In 1976, Julian Dodge Hayden (1911–1998) published his ideas about a pre-Clovis group of people who lived in the Southwest of the United States some 30 to 40,000 years ago.
Hayden described a "pre-Paleoindian" human culture that lived during the Malpais pluvial, a humid period that spanned between 20 and 32 ky ago. Later, these people developed a stone tool culture known as San Dieguito, in the Sierra de Pincate area in the north of Sonora, Mexico. The Malpais stone tools were first described by Malcom Jennings Rogers (1890-1960) in the 1920s, who used the Malpais to name them, and later adopted the San Dieguito term for them. Hayden adopted it.
"The phase sequence goes: Malpais, which refers to the earliest inhabitants with a pre-projectile point horizon who entered the area 30,000–40,000 years ago and who used a basic flake and core tool technology (flakes and ‘choppers’); followed by San Dieguito I, II, and III phases. San Dieguito describes a long-lived industry widely spread over southeastern California, northern Baja California, northern Sonora, Mexico, southern and western Arizona, and southern Nevada (Huckell 1998). Bruce Huckell has identified a San Dieguito site in proximity to Organ Pipe at Dateland on the Lower Gila River in southwestern Arizona (1998:145)." source
But, how old are these San Diegito stones? Hayden gave them an age of 30,000 to 40,000 years based, not on radiocarbon or other technological tools. Hayden estimated the age based on their appearance. The objects have a glassy, dark "desert varnish", the outcome of thousands of years of exposure to the wind, the sun and rain. It is caused by silica leaching from the rocks, and is also found on Martian rocks (read all about desert varnish in this Science article). There have been several attempts to use desert varnish for dating purposes, but none of them are reliable. One research paper states that rock varnish forms at a rate of 1-40 microns every 1000 years.
Hayden claimed that: "Two degrees of thickness of desert varnish on pavement and associated artifacts provide evidence that man was present during a pluvial period preceding an altithermal episode centering around 17,000 B.C., as well as throughout the well-known Pluvial period. The formation of caliche below ground level helps place excavated artifacts in sequence. M. J. Rogers' term “Malpais” is revived and assigned to the earlier pluvial occupation, basal to “San Dieguito Phase I (SD I)” of the Pluvial."
Hayden's work is the following: Hayden, Julian D. (1976), Pre-Altithermal Archaeology in the Sierra Pinacate, Sonora, Mexico. American Antiquity 41(3):274–289. 🔒
The term altithermal is a formal one referring to the warming up caused by the end of the last Ice Age, so this is Pre-Altithermal, or preceeding the last glacial period.
An interview with Hayden in 1988 provides some insight into his research:
"... the desert pavement. Sierra Pinacate is a volcanic complex composed of basalt left by the ejecta and lava flows of an ancient eruption. When the soil is blown away during periods of drought, the basalt stones drop down upon a bed of clay, which will not support plant life. This is the desert pavement. Once formed, it is more or less permanent except for human disturbance. Tools dropped on the desert pavement remain lying atop it and are thereby identified as being more recent; tools within or troiecting throuch the navement are necessarily as old projecting through the pavement are necessarily as old or older than it is, as the composition of the pavement prevents anything from sinking through it.
...
Rogers himself had noted in the Colorado desert back in the 1930s that San Dieguito I tools, bifacially flaked and thinly varnished differed from other tools flaked and thinly varnished, found in the same area which were unifacially flaked and heavily varnished. Indeed, it was the latter to which he nad tentanvely appiled the name Maipais before abandoning the notion of a basal stage as untenable, in the absence of a reliable non-subjective technique of dating. Hayden had noticed a similar phenomenon at Ventana Cave. When he got to Sierra Pinacate, Hayden not only found tools which differed in their amount of varnish found tools, but observed that the SD-I tools, as well as those of a more recent culture called Amargosan, lay on top of the desert pavement. The heavily varnished unifacial tools either projected from the similarly varnished pavements or had been dropped upon still older pavements, establishing a sequence. Hayden therefore revived the name and theory of a basal Malpais Complex.
At this time another piece of the puzzle fell into place. From Malpais times until recently, a number of tools were made out of shell brought inland from the Bay of Adair. East of the present shoreline, Hayden found aeolian dunes with food shell remains: occupied dunes. The weathered surface shell yielded a radiocarbon date of 33,950±1250 years B.P., the lower level dates in excess of 37.000 vears B.P. It has been only in the last few thousand years that the sea level has risen and brought the shoreline close enough to harvest shellfish conveniently from the bay within traveling distance from the dunes.
...
[this helped] Hayden to realize with some assurance that he was dealing with two stages within the Malpais Complex: Malpais II, older than 17,000 years B.P., and Malpais I, an earlier from 22,000 years B.P. to at least 28,000 years B.P., and quite possibly exceeding the 33,000-37,000 year old shell dates. Varnish ages are, of course, minimum ages marking varnish-forming periods, with no implication or indication of how long the artifact had been made prior to varnishing."
This is his work on the shells at Adair Bay: Hayden, Julian D. [unsigned] 1981 [C-14 ages of foodshell from occupied dunes of the Bay of Adair, Sonora, Mexico] American Antiquity 46: 933.
Hayden's publication was controversial: "Hayden's discussion while certainly provocative, suffers from the same shortcomings as those found in the work of his mentor, M. J. Rogers. He presents his conclusions in some detail with only very general references to the data upon which it presumably rests. Further the use of desert pavement patination (either desert varnish or oxidation) and caliche as dating criteria are no more verified now than they were when Rogers attempted to use them. Independent verification of the paleoclimatic material might bolster his case, if he could demonstrate a correlation with the cultural remains." (Crabtree, 1981).
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