The Geology That Created The Confusion
The geological complexity of the Punín ravine — the feature that caused the century-long controversy about the age of the human remains found there — is worth understanding in some detail, because it is a case study in how geological context can mislead researchers and how those misleading conclusions get corrected.
The Andes are geologically young and tectonically active. The mountains of Ecuador were formed and continue to be shaped by the subduction of the Nazca Plate under the South American Plate — a process that has produced not just the Andes themselves but the active volcanic zone that includes Chimborazo and numerous other Ecuadorian volcanoes. The geological dynamism of the region expresses itself in frequent earthquakes, ongoing volcanic activity, and the rapid erosion and redeposition of sediment that comes with high-gradient rivers, intense rainfall, and steep slopes.
In this environment, sediment contexts are often mixed — material from different time periods and different depositional environments gets combined by the processes of erosion, mass movement, and redeposition. A ravine in the Chimborazo region is exactly the kind of geological feature where this mixing is likely: water running through the ravine erodes the surrounding slopes, carrying material from different depths and different depositional contexts and depositing it in a mixed layer at the ravine floor or within the ravine walls.
The Pleistocene fossils found in the Punín ravine — the mastodons and giant sloths — were genuine Pleistocene material, deposited in sediment layers that had accumulated during the Ice Age. But the human skulls, found in apparently similar sediment in the same ravine, were not the same age as those fossils. The geological processes of the post-glacial period had moved and remixed sediment in ways that brought material of very different ages into proximity.
This kind of geological mixing is well-known in modern geoarchaeology — the subdiscipline that studies the specific geological processes affecting archaeological sites. Modern geoarchaeological investigation of the Punín ravine established that the apparent coherence of the sediment layer that contained both the megafauna fossils and the human remains was misleading: the layer was mixed, and the human remains were far younger than the fossils that appeared to be their geological contemporaries.
For the researchers of 1923 and 1925, who did not have the benefit of modern geoarchaeological methods and who were operating in a tradition that interpreted sediment context as a reliable indicator of age, the misleading appearance of the geological context was a genuine source of error. They were not being careless; they were applying the methods available to them. But those methods were insufficient for the specific geological complexity of the Punín ravine, and the conclusions they drew were wrong.
The correction came incrementally, as improved geological and dating methods were applied to the site and as the direct dating of the human skeletal material established their actual age independent of the sediment context.