Ordovician, marine, around 50 m thick. The Maravillas was deposited in a deep-water, basinal environment (Turner et al. 2011). The formation is exposed along the northern margins of the park northward (Persimmon Gap and Dog Canyon areas within BIBE) and is convolutedly deformed in some areas by pre- and post-Cretaceous thrusting. The formation contains dark brown/blackish cherts and thin conglomerate lenses, and a few limestone beds Fossils from BIBE include graptolites, brachiopods, bryozoans, and conodonts. Extensive deformation and poor exposures make sectional thickness measurements and definition of individual members within BIBE difficult.

Silurian-Devonian, marine, only 20 m thick. The origin of both the novaculite and chert members leads to contrasting interpretations of water depth during deposition (e.g., Folk and McBride 1978). This formation contains chert and silicious shale with thin but conspicuous, white novaculite beds. The unit is modestly exposed near Persimmon Gap near the entrance of the park however no fossils have been reported from BIBE.

Mississippian - Pennsylvanian, marine, variably thick from 15-200 m. Deep-water sediments, thin to thickly bedded sandstone and dark gray, brown, and black shale. Several small outcrops are situated in the northernmost part of BIBE. No fossils have been collected from the park however nearby areas have produced conodonts, foraminifera, and a few Pennsylvanian Period plant fossils (King 1937).

Marine, massive, about 100-150 m thick. Primarily a massive limestone but contains clay, minor sandstone, marl, and conglomerate deposited in near-shore tidal and sub-tidal marine environs (Maxwell et al. 1967; Busbey 1989). This unit is exposed in several areas of BIBE including Persimmon Gap and Dog Canyon in the north, Marufo Vega trail in the southeast and Santa Elena Canyon in the southwest (Turner et al. 2011). These outcrops are generally exposed in areas which have been subjected to Cretaceous Laramide folding and/or the development of horst and graben structures emplaced during Miocene time. Invertebrate fossils include ammonites, oysters, gastropods, and echinoids. Rarely, dinosaur fossils have been found elsewhere in Texas from this formation (Upchurch et al. 2004). Although dinosaur trackways are somewhat common in the Glen Rose of Texas (e.g., Bird 1985) none have been reported in BIBE. However, several theropod dinosaur tracks are preserved along the Rio Grande in the Glen Rose Formation of Mexico just east of the park within the lower canyons (photos shown to the author by D. Corrick, BIBE Geologist).

Marine, generally 20-45 m thick. Lagoonal sediments (Busbey 1989) containing thin nodular limestone with marl beds This formation can be seen in several areas of BIBE including Heath Creek, along the Marufo Vega Trail in the east, and Santa Elena Canyon in the southwest where folding and faulting have exposed it (Turner et al. 2011). Common invertebrate fossils in this formation include gastropods, oysters, and echinoids. Ammonites have also been reported.

Marine, massive, from 100-150 m thick. Open lagoon, tidal flat, and rudistid biostromal facies (Busbey 1989). Generally, a massive, dense limestone with abundant rudistids. This karstic formation also contains lenticular cherts and minor marl beds. Within the park, it is exposed in areas of tectonic folding and faulting such as Santa Elena Canyon in the southeast and Marufo Vega Trail, and Sierra del Caballo Muerto in the east (Turner et al. 2011). Typical invertebrate fossils include bivalves and gastropods although recovery of them from the hard matrix is difficult which makes their identification problematic.

Marine, around 25-30 m thick. Transgressional marine sediments containing shale, marl, thin nodular limestone ledges (Maxwell et al. 1967; Busbey 1989). The formation is exposed in eastern and southwestern areas of the park including portions of the Sierra del Carmen, as well as Santa Elena Canyon where faults and folding have exposed it. Common invertebrate fossils include oysters, echinoids, gastropods, and numerous types of ammonites.

Marine, massive, up to 225 m thick. Open shelf carbonate environments (Busbey 1989). The Santa Elena is a massive, karstic limestone, hard, with some finely crystalline bedding along with nodular chert masses. Upper portions of this formation contain massive limestones with interbedded marls that weather to form a terrace-like topography. The formation can be found in eastern and southwestern parts of the park (and surrounds) such as the Sierra del Carmen, Santa Elena Canyon, and Sierra Ponce where faulting and folding have exposed it (Maxwell et al. 1967; Turner et al. 2011). Common invertebrate fossils include rudists with other pelycopods and gastropods being uncommon.

