Otter 1963:12), tanning vats, sweathouses, pottery kilns, and holes for grinding gold-bearing quartz (Stewart 1929:428).

What is interesting is that Native Americans, without the use of bedrock basins, accomplished all of these activities in other areas. This leads to the assumption that these basins could have been indeed useful but not essential. Could these basins have been a feature, that if convenient and associated with other essentials such as water and good topography, fit into a good site for a seasonal base camp? Were these basins just another feature of the landscape that proved to be useful to the Native American population?  One basin site on Mt. Home without any other archaeological evidence is so far from a water source as to make the site a poor candidate for occupation.  Further study of the basin sites without archaeological evidence nearby may lead to greater knowledge concerning the association of these basins with prehistoric occupation.

A better understanding of the uses of the rock basins may also lead to more convincing theories as to their formation. If the basins were used for a critical purpose where no suitable alternative could be found, then the exhaustive time and labor to hand form one of these basins may be justified. But, if for example, a basket would serve the same purpose as a basin, then why make a basin?

The Sequoia Connection

Although some basin sites do occur outside the range of giant sequoia (Sequoiadendron giganteum) there are so many sites in close proximity to giant sequoia groves that it has led to speculations as to a possible connection between rock basins and giant sequoias (Fig. 7).  Concentrations of basin sites occur within the Mt. Home giant sequoia grove and also in giant sequoia groves within the Sequoia and Kings Canyon National Park and Sequoia National Forest. Mapping efforts that are currently underway should further clarify the spatial relationships between rock basin sites and giant sequoia groves.

Giant sequoia groves are distributed in the Sierra Nevada from Placer County to Tulare County. The vast majority of giant sequoia acreage lies in Fresno and Tulare Counties. The more northerly groves are small and separated by greater distances. The present distribution of giant sequoia is not well understood. Fossil records indicate that the range of giant sequoia and its close ancestors once covered much of the Western United States.

It is thought that a migration occurred to the western slopes of the Sierra Nevada before the mountain range was uplifted to its present height ( Hartesveldt et al.1975:66-73). The uplifting of the Sierra Nevada changed the climate east of the mountain range to one that was colder, dryer and not capable of supporting giant sequoia. The remains of the giant sequoia population can be seen in the disjunct groves that occur today.

Pleistocene glaciation may have played a role in the present distribution of giant sequoia (Hartesveldt et al.1975:73). Herein lies the possible connection between the range of giant sequoia and rock basins. If sequoia groves are survivors of Pleistocene glaciation, indicating that little or no glaciation was present in existing grove locations, then the glacial theories for basin formation may not be as plausible. If the basins pre-dated the glacial activity then the lack of glaciation may have preserved the basins in their present locations. Basins occurring in other areas of the Sierra Nevada formed before the Pleistocene may have been scoured away from the granite rock by the glacial activity.

Another link between basins and giant sequoia concerns their possible use as tanning vats. Giant sequoia contains a large amount of tannin. Almost pure tannin occurs as a granular substance in the giant sequoia cones. This tannin would have been useful in tanning hides and hence the tanning vat theory.

Unfortunately, the reasons for the current distribution of giant sequoia may be just as elusive as the formation of the rock basins. The true link, if any, between rock basins and giant sequoia may be very difficult to determine (Figs. 10-12).

Conclusion

This report offers some facts concerning the rock basins and it also presents some questions regarding the formation and uses of the basins. This seems to be the theme of previous reports concerning these interesting features. The questions are not easily answered. We should continue to bring out as many facts as possible about these basins to help to refine and focus our attention on the important questions.

We should continue to study the basin sites to determine more factual information about their distribution, size and archaeological context. Experiments have been proposed to recreate the formation of basins by manmade methods. These experiments may not answer the formation question but at least we would know if man could reproduce a typical basin. Experimental work should also be proposed that might help determine the function or uses of these basins. This question to many is more important than that of formation.

To some, the answering of questions is secondary to the beauty and mystic that these basins place on the landscape. The basins are certainly a unique feature that draw our attention and exercise our minds. In some ways the basins would be spoiled if we knew all the answers to their formation and uses. It is sometimes nice to have these unexplainable features to reflect and to ponder.

References Cited

Barnes, Eric K.
1984 Sierra Sub-Glacial Potholes: Their Significance in California Geology, With a Note on Archaeology.

Dillon, Brian D.
1992 Excavations at the Sunset Point Site (CA-TUL-1052) Mountain Home Demonstration State Forest, Tulare County, California. CDF Archaeological Reports, Number 11.

Foster, Daniel G., Eric Kauffman, Richard Jenkins, John C. Betts.
1991 Archeological Testing at the Salt Creek Ridge Site (CA-TUL-472) A Southern Sierra Rock Basin and Bedrock Mortar Encampment on case Mountain, Tulare County,              California. California Department of Forestry Archeological Reports, Number 5.

Hartesveldt, Richard J., H. Thomas  Harvey, Howard S. Shellhammer, Ronald E. Stecker
1975 The Giant Sequoia of the Sierra Nevada, U.S. Department of Interior, National Park Service, Washington, D.C.

Hill, Mary
1975 Geology of the Sierra Nevada, University of California Press, Berkeley, California.

Norquist, Gary
1997 Life in a Stone Pool, Endangered Species Bulletin Vol. 12, No. 3,  Fish and Wildlife Service, U.S. Department of Interior.

Otter, Floyd
1963 The Men of Mammoth Forest. Ann Arbor: Edwards Brothers, Plotnicov, L., and Albert B. Elsasser.

1959 Additional Notes on Granite Basins in Sequoia National Park. In: Archaeological Survey of Sequoia-Kings Canyon National Parks, Albert B. Elsasser, Appendix I. MS on file at the National Park Service.

Schutt, Harold G.
1962 Prehistoric Rock Basins. Visalia: Los Tulares, Quarterly Bulletin of the Tulare County Historical Society 54:1-2.

Size, William B. and Nayla Khairallah.
1989 Geology of the Stone Mountain Granite and Mount Arabia Migmatite, Georgia. Georgia Geological Society Guidebooks, v. 9., no. 1, April 1989

Stewart, George W.
1929 Prehistoric Rock Basins in the Sierra Nevada of California. American Anthropologist 31(3): 419-431

Storer, Tracy I. and Robert L.Usinger.
1963 Sierra Nevada Natural History. University of California Press, Berkeley, California.

Wallace, William J.
1993 The Great Indian Bathtub Mystery Solved?  Center for Archaeological Research at Davis, Publication No. 11.

Weinberger, Gay
1981Indian Slides and "Bathtubs" --- Archaeological Enigmas of the southern Valley and Foothill Region. Paper presented at the Annual Meeting of the Society for California Archaeology, Bakersfield.