THE DISTRIBUTION and abundance of peat bogs in North America is largely determined by the boundaries of Pleistocene glaciation. The occurrence of innumerable lakes within the glaciated region is responsible for their great number, and sufficient precipitation over much of this region permits the maintenance of a constant lake level or water table in the bog. Apparently most of the environmental conditions have been at an optimum for hydrarch plant succession culminating in the formation of peat deposits. Climate is perhaps one of the chief ecologic factors concerned with the development and continued accumulation of peat and other types of pollen-bearing sediments. The proximity of the Pacific Ocean and the presence of the Coast Range, Olympic Mountains, and the Cascade Range, permit the existence of a great diversity of climates, and hence, many plant formations in the Pacific Northwest. All of Oregon, and most of Washington and Idaho lie south of the boundaries of Pleistocene continental glaciation, and, consequently, peat deposits and other types of sediments suitable for pollen analysis are not common over much of this vast re*gion. Where the climate is not too dry, the absence of ponded water has inhibited the initiation of bydrarch plant succession and subsequent accumulation of peat. The greatest number of peat deposits is in the Puget Lowland of western Washington, which was glaciated and where the climate is favorable for maximum bog development. Peat deposition has also occurred in montane glacial lakes and along the Oregon Coast, where the erosion cycle of a submerged shoreline has resulted in the formation of many lakes (Hansen, 1941c). In the latter region, sand dune lakes also support various stages of bydrarch plant succession, but they are usually recent or of uncertain age. East of the Cascade Range in Oregon and Washington the few existent lakes are usually too alkaline to permit hydrarch succession, or fluctuating water levels do not encourage peat deposition. In other areas the mature topography with its efficient drainage prevents the existence of standing water and subsequent hydrarch plant succession. Tule swamps are rare, and those examined overlie shallow strata of black muck, unsuitable for pollen analysis. LOCATION AND CHARACTERISTICS OF THE BOG.The peat deposit of this study is located about 13 miles west of Bend, Oregon, in section 16 of T. 18 S., R. 10 E. on the Three Sisters quadrangle. The elevation of the bog is about 5,240 feet above sea level. The peat and other pollen-bearing sediments have accumulated in Tumalo Lake, a small lake that was apparently ponded by the terminal moraine of