The Bridge Timber Mountain area in south-central La Plata County, southwestern Colorado lies mostly in the northwestern part of the Central San Juan Basin but contains a segment of the bounding Hogback 'monocline' and Four-Corners platform.
The area contains rocks of late Cretaceous through early Eocene age, as well as Pliocene, Pleistocene, and Recent terrace and pediment gravels. The Pictured Cliffs sandstone of late Montana age is the latest marine formation present. Retreat of the Cretaceous seas from the area marked the beginning of Laramide orogenic activity and the earliest stages of deformation which produced the modern San Juan Basin. The Fruitland formation and Kirtland shale were deposited in brackish water and on coastal plains left by the retreating Cretaceous sea. Beds of the Farmington sandstone member and upper shale member of the Kirtland shale show evidence of a new source of sediments to the north or northeast distinct from the southwestern source area of older Cretaceous rocks. The McDermott 'formation', composed mainly of volcanic debris, is considered to be a local lower member of the Animas formation. Beds of the upper member of the Animas formation of Cretaceous and Paleocene age are considered to extend entirely across the area and into New Mexico. Overstep of higher sandstone and shale beds of the upper member across lower conglomeratic beds shows that folding on the Hogback 'monocline' began during deposition of the upper member. Beds of the upper member of the Animas formation grade laterally southward into Paleocene beds of the Nacimiento formation, but upper Nacimiento beds overstep folded beds of the Animas formation on the Hogback 'monocline' at the north end of Bridge Timber Mountain. The San Jose formation of Paleocene and Eocene age is conformable with the Nacimiento formation except at the north end of Bridge Timber Mountain where upper San Jose beds overstep all older tilted beds down to the Fruitland formation. The heavy sandstone facies of the Nacimiento and San Jose formations are correlated with similar facies of these formations on the east side of the San Juan Basin. Folding along the borders of the Central basin was completed prior to deposition of the youngest San Joss beds, and they were probably widely distributed outside of the Central Basin in Eocene time. In Pliocene time, the San Juan region was beveled by the San Juan peneplain. Rejuvenation of the San Juan Mountains in late Pliocene time caused erosion in the mountains and deposition of the Bridgetimber gravel in the San Juan Basin. Uplift in Pleistocene time caused large-scale erosion in the Bridge Timber Mountain area and gravel-covered terraces represent the various stages of uplift and erosion.
The stratigraphic relationships of uppermost Cretaceous and lower Tertiary rocks in the Bridge Timber Mountain area are similar to recently described relationships of equivalent rocks in other parts of the San Juan Basin. The southwestern lobe of the Pictured Cliffs sandstone was derived from older Cretaceous source areas to the southwest and deposited in the seaway which was retreating northeastward. The northeastern lobe consists of reworked Cretaceous sediments eroded from the flanks of the rising San Juan zone and Sangre de Cristo upwarp and deposited in an arm of the sea which was isolated by uplift of the mountain masses. This arm of the sea was forced to retreat to the southeast as sediments of the Fruitland, Kirtland, Animas, and Ojo Alamo formations were deposited in' the basin. The Animas formation which was derived from hi6hlands to the northeast spread progressively to the southwest and interfingered with lesser amounts of Fruitland and Kirtland sediments derived from the southwest. In latest Cretaceous or earliest Paleocene time folding began along the Hogback 'monocline' in northern and western San Juan Basin and sediments were eroded from the uplifted platforms around the margin of the Central Basin and rede