stoneflies, are pollution-intolerant and thus their presence is an indicator of good water quality. They were found during a 1989–90 survey of the lower Oatka, but in lower numbers than expected. The stream is thus considered "slightly impacted" by pollution. Many of those insects constitute the food supply for various trout species. The lower Oatka is considered a blue-ribbon brown trout fishery, with some brook trout populating the water as well. The state fish hatchery on Spring Creek near Caledonia stocks the stream annually, and there is also evidence of a wild trout fishery on the stretch between Bowerman and Wheatland Center roads. A 1999 survey estimated the creek's total trout biomass at 30 pounds per acre (33 kg per hectare) bass, walleye and northern pike have also been taken near the Genesee. Geology Buttermilk Falls divides Oatka Creek into two distinct geological regions. The upper stream's bedrock is local sedimentary rock formations, with the shales and sandstones of the Sonyea Group, shale-limestone Genesee Group and Tully Limestone dominating from the headwaters to Pavilion. Between there and Le Roy the shales and thin limestones of the Hamilton Group underlie the creek. North of Le Roy, the Oatka becomes a losing stream, with water disappearing into the ground. In the warmer months this results in the creekbed drying up above Buttermilk Falls, and slowly re-emerging further down from springs and seeps. After it does, and turns to the east, it flows over the Akron Formation, Bertie Group of dolomitic shales, and the Salina Group. The latter is diverse, including evaporites such as halite and gypsum in addition to its limestones and shales. In the lower section of the creek there is significant groundwater entering the creek. As it discharges, it weathers the gypsum and limestone. This results in increased sulfates in the water, and bicarbonates and calcium have also been found near the falls. These concentrations are in winter and spring, when heavier precipitation and runoff offsets the impact of the groundwater. History As the glaciers retreated 12,000 years ago the landscape they left behind on the Allegheny Plateau was a rolling expanse of drumlins and depressions in which meltwater and precipitation could accumulate. It was ideal topography for stream formation, and these bodies of water eventually combined and became the Oatka, eroding the Oatka Valley. Fertile soil from the highlands accumulated in the valley, and the land eventually reforested. The