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of the section, contains a notable fauna and the geologic age of the formation has been much discussed. General opinion places it among the lowest or as the lowest of the divisions of the Pampean and Ameghino regards it as Miocene” (Hrdlicka 1912, pp. 361–362). This lower Pampean formation is the Montehermosan, now regarded as Middle to Early Pliocene, and above it lies the Puelchean, which could be Early Pleistocene or Late Pliocene. According to Florentino Ameghino, the layer from which both he and Hrdlicka extracted stone implements during their excursion to Monte Hermoso represented the “superior part” of the Puelchean formation (Hrdlicka 1912, p. 105).

Describing the Puelchean, Willis stated: “The Puelchean consists of the stratified, slightly indurated, gray sands or sandstone, both above and below the volcanic ash, marked by very striking cross stratification and uniformity of gray color and grain. The writer regards it as an eolian formation. Later in the season, when studying the section exposed along the Rio Colorado from the delta to Pichi-Mahuida, he observed a very similar sandstone, which might be correlated with the Puelchean on grounds of lithologic identity. It is a thick widespread formation which is regarded as a Tertiary sandstone. The Puelchean, if the same, represents only a thin edge of it” (Hrdlicka 1912, p. 363).

We may note that the Puelchean sands, which Willis, with some hesitation, accepted as Tertiary, are characterized by their gray color, quite different from the underlying Montehermosan loess, which is yellow-brown. Willis then described the so-called recent topmost layer, apparently included by Ameghino in the Puelchean formation, as “a layer of 15 to 40 cm. [6 to 16 inches] thick composed of gray sand, angular pieces of gray sandstone and pebbles, some fractured by man” (Hrdlicka 1912, p. 362).

Willis elsewhere remarked that the top layer of gray implement-bearing sand is separated from the lower layers of the Puelchean by an “unconformity by erosion” (Hrdlicka 1912, p. 363). An unconformity is a lack of continuity in deposition between strata in contact with each other, corresponding to a period of nondeposition, weathering, or, as in this case, erosion. The unconformity may represent a long break in time, or a very short one. So by itself, the presence of an unconformity by erosion should not allow Willis to so greatly separate the top layer of gray gravelly sand from the underlying formation. He appears to have used the mere presence of stone tools to perform the desired operation, which was necessary to save Hrdlicka the deadly embarrassment of having discovered stone implements in an unexpectedly old formation in South America. For judging how much time might be represented between the formations lying above and below the line of unconformity, the surest indicator is faunal remains. Willis, however, did not mention them. It is thus unclear how much time might be represented by the unconformity.

Willis then stated about the upper layer of gray gravelly sand and the underlying Puelchean: “The two are identical in constitution; they are both eolian and may exhibit similar structures; the Recent formation may be consolidated almost or quite to the firmness of the older one. The unconformity between them suffices to establish the difference in age and is unmistakable when clearly shown in section” (Hrdlicka 1912, p. 363).

Let us carefully consider just what Willis is asking us to accept. First of all he admitted that the two strata are identical in composition, which would seem to be very much in favor of Ameghino, who considered the top layer to be part of the Puelchean. And, given the evidence, why not?

But for argument’s sake, let us accept Willis’s version. The unconformity by erosion that he proposed would mark a gap of 1–2 million years, as the upper gray layer is supposedly recent, while the gray sand layer below it, identical in composition, is referred to the Early Pleistocene–Late Pliocene Uquian formation.

Another, and perhaps more likely scenario, would be that the two