Nature records the passage of time in a variety of ways. One of these records, preserving the evidence of changing environment and developing life, is seen in the successive layers of the earth's crust. Another cyclic process, the destruction of existing land features, is reflected in the alternate cutting and filling of valleys, leaving terraces and various types of sediments as clues of nature's changing moods. In both of these, whereas the relative order of events may be established with certainty, the actual elapsed time involved can be but roughly estimated. More precise records of the passage of time are to be found in the annually deposited layers of clay, known as varves, on the floors of glacial lakes, and in the yearly growth rings of certain trees. Science has learned how to read both of these natural time pieces.
The study of varves has contributed greatly to our knowledge of some glacial events, particularly the time required for the retreat of the last ice sheet as in Scandinavia. Tree‑rings, on the other hand, have given us much useful information concerning climate and, more importantly from the viewpoint of the southwestern archaeologist, exact knowledge of the age of the now ruined villages which were thriving before the advent of written history.
Almost every archaeological report concerning the Southwest leans heavily in one way or another on the results of tree-ring studies, an indication of the importance assigned to this unique method of reading time.
In 1929 when the charred beam was found in the Showlow Ruin by the Third National Geographic Society Beam Expedition which united Dr. Douglass' sequence of modern rings reaching back to about 1300 and his floating series of 585 rings, only three ruins had been allotted pre‑Spanish dates. The first of these was Kawaika with a date of 1495, and the second, Kokopki, with a date of 1410, both situated in the Jeddito area of northeastern Arizona. The third of this trio was the Chaves Pass Ruin located •about 30 miles southwest of Winslow, dated at 1381. Kawaika has the distinction of being the first prehistoric ruin dated by dendrochronology.
Twenty‑six ruins contributed to the constructing of the 585 year record which, prior to 1929, was floating and had not been united with the modern calendar. Immediately on the joining of these two sequences, these 26 ruins were assigned precise dates. This dramatic incident has been well described by Dr. Douglass on several occasions1 and is one of the highlights in tree-ring research.
With the tree-ring calendar now complete from modern times back to about 700 A.D., the stage was set for dating many more ruins which fell within that span of time. Archaeological activity in general was spurred by these findings, and from 1929 until the outbreak of the war, many ruins were fitted into the Christian calendar. The list now includes well over two hundred sites. p11 Several inventories have already been given,2 but since these are not readily available to many persons, a selected list of some of the better known sites is given herewith.
Chaves Pass, 1381‑
Elden Pueblo, 1162+
Medicine Fort, 904‑1063
Turkey Hill Pueblo, 1168‑1278
Tusayan Ruin (Grand Canyon), 1170+X‑1205+X
Walnut Canyon Cliff Dwellings, 888‑1094
Broken Flute Cave, 354+X‑647+X
Kiet Siel, 1116‑1286
Mummy Cave, (early occupation), 348‑702
Mummy Cave, (late occupation), 1253‑1284
Vandal Cave, 608‑683
White House Pueblo, 1060‑1275
Kinishba Pueblo, 1238‑1306
Pinedale Pueblo, 1150‑1375
Showlow Ruin, 1174‑1382
Sierra Ancha Cliff Ruins (13 sites), 1248‑1348
Northwestern New Mexico:
Aztec Ruin, 1110‑1121
Gobernador Canyon, 1723X‑1752X
Pueblo Bonito, 828‑1130
Pueblo Del Arroyo, 1052‑1101
Santa Fe‑Jemez Area:
Pecos Pueblo, 1348‑1612
Pindi Pueblo, 1217+X‑1348
Tyuonyi Pueblo, 1423+X‑1513
Unshagi Pueblo, 1402‑1605
Balcony House, 1190‑1272
Cliff Palace, 1175‑1273
Lowry Ruin, 987+X‑1104+X
Earth Lodge A (Mesa Verde), 612
Spruce Tree House, 1019+X‑1274
Square Tower House, 1194‑1259
Step House (Earth Lodge), 625
It should be borne in mind that the dates given for the foregoing ruins do not necessarily limit the full span of the occupation. Too rigorous an application of the dates is one of the pitfalls in archaeology. But as the list of dated ruins lengthens and as the same culture pattern is found over and over in approximately the same limits of time, our understanding of the development of southwestern peoples is immeasurably improved.
