"My assignment for the so-called mushroom cult, earliest 1,000 B.C., is based on the excavations of Kidder and Shook at the Verbena cemetery at Kaminaljuyu. The mushroom stone found in this Pre-Classic grave, discovered in Mound E-III-3, has a circular groove on the cap. There are also a number of yet unpublished mushroom stone specimens in the Guatemalan Museum from Highland Guatemala where the pottery association would indicate that they are Pre-Classic. In each case the mushroom stone fragments has a circular groove on the top. Mushroom stones found during the Classic and Post-Classic periods do not have circular grooves. This was the basis on which I prepared the chart on mushroom stones which was then subsequently published by the Wassons. Based on Carbon 14 dates and stratigraphy, some of these Pre-Classic finds can be dated as early as 1,000 B.C. The reference is in the following"...(see Shook, E.M. & Kidder, A.V., 1952. Mound E-III-3, Kaminaljuyu, Guatemala; Contributions to American Anthropology & History No. 53 from Publ. 596, Carnegie Institution of Washington, D.C. (letter from de Borhegyi to Dr. Robert Ravicz, MPM archives December 1st 1960 )