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An abstract of the thesis of Shaun Andrew Marcott for the Master of Science in Geology presented August 11, 2005. Title: A Tale of Three Sisters: Reconstructing the Holocene glacial history and paleoclimate record at Three Sisters Volcanoes, Oregon, United States. At least four glacial stands occurred since 6.5 ka B.P. based on moraines located on the eastern flanks of the Three Sisters Volcanoes and the northern flanks of Broken Top Mountain in the Central Oregon Cascades. The youngest of these advances was the Little Ice Age (LIA) glaciation, which reached its maximum advance 150-200 yrs. B.P. and is defined by the large sharp crested and unvegetated moraines adjacent to the modern glaciers. In isolated locations less than 100 m downslope from these moraines, a second set of sparsely vegetated lateral moraines marks the LateNeoglacial stand of the glaciers between 2.1 ± 0.4 and 7.7 ka B.P. A third set of EarlyNeoglacial end moraines is 300-700 meters downslope of the modern glacier termini, and postdates 7.7 ka B.P. From SST temperature data (Barron et al., 2003) and a speleothem record (Vacco, 2003), we infer that this advance occurred between 4.5 and 6.5 ka B.P. Finally, the Fountonnor stand is marked by moraines 500-900 meters downslope of the modern glacier termini, and we infer these are latest Pleistocene or early Holocene. Modern equilibrium line altitudes (ELAs) at the Three Sisters and Broken Top are approximately 2500 – 2600 m. During the LIA, the ELAs were 40 180 m lower, requiring cooler mean summer temperatures by 0.7 – 1.0°C and winter snowfall to increase by 10 – 60 cm water equivalent. The average Early-Neoglacial and Fountonnor ELAs were 130 – 300 m and 290 – 320 m lower than modern glaciers, respectively, requiring air temperatures to be 0.7 – 1.6°C and 1.5 – 1.7°C cooler during the summer and winter snowfall to be 40 – 100 cm water equivalent and 90 – 100 cm water equivalent greater. Acknowledgments I would like to express my gratitude to my boss and advisor, Andrew Fountain, for his support throughout this thesis and for protecting me from the glacier database project. Through our stimulating discussions on all varieties of subject matter, I feel that I have learned a great deal about life and science. Thank you. I would like to thank Jim O’Connor for his patience in answering my never-ending emails about the geology of Three Sisters and for all the insight he has given me during this project. Without his help, this project would have never left the ground. I would also like to thank my other committee members, Scott Burns and Alan Yeakley, for their insightful comments that helped improve this thesis. Because no master’s thesis that involves fieldwork is done alone, I would like to thank Matt Brunengo, Jon Ebnet, Ashleigh Fines, Matt Hoffman, Seth Moorehead, Peter Sniffen, and Andy Urich for helping lug all my gear up and down the mountains. Without your assistance, my back would surely be broken and very little fieldwork could have been accomplished. I would also like to thank Dr. Doug Clark and Niki Bowerman of Western Washington University for taking five days out of their busy schedules to help collect the lake cores during the spring of 2005. They, as well as all my field partners, embody the altruistic character that is becoming rare in science today. I am forever grateful to you all. Thanks to Willie Scott for sharing all of his knowledge about the glacial history of Oregon. Without his comments and suggestions, this thesis would have taken much longer and would have suffered many setbacks. Thanks to David Dethier