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• ~ ` 3- is underlain by recent alluvial fan deposits of the Alamosa Formation of Quaternary Age. The Alamosa Formation is at least 4,000 feet thick at the lowest point of the Alamosa Basin. The deposition of the Alamosa Formation was contemporaneous with extensive block faulting along the Sangre de Cristo mountain front. At least 30,000 feet of largely unexplored fill material underlies the Alamosa Formation at the deepest part of the Alamosa Basin. The San Luis Hills are a physiographic subdivision of the San Luia Valley and constitute a rugged mass of hills and mesas 500 to 1000 feet high extending from near the town of Antonito to near the town of Blanca. These hills are dominantly andesitic volcanics which can be correlated with the mid Tertiary volcanic series of the San Juan Mountains. The hills were faulted upward possibly by extensions of the faults bordering the west flank of the Sangre de Cristo Range. After dissection of the hills by erosion, they were surrounded by basalt flows of the Hinsdale series which in most areas is now covered by alluvium of the Alamosa Formation. The Taos Plateau is an extensive plateau like area lying south of the San Luis Hills and extending into Colorado south of the town of Antonito. The plateau is largely underlain by basalt of the Hinsdale series. The Costilla Plains physiographic subdivision lies between the San Luis Hills and the Sangre de Cristo Range and extend southward into New Mexico. Alluvial deposits blanket the eastward extension of the Hinsdale basalt and is largely undissected. The east boundary of this subdivision probably marks the trace of the normal fault along the edge of Sangre de Cristo Mountains. While the surface of this physiographic subdivision resembles the Alamosa basin it is derived from erosion rather than deposition. The Culebra Reentrant differs from the rest of the San Luis valley in diversified and mature topography without being buried by basalt flows. The area is topographically diverse consisting of three parts: (1) a relatively elevated belt of foothills near the Sangre de Cristo Mountains (2) a central graben subsequently eroded and (3) a prominent Horst - the San Pedro~Mesa.