Clue No. 2 is the clay, or rather, this is a series of clues, because the clay proves on investigation to be complex and to include several distinctive superposed layers. The lowest layer visible, as far as it has been excavated, is massive, yellowish gray, and somewhat gritty. We found no traces of bone in this. At its top in some places but not in all is a layer of dripstone (cave or “Mexican” onyx) from which rise stalagmites, buried by the overlying layers of clay. The next higher clay layer, sometimes absent but in other places two feet or more thick, is very smooth and fine, without grit, and is deposited in thin, horizontal layers. There are no bones here, either, except occasionally right at the top where they probably sank in from above when the clay was less compact. The top of this is sharply distinguished from the overlying bed but it has no layers of dripstone so far as we saw. Next higher is a bed of clay quite variable in thickness but averaging 18 to 20 inches, also fine and plastic, but without layers and containing many scattered chunks of limestone and of dripstone. Almost all the bones are in this bed of clay, which we called “the peccary layer.” Above it there is occasionally, but not usually, a thin layer of dripstone. At the very top is a bed, usually less than a foot thick, of relatively loose, granular, earthy clay. In places it fills holes extending down into the lower layers. A few very small bones were found in this bed. In some places where there is a small unfilled space above this top layer there are small stalagmites on it, and where these occur they are usually set on small plaques of dripstone.
Our major clues are the bones themselves, not only because of what they are but also because of how they occur. As I have said, almost all the bones are in the “peccary layer.” You cannot dig long in any part of that particular stratum without finding bones, but they do usually tend to be more common toward the bottom of the layer. Even when several are found together, they are just piled up at random. No two bones of the same animal are found together. Most of the long bones are buried in a more or less horizontal position, but some are oriented without regard for the natural bedding of the deposit and they may even be vertical. Small, solid individual bones are usually whole, but the longer and more fragile bones are usually broken. We did not find a single complete rib. A few of the bones have tooth marks and had been gnawed before being buried here. Bones of the extinct peccary are by far the most common, but there are also a few bones and teeth of other extinct animals and of some living species in this layer; I will give the list later.
The rare bones in the highest layer tend to occur in a few pockets, scattered but sometimes with the remains of one individual near each other. Except for one or two bones apparently washed out of the peccary layer, there are no extinct animals in this bed and most of the bones belong to small, burrowing rodents.
Those are the main clues. This is my proposed solution, so far as it has yet been carried: The very first thing that left traces here happened so long ago that it is only indirectly involved in our problem of the bones. This was the deposition of the limestone, which occurred in a sea that covered this site about 300,000,000 years ago. Much later, perhaps only a million years or so ago (the event has not been very exactly dated, and it took a long time), the cave was formed. The sea had withdrawn long since and the region had been uplifted gently. Water began to percolate along the cracks and seams of the limestone and as it went, it slowly but steadily dissolved the rock. Eventually it formed a large underground channel which was, and is, the cave. At this stage the cave was free of any extensive deposits of clay, and it probably had a subterranean stream or river at the bottom. This probably reached the surface some distance away and eventually flowed into the Mississippi.
Somehow the exit from the cave became clogged and the clay and silt brought in by streams from the surface, instead of being washed on through the cave and out again, began to pile up in the cave. These sediments eventually filled the cave up to within a few feet of its ceiling. Then for a long time there was no particular activity except the slow dripping of lime-filled water within the cave, developing dripstone deposits here and there on the top of the silt which now formed the floor of the cave. This floor was not even but contained shallow depressions. The next recorded event, which probably occurred during a particular rainy period of the Ice Age, was the filling of these depressions with water, forming within the cave a lake, or a series of small lakes. Tiny, insoluble clay particles were slowly washed into this standing water and they accumulated at the bottom, forming the bed of horizontally banded clay that we found below the peccary layer.
Now came what is for us the great event: the deposition of the bones in the cave. The evidence shows clearly that these animals did not live or die in the cave and it strongly suggests that this was not the first place in which they were buried. The animals probably fell into a sinkhole or fissure somewhere near the cave, perhaps a hole that had been an entrance to the cave but had been sealed off from it by the older accumulation of clay or by a fall of rock. The exact spot has not been found and search for it would not be very hopeful now that the whole region has been built up as part of a great city. The bones of many animals, hundreds certainly and perhaps thousands, piled up in this sinkhole or fissure and were buried there in mud and clay that washed in over their bones. Then the accumulation—clay, bones, and all—was somehow washed into the cave. There are several ways in which this could have occurred. Perhaps the most likely is that the sinkhole or fissure filled up with water above the clay and bones, that this water found an outlet into the cave, and that it suddenly flushed the whole deposit into the cave and spread it out over the older clay deposits of the cave. The nature of the peccary layer in the cave suggests that it came there rapidly, perhaps in an hour or two—one dramatically rapid event in a sequence where most changes can only be measured in terms of thousands or hundreds of thousands of years.
After this sudden change, things quieted down again. A little more clay was washed in from time to time. Rodents occasionally wandered into the cave, rooted around a bit in the top clay, and died there. These later events did not matter much so far as our interests go, until the final event of the reopening of the cave by man. It is surprising that the discovery of prehistoric animals here was delayed until 1946. When the brewery cleared part of the cave, many tons of clay were removed and in this there must have been thousands of bones. So far as is known, no one paid any attention to them. Presumably they were carted off with the clay, dumped somewhere, and buried again: their third burial.