Travelers have long reported bones of mammoths and mastodons in Mexico. The first to do this was Cortés’s young lieutenant, Bernal Díaz. To be sure, he thought that the bone the Tlascalans showed him belonged to one of their gigantic ancestors, as they said it did. In his book, _The True History of the Conquest of New Spain_, he wrote: “So that we could see how huge and tall these people had been they brought a leg bone of one of them which was very thick and the height of a man of ordinary stature, and that was the bone from the hip to the knee.” If we try to visualize Cortés and Díaz inspecting this mammoth bone, the scene adds, if possible, to the extraordinary picture that the conquest presents. “I measured myself against it and it was as tall as I am although I am of fair size.”[35] Since the days of Cortés more and more fossils of elephants have been found south of the border. Indeed they crop out of eroded arroyos, drainage ditches, and old lake beds only a few miles from Mexico City itself. Until November, 1945, little had been done to place early man in Mexico or to link him with its many fossils. Then Helmut de Terra—who had worked long in eastern and southeastern Asia—went to the Valley of Mexico to study the glacial moraines on neighboring mountains, and to link them with the soils of the old lake beds in which fossils had long been found. Expanding the work of A. R. Arellano and Kirk Bryan, he established the fact that a certain layer of caliche, or soft, earthy limestone, had been laid down upon the shrunken shores of Lake Texcoco after the glacial ice had retreated up Popocatepetl and other mountains. Under the caliche—which he dates as beginning to form between 9,000 and 11,000 years ago—he found a wet, silty clay which contained, here and there, the bones of elephants and a few rudimentary tools of stone and bone. Then in February, 1947, he decided to hunt for larger objects in an area near Tepexpan with an electrical device used for detecting valuable metals and military mines but never previously used for archaeological purposes. The resistance of a certain section of the clay to the passage of an electrical current between two metal stakes driven into the ground told him that a foreign body lay a little below the surface. He dug, and unearthed a skull and a considerable part of the skeleton of a man who was probably hunting an elephant or running from one when he became mired in the bog of the lake shore. Because of the lake sediment in which de Terra found the bones, he dates Tepexpan man between 11,000 and 12,000 years ago.[36] Another anthropologist, Glenn H. Black—basing his opinion on published photographs of the excavation—has suggested the “possibility, even probability” that Tepexpan man got into old lake sediment through “intrusive burial,” which means that he was interred by his fellows at a much later date.[37]