Many of the outcrops in the Lake Meredith National Recreation Area are Permian in age, and due to the nature of these sediments, they have no fossils associated and are not considered in this discussion. The Texas Panhandle is one of the prime localities for Cenozoic vertebrate fossils in North America. No less than three Land Mammal Ages are named from localities in our area. These are the Clarendonian, Hemphillian, and Blancan Mammal Ages, named for localities in Crosby County, Hemphill County, and near Clarendon in Hall County. (See Wood et. al. 1941)

Triassic

      Triassic Dockum Group which reaches a maximum thickness of more than 1000 feet. These sediments consist of vari-colored shales interbedded with vari-colored sands. Petrified wood is common in some of these sediments, and some are thought to represent the Chinle formation which is the formation in Arizona in which the petrified forest is found. The Triassic lasted from 245 to 208 million years ago. In our area it is best represented by outcrops with petrified wood, especially in the Tincup Creek area.

Tertiary

      There is a break in deposition between the Triassic and overlying rocks, but after this unconformity, the Tertiary Ogallala Formation outcrops and thickens as it moves upslope and away from the recreation area.

The LX Member

      The LX Member of the Ogallala is named for the LX Ranch where its broad exposures are present along the divide between Chicken and Bonita Creeks. The LX Member crops out only along the eroded divides separating Bonita, Chicken, Coetas, and Mullinaw Creeks and on the north side of the Canadian River, along Corral and Ranch Creeks. This Member consists of 40 to 110 feet of massive, unconsolidated, pink to buff and sometimes yellow and greenish-gray silty, fine to medium grained sand that conformably overlie the consolidated channel sands of the Potter Member or where the Potter is absent, the older Triassic or Permian Rocks. The upper 20 feet of the LX Member contains vertebrate fossils representative of the Clarendonian Land Mammal Age.

The Coetas Member

      The limestone beds show distinct ripple marks and often include gastropod molds and siliceous plant fragments. Two small fish imprints were obtained from the thin flaggy beds. The flaggy limestone units often contain Clarendonian vertebrate fossils.

Pleistocene

      Above the Tertiary outcrops, Pleistocene rocks and sediments outcrop locally, especially near Sanford Dam. Associated with these Pleistocene sediments is a marker bed, the Perlette O’ bed of Volcanic Ash which is associated with the final eruptions at Yellowstone Caldera, and date to about 610,000 years ago. Younger beds which outcrop in places around the lake are Rancho La Brean in age and date to 40,000 years ago. These have yielded a Bison latifrons skull from the area near Plum Creek, and scattered remnants of Imperial Mammoth and Giant Tortoise.
      In real-time dating, the Pleistocene mammal record from northwestern Texas includes four intervals. First from 10,000 Years ago, the arbitrary termination of the Pleistocene, to 40,000 Years ago, the approximate limit of carbon dating. Second, the long interval from 40,000 Years ago to 610,000 Years ago, lacking time markers. Third, a short period with deposits associated with the Lava Creek B (Pearlette Type O) volcanic ash. Last, the long interval from approximately 610,000 Years ago to approximately 1.2 million Years ago, from the Pearlette ash fall back almost to the beginning of the Pleistocene Epoch. (Dalquest and Schultz, 1992)
      Two ages are represented - the Irvingtonian dating from about 1.3 million years ago to about 40,000 years ago and the Rancho LaBrean age dating from about 40,000 to 10,000 years ago. The Holocene/Modern Period is after 10,000 years ago. Two fossil localities near Lake Meredith and Sanford Dam have yielded vertebrate remains.

Merchant Ranch

      A small eastward-flowing tributary to Big Creek, approximately 1 mile north of the Canadian River and 2 miles north of the town of Sanford, Hutchinson County, Texas. Parts of a mammoth skeleton and other fossils were excavated from a bluish to yellowish limy clay overlain by a thick deposit of eolian sand. The clay may have been deposited in a pond or marsh in which the mammoth died. These were Late Pleistocene, possibly late Irvingtonian, based on referral of a muskrat jaw to Ondatra nebracensis.
      The second location , known as the Sanford-Big Creek Local Fauna is found in a roadcut on the north side of Farm-to-Market Road 687, one half mile west of Big Creek, approximately 2 miles north of the town of Sanford in Hutchinson County. In this area, Late Pleistocene strata are exposed for a horizontal distance of about 380 ft on the north side of the road. The exposed section consists of about 9 ft of unconsolidated, rusty brown medium to coarse fluvial sand containing much gravel and cobbles up to 6 inches in diameter. The unit, presumably of late Pleistocene age, grades rapidly upward into a 3 feet thick ash-gray silty clay which contains abundant freshwater snails, pond turtle, and a few microvertebrates, which constitute the Sanford-Big Creek local fauna. This layer is covered by up to 2 feet of gray to brown eolian silt and soil. It is assumed to be Late Pleistocene in age based on occurrence of Microtus pennsylvanicus, the meadow vole. A cool wet period (glacial?) is suggested by the presence of several species of freshwater snails which today are northerly forms enjoying cooler summers and more effective rainfall. Microtus pennsylvanicus today ranges no farther south in the Great Plains than extreme north-central Kansas.
      The Holocene is represented in some of our prehistoric ruins, and includes a study by Lathel Duffield outlining the vertebrates associated with the Plains Village period. The Alibates ruin, Sanford Ruin, and the Roper site were included in this study of Holocene vertebrates by Duffield (1964 ).

---