So it was at Agate only a year or so after the day of our visit. The river died for a while. As with many rivers much of its water flowed beneath the surface, through the sand and gravel of the bed. When the ancient Niobrara died there was still water moving through the sands and filling the low spots in its bed. Some animals could dig down to it and survive, others could stake claims to the diminishing water holes. So the thirsty, suffering herds of _Menoceras_ went to the river and found no water. The strong held the water holes. The smart dug into the sand and made their own water holes. The rest died. They died by the hundreds, and thousands. Mixed with the carcasses of _Menoceras_ were other victims: occasional chalicotheres, giant pigs, oreodons, cats, dogs, and a variety of equally thirsty smaller animals. Perhaps most of the animals went farther up or down stream, or perhaps they chose not to die at the river. Whatever the pattern of dying might have been, we know that _Menoceras_ left untold numbers of skeletons on the broad, flat, and dry bottom of the ancient Niobrara.
Finally the rains fell in the mountains to the west. The river filled with water again and ran in sheets across the plain. At Agate the millions of _Menoceras_ bones and lesser numbers of the bones of other animals were swept for a few hundred meters downstream and into some sort of backwater or river lake—possibly a great meander, or an oxbow lake. There, like a gigantic mass of jackstraws, they were piled in a tangled mat 30 centimeters (12 inches) thick, covering an unknown number of hectares. All we really know is that they were moved far enough to get thoroughly jumbled, but not far enough to be badly broken or much eroded by the action of the water.
The mass of bones was soon buried by the sands and silts dropped by the reborn river, and by wind-carried debris swept off the parched land. Once buried, the bones were partially petrified by mineral water flowing beneath the surface. The land was built up a few hundred meters by sediments continually brought down from the mountains to the west. Eventually, continued uplifts of the Rockies and the Great Plains combined with erosional cycles to leave the modern Niobrara River. The two erosional remnants known today as Carnegie and University Hills were produced by the cutting of the modern river system. On the sides of these hills were exposed the tangle of bones which marked the site of ancient tragedy.