Marine, fissile, around 1-35 m thick. A regressive marine environment facilitated development of this shaly, shallow-water facies (Busbey 1989). This formation consists mostly of claystone with interbeds of limestone and friable sandstone. It is exposed in the eastern and southwestern portions of the park including Mesa de Anguila, Dog Canyon, Alto Relex, and Sierra del Caballo Muerto (Turner et al. 2011). Invertebrate fossils include oysters, echinoids, and gastropods.

Marine, 20-30 m thick. Shallow, inner-shelf environment. This formation primarily crops out in eastern, southern. and southwestern areas of the park such as Dog Canyon, Dagger Mountain, Mariscal Mountain, and Mesa de Anguila (Turner et al. 2011). Invertebrate fossils are rare in fine-grained limestones and more common in marls including echinoids, gastropods, and bivalves. West of the park along route 170, the Buda/Boquillas limestone contact interval harbors the typical reddish tint of cinnabar.

Marine, massive to shaley, from 220-245 m thick. Foraminiferal limestone and shale deposited in relatively shallow, open marine (platform) conditions (Lehman 1989b; Cooper et al. 2017). This formation contains two members including the lower Ernst Member and upper San Vicente Member (Maxwell et al. 1967). The Ernst Member contains silty limestone flags, siltstone, and calcarious clay while the San Vicente Member contains chalk, marly clay, and shale. It is exposed widely in the park in areas such as San Vicente, Hot Springs, Mariscal Mountain, McKinney Hills and Mesa de Anguila (Turner et al. 2011). The Boquillas Formation is very fossiliferous. Fossils include invertebrates such as cephalopods, bivalves, and echinoids as well as a few vertebrate fossils from mosasaurs, fish, and sharks. Even soft-bodied organisms (squids) have been discovered in the Boquillas.

Marine shelf, 70-200 m thick. Calcareous clay shale and chalky limestone with concretionary intervals. The Pen Formation was deposited upon a shallow marine shelf. This unit also includes a westerly-thinning wedge of dark gray marine shale within the overlying Aguja Formation (e.g., Lehman 1985). This formation is widely exposed in the park in areas such as San Vicente, Mariscal Mountain, Maverick Mountain and the McKinney Hills (Turner et al. 2011). Invertebrate fossils include echinoids, bivalves, gastropods, and ammonites. Vertebrate fossils are uncommon but include fragmentary sharks, fish, and mosasaurs. However, shed shark teeth and fish vertebrae are common throughout the formation. Rarely, re-worked dinosaur bones (resulting from floods washing carcasses seaward) are also encountered (pers obs. by the author).

Originally named "Rattlesnake Beds" by Udden (1907), these strata were later re-named the Aguja Formation as the previous name was already in use elsewhere. Nearshore marine, deltaic, and continental facies including paralic, estuarial, and coastal marsh and swamp deposits (Maxwell et al. 1967; Lehman 1985), 120-280 m thick. The coastal Aguja Formation records fluctuating periods of marine transgression and shoreline progradation. Transgressive and regressive marine Aguja facies include thicker, well-indurated marine sandstones, poorly developed coals, lignitic shales, and thin cross-bedded fluvial channel sandstones. The upper part of the formation contains coastal floodplain mudstones; some with incipient paleosol development. The Aguja Formation is widely exposed in BIBE in areas such as Dawson Creek, Rattlesnake Mountain, San Vicente, and McKinney Springs (Turner et al. 2011).

Some facies within these units are very fossiliferous while others are not. Plant fossils are locally abundant in the Aguja Formation. These usually include fossilized woods from conifers, palms (monocots), and flowering plants (dicots). Rarely, tree stumps are found upright, situated in their original growing positions. Fossil leaves have been found in a couple areas preserved as carbonate films within mudstone horizons or, in one area, as impressions within reworked volcanic ash. This ash bed and its fossils are currently under study by the author (S.W.). Aguja invertebrate fossils include bivalves, gastropods, cephalopods and rarely, crustaceans. Trace fossils from some of these taxa are also relatively common (e.g., Ophiomorpha burrows).

Occasionally, vertebrate fossils (and microfossils) are also found at various stratigraphic intervals in strata representing numerous environs. Taxa include sharks, fish, turtles, crocodilians, as well as dinosaurs among other reptiles. Very rarely, small fossil mammals are encountered (mostly teeth) as are dinosaur eggshell fragments. The vertebrate fossil assemblage of the Aguja Formation is the most inclusive of its kind reported from southernmost North America.

