The Late Tertiary represents warm Earth conditions immediately prior to the development of extensive northern hemisphere glaciation, and this period in Earth history may therefore provide the best available analogy for the projected outcome of continued global warming. There are few interior continental sites of Late Tertiary age from the eastern half of North America and subsequently very little is known about the conditions characterizing climate. The Early Pliocene (∼5 Ma) Pipe Creek Sinkhole (PCS) includes the sediment fill of a complex karst environment that developed in north-central Indiana, USA. The PCS stratigraphy has multiple facies including: (1) Terra rossa with δ ¹³ C values averaging -20 ±0.7‰ PDB, which are interpreted to represent sediment deposited in a closed cave system under high summer temperatures with well-drained soils. (2) An in-situ paleosol at the top of the terra rossa, which formed after the cave opened to the land surface. (3) Dark-colored, organic-rich pond sediments cap the other facies, and include the abundant vertebrate fossils that are broadly consistent with those inhabiting an open ecosystem such as a savannah or parkland. However, pollen analysis from the pond sediments includes dominantly pine pollen and no grass pollen.

The Gray Fossil Site (GFS) includes an exceptional latest Miocene to earliest Pliocene (4.5 to 7 Ma) sinkhole lake deposit that contains incredibly well-preserved sediment, fauna, and flora. The uppermost ∼2.8 m of lacustrine stratigraphy is characterized by autochthonous rhythmites that regularly alternate between coarse-grained and organic-rich (A) laminae and fine-grained, silty-clay (B) laminae, and are interpreted to represent annual varves deposited in a monsoonal precipitation pattern. Equivalent periodicities at 23.0/24-years and 5.1/4.4-years were discovered from the spectral analysis of thickness measurements of annual growth bands in fossil wood and the lacustrine rhythmites, which confirms an annual association for the rhythmites. The dominant 23/24-year climate cycle is interpreted to represent the Hale-cycle, which is thought to have modulated precipitation. The strong expression of the Hale-cycle suggests the importance of considering the Earth-Sun climatic relationship for understanding climate change and global warming.