As air masses rise over orographic barriers, rainout results in a decrease in d18O that has a quantifiable relation with elevation. Thus a comparison of d18O of materials preserved in ancient and adjacent basins can be used to estimate the relief of mountains between them.

For the early Eocene, average d18O values for riverine bivalve carbonate, river fish scales, and mammalian tooth enamel phosphate are ~1.1 per mil higher for the Powder River basin (WY) than the Bighorn basin to the west, with estimated river d18O values of ~-7 to -9 per mil and –9 to –10.5 per mil, respectively. Global climate model simulations indicate predominant wind directions from the east and south. Thus this inter-basinal difference in d18O is likely due to the ancient Bighorn Mountains standing ~ 250-520 m between the basin floors.

Anomalously low d18O values for bivalve carbonate indicate that Bighorn basin river waters may have been as low as ~-21 per mil. Rather than representing permanent snow and hence much higher relief, it is proposed that these rare, low d18O values reflect (1) episodic changes in atmospheric circulation which introduce highly distilled moisture from Pacific sources to the Bighorn basin; or (2) transient cooling events that lead to cooler winter temperatures and winter accumulations of higher elevation snow.