Such ruins as Casa Grande near Florence, Los Muertos near Tempe which was excavated by the Hemenway Expedition of 1887‑88, Snaketown on the Gila River west of Sacaton, and the University Indian Ruin east of Tucson, lie outside of the area where tree-rings can be effectively used. But approximations of the times when these villages were occupied may be assigned on the strength of traded materials found in them, notably pottery of known age. Thus Casa Grande and Los Muertos and the University Indian Ruin may be assigned to the 14th century, and Snaketown is known to have had a much longer life encompassing the first millennium of the Christian Era.
This method of extending tree-ring dating has served the archaeologist well, particularly since the ruins of southern Arizona yield only information which permits relative but not precise dating.
The years from 1901 to 1929, when the archaeological dating in the Southwest was secured, were arduous ones for Dr. Douglass. With the archaeological applications of his method demonstrated, interest at once became widespread, so much so, in fact, that a number of persons began to study under Dr. Douglass to learn and gain experience in the actual dating of wood. In June, 1934, a group of them met at Flagstaff and founded the Tree‑Ring Society. This organization at the same time also created and undertook the publication of the Tree‑Ring Bulletin, the first issue appearing in July, 1934. The Tree‑Ring Bulletin, a quarterly now in its tenth volume, is an outlet for brief technical papers on a variety of subjects related to dendrochronology. In it have been released many of the dates for Southwestern ruins.
Currently the paid membership of the Society is about one hundred, and some subscribers are as far away as South Africa and Australia.
Dr. Harold S. Colton from the Museum of Northern Arizona was managing editor of the Bulletin from its inception to April, 1937. It was then transferred to the University of Arizona where it is currently managed by Mr. Edmund Schulman with Dr. Douglass as editor-in‑chief.4
p13 The administration of the University of Arizona, well aware of the scientific merits of tree-ring studies, created the Laboratory of Tree‑Ring Research in January, 1928, with Dr. Douglass as its head. Office, storage, and workroom space was provided in the Stadium. The object of this Laboratory is to establish a central point where research activities in this field may be continued, and to safeguard the equipment and very extensive collection of specimens.
A measure of the extent to which studies in dendrochronology have been carried may be seen in a bibliography compiled by Mr. Schulman and printed in the Tree‑Ring Bulletin, Vol. 6, No. 4. Listed are more than four hundred titles, ranging from such fundamental headings as How a Tree Grows to the highly technical climatic aspects of the study.
For the guidance of those who may wish to read further on this subject a brief selected bibliography will be found at the end of this article.
It is, of course, common knowledge that the spade work in establishing the tree-ring system and most of the subsequent research with respect to archaeological dating has been done on the campus of the University of Arizona. Dr. A. E. Douglass, creator of this system, since 1906 has been energetically engaged in this study. In more recent years he has been ably assisted by a number of other people in the promotion of various aspects of the problem. To commemorate this achievement and to make the principles of tree-ring dating as clear as possible to the layman, the Arizona State Museum has recently completed the installation of an exhibit devoted to the subject.
This display is housed in a case of table top type, •18 feet long. The principles of the tree-ring technique, answering such questions as the nature of the annual growth ring in trees, the effect of climate on ring size, cross-dating, how a chronology is built, the application of dendrochronology to archaeology and climatology, are illustrated in small unit exhibits on the deck of the case. Running lengthwise down the center is a light tunnel supporting on top a photographic panorama of tree time. This is a continuous picture, •35 feet long, recording the tree-ring pattern for the Pueblo area from 11 A.D. to 1936. The sides of the light tunnel carry •5×7 inch kodachrome transparencies of the major dated ruins so placed that they correspond with the photographic tree-ring record above.
This exhibit has attracted wide attention and it is commended to those who have a serious interest in the science of tree-rings.
Although the accomplishments of tree-ring dating so far are very imposing, there still remains much to be done. From the archaeological slant the extension of the dating system to other areas is of first importance. Progress along this line has already been made by Dr. Florence Hawley for the Mississippi drainage. Louis Giddings, as a product of extensive field work in Alaska, has been able to establish a chronology for the far North and, by means of it, to assign dates to early historic and late prehistoric Eskimo remains. Giddings' work, furthermore, opened up a new vista in tree-ring analysis by demonstrating that the requisite qualities for cross-dating in Arctic wood depend on temperature stresses instead of differences in precipitation as here in the Southwest.
p14 Mexico, South America, and other areas of the world also beckon the tree-ring specialist to these now fields. A preliminary inspection by Mr. Edmund Schulman of the dating possibilities in Mexico was made last summer with encouraging results. But one should not overlook the greater issue involved in tree-rings, namely, climatology, which has been the primarily concern from the start.
