We have been studying a core of opportunity obtained as a result of geotechnical drilling in the Upper Truckee Marsh on South Lake Tahoe, California. The core is 51.8 meters long and provides new information about conditions in the Lake Tahoe basin toward the end of the last glacial. We have conducted sedimentological, environmental magnetic, palynological and paleontological studies of the core. Detailed particle size analysis shows that the upper 9 meters of the core consists primarily of coarse sand, which is underlain by 8 meters of silty sand. A silty clay layer spans the interval from 17 to 21 meters, and that is underlain by another 29 meters of silty sand. At the base of the core is a second silty clay interval. The magnetic intensity is generally higher in the fine-grained units than in the coarser-grained units although other magnetic properties do not vary significantly. The trends in magnetic grain-size mirror those in overall sediment particle size. The pollen spectrum is dominated by two types of pine and by fir, but there is little variation in either the concentration or relative abundances. Diatoms have not been found in the fine-grained portions of the core. Radiocarbon dating of the upper fine-grained interval gives an age of 14,000 BP for the top and 25, 000 BP for the bottom. The concentration of radiocarbon in the basal fine-grained interval is too low to provide an age. As indicated by the radiocarbon dates, this core provides information about conditions in the Lake Tahoe basin before, during and after the Last Glacial Maximum. Previous workers have suggested that the lower part of the Upper Truckee River, which drains into Lake Tahoe, was never glaciated. Our results support the idea that the valley was occupied by a glacial outwash plain that provided the coarse sediment that predominates in the core. We hypothesize that the fine-grained layer in the upper part of the core reflects a time when the canyon of the Lower Truckee River, into which Lake Tahoe drains, was blocked by one or more glaciers extending down from Squaw Valley or another drainage. This blockage raised the level of the lake above its natural sill and moved the depocenter of the outwash plain up the Upper Truckee Valley, resulting in deposition of finer- grained material at the coring site. Although a 10-15 meter rise in lake level dramatically changed the sediment being deposited at the coring site, it did not have a significant impact on the surrounding landscape, as evidenced by the lack of abrupt changes in the pollen and the magnetic properties. The presence of the basal silty clay indicates that the glacial blockage of the canyon of the Lower Truckee River was probably a recurring phenomenon.