Sometime about 220 million years ago, a meandering stream flowed here and plants grew along its banks. Something, as yet unknown, caused sediment to flood the area rapidly, which helped preserve the plants. Gulbranson splits open a grey slab of siltstone in the quarry to reveal amazingly well-preserved Triassic plant fossils, as if the leaves and stems had been freshly pressed into the rock only yesterday. "It's a mixture of plants that don't exist anymore," he says, "but we have some plants in these fossil ecosystems that we might know today, like ginkgo." On the one end are fossils from...