Dr. Douglass is presently pushing these studies forward with the hope that such phenomena as weather cycles and water supply changes, both vital to man, can be predicted. Direct application of the science to modern living is seen in such matters as working out the history of the run‑off of the Colorado River, a project requested by the Boulder Dam Management.
It is not too much to hope that in the years to come, highly significant results in climatics will be achieved.
Douglass, A. E. — Method of Establishing Rainfall by the Growth of Trees. Carnegie Institution Publications 192, 1914.
Climatic Cycles and Tree Growth. Carnegie Institution Publications 289, 1928.a
The Secret of the Southwest Solved by Talkative Tree Rings. National Geographic Magazine, December, 1929.
Dating Pueblo Bonito and Other Ruins of the Southwest. Pueblo Bonito Series I, National Geographic Society, Washington, D. C., 1935.
Advances in Dendrochronology, 1943. Tree‑Ring Bulletin, Vol. 9, No. 3, January, 1943.
Giddings, J. L., Jr. — Dendrochronology in Northern Alaska. University of Arizona Bulletin, Vol. XII, No. 4, Laboratory of Tree‑Ring Research Bulletin, No. 1.
Glock, Waldo S. — The Language of Tree‑Rings. Scientific Monthly, Vol. 38, June, 1934.
Principles and Methods of Tree‑Ring Analysis. Carnegie Institution 486. Washington, D. C., 1937.
Haury, E. W. and L. L. Hargrave — Recently Dated Pueblo Ruins in Arizona. Smithsonian Miscellaneous Collections, Vol. 82, No. 11. Washington, 1931.
Haury, Emil W. — Tree‑Rings — The Archaeologist's Time-Piece. American Antiquity, Vol. 1, No. 2. October, 1935.
Hawley, Florence — The Significance of the Dated Prehistory of Chetro Ketl, Chaco Canyon, New Mexico. University of New Mexico Bulletin, Monograph Series, Vol. 1, No. 1, July 1, 1934.
McGregor, John C. — Tree‑Ring Dating. Museum Notes, Vol. 3, No. 4, October, 1930. Museum of Northern Arizona, Flagstaff, Arizona.
Dating the Eruption of Sunset Crater. American Antiquity, Vol. 2, No. 1. Menasha, 1936.
How Some Important Northern Arizona Pottery Types Were Dated. Museum of Northern Arizona, Bulletin 13, Flagstaff, 1938.
Schulman, Edmund — A Bibliography of Tree‑Ring Analysis. Tree‑Ring Bulletin, Vol. 6, No. 4, April, 1940, pp27‑39.
Stallings, W. S., Jr. — A Tree‑Ring Chronology for the Rio Grande Drainage in Northern New Mexico. Proceedings of the National Academy of Sciences, Vol. 19, No. 11, 1933.
Dating Prehistoric Ruins by Tree‑Rings. General Series, Bulletin 8, 1939. Laboratory of Anthropology, Santa Fe, New Mexico.
1 The Secrets of the Southwest Solved by Talkative Tree‑Rings, National Geographic Magazine, Dec. 1929; Dating Pueblo Bonito and Other Ruins of the Southwest, National Geographic Society, Pueblo Bonito Series, No. 1, 1935.
2 See Tree‑Ring Bulletin, Vol. 4, Nos. 2, 3, 4, 1937; Vol. 5, Nos. 1, 2, 1938; Douglass, A. E., 1935, Dating Pueblo Bonito and Other Ruins of the Southwest, National Geographic Society, Pueblo Bonito Series No. 1, pp51‑54; McGregor, J. C., 1941, Southwestern Archaeology, pp372‑376.
3 The symbol +X signifies that the outermost rings of the beams in question were lost. The true culture date therefore is more recent than the dates indicated.
4 The subscription price is $1.50 per year. Single copies, $.50. Communications should be addressed to Mr. Edmund Schulman, Tree‑Ring Laboratory, University of Arizona, Tucson.
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