Hailu explains the nature of what we're seeing here. The quarries -- in fact, the rock making up all these low, brown-gray hills ringing the Changma Basin -- pertains to the Xinminpu Group, the same unit we saw out in White Pagoda. However, the nature of the Group is different in the two localities: at White Pagoda, it's floodplain and other fluvial-based deposits; here, it's a mix of fluvial and lacustrine (lake) deposits. The brown-gray sediment the quarry is in belongs to the Xiagou Formation, which is the lower of the two units. The upper unit, the Zhonggou Formation, has some exposures near the edge of the basin. The Xiagou Formation was deposited in what appears to be a fairly large, steep-sided lake near the middle of the Cretaceous, about 110-115 million years ago. Lake deposits are great for fossils because the waters tend to be relatively calm but have a fairly consistent input of sediment to bury them. So spectacular fossils form in lakes, especially big lakes like this. And that's why we're here! Fossil fish and insects have been known in the Xiagou Formation since the late 1970s-early 1980s; during a fish-recovery expedition by the IVPP in the early '80s, the foot of a bird was recovered. It was studied and named Gansus yumenensis in 1984. Gansus was unusual at the time -- it was very advanced in its morphology...that is, it was much more similar to living (and a few Late Cretaceous) birds than anything as old that was known in 1984, such as Archaeopteryx. But for some reason, no one went back to look for more...until Hailu did in 2002. Within a week or so of splitting lacustrine shales, he and his team found another bird fossil. In general, bird fossils tend to be pretty rare, particularly in the Mesozoic, and the best places to look for them tend to be in lake deposits. Finding another one so quickly suggested that there were many more waiting to be found, so Hailu and Da-qing organized an expedition to work the site; later, Matt and I were invited to participate, an opportunity we both jumped all over! Since then, nearly 100 new bird fossils have been found, most of which are more complete specimens of Gansus. That's how the Changma Project originated, and it's led to this tour! Our group here is the first set of non-paleontologists, and some of the very few Westerns, ever to be in the Changma Basin, and the first to work in our quarries!
ChangmaquarryXiagou Formationexcavationfossil birdGansus