Continental, 100-190 m thick. The formation can be found along the flanks of the Chisos Mountains and is well exposed along the drainages of Tornillo, Terlingua, and Dawson creeks, as well as Rough Run (Turner et al. 2011). This formation contains facies from inland floodplain environs. Sedimentary strata include well-cemented fluvial sandstones, rhythmically-bedded lacustrine deposits, and floodplain mudstones - some containing fairly well-developed paleosol and paleocaliche horizons (Lehman et al. 2018). Generally, fossils are uncommon throughout this formation; however, several discreet areas (and representative habitats) are quite fossiliferous (e.g., Lehman and Langston 1996). Fossil wood is common in the Javelina Formation and includes fossils from fan palms as well as conifers and flowering plants. Abundant prone fossil logs can be found along a few stratigraphic horizons while others harbor stumps in their original growing positions. Invertebrate fossils are very rare but include fresh-water gastropods and crustacean burrows.

Isolated, broken vertebrate fossils are somewhat common within scree along deflated surfaces atop fluvial sandstone hogbacks but are also found in-situ at local intervals within overbank mudstones. Vertebrate fossils include those from fish, turtles, pterosaurs, dinosaurs, and small mammals (represented mostly by teeth). Vertebrate fossils usually occur as isolated, fragmentary bones. However, a few dinosaur skeletons have been found partially articulated or with bones in close association. Although typically well-preserved, Javelina Formation fossils are seemingly not as numerous as those of the underlying Aguja Formation. As such, I surmise that the paralic Aguja environment favored a greater variety (and populations) of vertebrate species and/or the paralic environment was more conducive the burial and preservation of remains. Lehman et al. (2006) obtained a radiometric date of around 69 Ma. for the middle of the formation

Continental, around 40 m thick (widely variable) (e.g., Lehman et al. 2018). The Black Peaks Formation contains inland flood plain deposits with interstitial fluvial sandstones. The formation is exposed widely in BIBE especially near Grapevine Hills, Dogie Mountain, and Tornillo Flat. Paleosols are sometimes well developed, appearing as somber red and black bands which are, in places, interrupted stratigraphically by fluvial sandstones. The bottom third of the formation is Cretaceous in age. Plant fossils present in the lower Black Peaks Formation including conifers and flowering plants. Invertebrate fossils are virtually unknown; however, freshwater ?crustacean burrow structures have been observed. Vertebrate fossils are uncommon in this portion of the formation but include those of fish, reptiles, as well as dinosaurs (especially those of the huge titanosaur Alamosaurus). Usually, vertebrate fossils are found isolated, weathering out of fluvial channel sandstones. Rarely, associated dinosaur bones have been located eroding from overbank mudstones.

The Cretaceous-Paleogene (K-Pg) boundary is situated in the lower third of the Black Peaks Formation although its exact stratigraphic position remains obscure. It has been defined within a two-meter section near the Grapevine Hills (Lehman and Coulson 2002). However, it has not been this well-defined elsewhere in BIBE (see discussion in Lehman et al. 2018, p. 2225). It is possible that there was a depositional hiatus during the K/Pg time interval and that the K/Pg boundary is only preserved in very localized lenses of deposition (if at all) within the park.

Continental, up to 400 m thick (widely variable). The Black Peaks Formation straddles the K-Pg boundary. The Paleogene portion of the formation contains inland floodplain deposits with thick, fluvial sandstones. It is exposed near Dogie Mountain, Grapevine Hills, and Tornillo Flat (Turner et al. 2011). Paleosol horizons are often striking, appearing maroon, black or somber gray sometimes with interstitial, tan fluvial channel sandstones. Paleosols within the Cretaceous, Aguja and Javelina formations are often poorly developed. However, they become increasingly better developed higher in section with the Black Peaks having the most conspicuous forms. Typical vertebrate fossils include garfish, turtles, and mammals.

Plant fossils (mostly conifers) are rarely found in the lower part of the formation but are more common higher in section. Two, closely-space stratigraphic intervals of very large fossil dicot logs (Paraphylanthoxylon) in the middle portion (Torrejonian-Tiffinian) of the Black Peaks section (informally called the "log jam sandstone") suggest the post K-Pg resurgence of trees during this time. This fossil log horizon is conspicuous in many areas of the park and is a useful stratigraphic marker (Lehman et al. 2018).

Continental, varies from around 30 to 70 m in thickness (e.g., Lehman et al. 2018). This relatively thin formation is very limited in area with all known outcrops in the Tornillo Flat region of BIBE and represents the final infilling of the Tornillo Basin (Turner et al. 2011). The inland floodplain formation contains variegated mudstone-dominated facies along with coarse fluvial sandstones and conglomerates. Vertebrate fossils include those from several mammalian taxa. The fossil bone exhibit in BIBE is situated atop fluvial channel sandstones of the Hannold Hill Formation (Exhibit Ridge Sandstone Member) where numerous specimens of Coryphodon were excavated and displayed as part of the park's original Fossil Bone Exhibit.

