Wyoming Fossil Fish in Matrix Eocene Period, 58-36 million years B.P. A Knightia sp.(?) fossil fish in an irregular matrix. 177 grams, 12.5 cm wide (5 in.). From Wyoming (Green River Formation), USA. From a Lincolnshire, UK, collection. Seasonally, some 58 million years ago, in the Green River region of Wyoming, torrential rains occurred in the mountains. The rain would flow down the mountain sides, forming wide, shallow lakes in the valley below. During the dry season, these lakes would dry up due to evaporation (similar to certain parts of Africa today). Millions of fish died in this short period of time. This climatic cycle recurred over a 20-thousand-year period. Layer upon layer of mud was deposited over these years, entombing huge numbers of fish. At a much later date volcanic activity uplifted this area to expose the fossil layers. [No Reserve]