Near the middle of the Tecovas [Formation] there is a bed of white, crumbly (friable) [sandstone]. Averaging about 15 feet in thickness, this sandstone contains many [joints] (small crack-like fractures) along which no appreciable movement has taken place ([fig. 8]). There are two distinct sets of these joints which intersect each other at right angles. The distinctive joint patterns, the color, and the friability of this sandstone clearly differentiate it from the harder, darker, and more coarse-grained sandstones of the overlying Trujillo Formation ([p. 22]).
The upper part of the Tecovas consists of a layer of orange [shale] which overlies the middle [sandstone] unit and is in contact with the lower part of the Trujillo [Formation].
Fig. 12. Taken from the northwest rim near Coronado Lodge, this photograph shows the four major [rock] units exposed in the park: (1) The Quartermaster [Formation] which forms the lower wall and canyon floor; (2) Tecovas Formation; (3) Trujillo Formation which caps the mesas; and (4) Ogallala Formation.
The [fossils] which have been found in the Tecovas [Formation] suggest that these [rocks] were derived from [sediments] deposited in swamps and streams. Unlike the marine deposits of the Quartermaster, the rocks of the Tecovas were formed from continental deposits laid down on the land. Fossils found in the canyon include the bones and teeth of the extinct semi-aquatic reptiles known as phytosaurs ([fig. 13]) and bone and skull fragments of a primitive amphibian called Buettneria ([fig. 14]). Coprolites (the fossilized excrement of animals), pieces of petrified wood, and the teeth and bones of lungfish have also been reported from the Tecovas.
Fig. 13. The skull of this crocodile-like creature called a phytosaur is typical of the reptiles that inhabited the Palo Duro area during the Triassic [Period]. (Photograph courtesy Panhandle-Plains Historical Museum.)
A number of [minerals] including hematite, an iron mineral, and psilomelane, a barium-magnesium oxide, occur in the Tecovas. Hematite is an ore of iron and psilomelane a manganese ore, though neither of these is present in commercial quantities in the canyon.
The Tecovas also contains a number of concretions which range from a fraction of an inch to as much as 6 inches in diameter. These spherical masses are generally harder than the fine-grained shaly sands in which they are found and were thus left behind when the surrounding [rock] was eroded away. Some of these concretions are marked by cracks or veins filled with the [mineral] [calcite]. Concretions bearing this type of structure are called septaria, or septarian concretions.
[Geodes] are also found in the Tecovas [Formation]. These are rounded concretionary [rocks] with a hollow interior that is frequently lined with [mineral] crystals. Well-formed crystals of clear [calcite] have been found in many of the geodes from the Tecovas.