Continental (upland), up to 350 m thick. This formation is exposed in the north-central portion of BIBE especially on Tornillo Flat (Turner et al. 2011). It contains rocks from a sandy, braided fluvial system with associated flood plain deposits (e.g., Rigsby 1986; Runkel 1988) which rest unconformably on the Hannold Hill Formation. Thick sandstones and conglomerates comprising the conspicuous Big Yellow Sandstone Member are present in the lowest part of the Canoe Formation with gray and variegated mudstones situated a bit higher in section. These paleosol horizons (along with interstitial sandstones and tuffaceous mudstones) make up a large portion of the Canoe Formation section above the Big Yellow Sandstone.

Vertebrate fossils are widespread within the formation in BIBE as well as areas northwest of the park in the Devil's Graveyard Formation which is temporally coeval with the Canoe Formation (e.g., Runkel 1988). The reader is cautioned that the Devil's Graveyard Fm. is not exposed within the park so its reported taxa are not included herein. Vertebrate fossils in the Canoe include those from mammals, turtles, and crocodilians. Fossilized wood is also common in the Big Yellow Sandstone including not only Eocene conifers and dicots but reworked and abraded, fossilized Cretaceous wood fragments exhumed during entrenchment of the younger, Eocene fluvial system. A striking example of its fossil ensemble includes a dense 'forest' of at least 92 fossil tree stumps in their original growing position observed by the author near the McKinney Hills. Whether these represent conifers or dicot trees is not yet known. However, these stumps (~10 to 15 cm in diameter) are the remains of smaller trees that apparently grew on islets within the confines of the braided fluvial corridor.

Continental (upland), from 500-700 m thick. The Chisos Formation is exposed in many areas of BIBE along the flanks of the Chisos Mountains (Turner et al. 2011). This widely variable formation contains lavas, tuff, tuffaceous sandstone, clay, and conglomerates. Vertebrate fossils include turtles and large mammals while invertebrates include fresh-water gastropods and snails). Fossil wood is present but not common.

Please note that the Burro Mesa Formation is not considered valid by all researchers and so both are included together here. Continental (volcanic), from 300-500 m thick. These typically massive, volcanically-derived strata are exposed in the central and southwest portions of BIBE in the Chisos Mountains and near Burro Mesa. They contain lavas, flow breccias, conglomerates, tuff, and tuffaceous sediments from various localized eruptive events and are apparently non-fossiliferous.

Continental (bolson deposits), up to 300 m thick. The formation is exposed on the west side of BIBE near Castolon (Lehman and Busbey 2007; Turner et al. 2011). Originally identified by Maxwell et al. (1967) as 'older gravels', the Delaho has two members including the lower member and Smokey Creek Member. These contain pink friable sandstone and gray conglomerate representing mid and distal alluvial fan deposits that accumulated in a fault bounded basin in the western half of BIBE (the Delaho Bolson). Vertebrate fossils include those from small and large mammals as well as from several reptiles including a unique Gila monster.

Continental (bolson deposits), up to 150 m thick. This formation is exposed in the east-central portion of BIBE near Banta Shut-In. These include pink fine-grained sandstone, siltstone and red mudstone which represent distal alluvial fan facies in the eastern half of BIBE (Estufa Bolson). Vertebrate fossils include amphibians, reptiles, and mammals (including those from canids, camels, and primitive horses). This formation is exposed in areas along Tornillo Creek that are not easy to reach and it is likely that its fossiliferous nature has yet to be fully realized.

Continental (bolson deposits), variably thick up to 300 m. These formations are exposed in the western portion of BIBE near Sotol Vista and along the flanks of Tornillo Creek east of Dugout Wells and consist mostly of bolson deposits. They were originally identified as 'older gravels' by Maxwell et al. (1967) and consist of proximal alluvial fan facies which overlie the Delaho and Banta Shut-In formations (Turner et al.2011). Primarily these contain larger sand and gravel clasts eroded relatively recently from the volcanic and plutonic rocks of the Chisos Mountains. However, they also contain scree from Paleozoic and Late Cretaceous strata exposed along the margins of the ancient bolson. The fingers and Estufa Canyon formations represent the youngest deposits within the Delaho and Estufa bolsons and have yet to produce fossils.