The Glass Mountains are located in southwestern Texas, U.S.A., at approximately 30°30' N and 103° W, about 370 km (about 225 mi) ESE of El Paso, Texas, in an area marked by the towns of Fort Stockton and Marathon. The mountain range has been described as about 24 miles (about 39 km) long and between 6 and 15 miles (about 10 and 24 km) wide. It runs from southwest to northeast, with a dip slope to the northwest and erosion cliffs to the southeast. The western part of the range has higher peaks and deeper canyons than the eastern part, which tends to be more even. The appearance of distant limestone scarps is a probable origin for the Spanish name "Sierra del Vidrio," from which the English name is translated. The mountains include limestone, shale, sandstone, dolomite, marl, and other rocks, sediment, clay, and sand.
This mountain range is part of a relatively large geographical area marked by the presence of geological features dating from the Permian period, dated 286 to 245 million years ago. This area, called the Permian Basin, underlies parts of western Texas and southeastern New Mexico in the southwestern United States. Sediments from that era cover a thickness of some 12,000 feet (about 3.7 km). The Permian features in the southwestern United States are mostly marine in nature; when the Permian period began, what is now the Glass Mountains were underwater. As time went on, the ocean evaporated, and when the Permian ended, the former ocean was dry.
The Glass Mountains are located on the southern edge of the Delaware Basin, which forms much of the western part of the Permian Basin. The border of the Delaware Basin is marked by the Capitan Reef, an ancient coral reef in Texas and New Mexico; one of the areas where it protrudes is the Glass Mountains, on the southern boundary of the Delaware Basin. The Delaware Basin includes an estimated billion years of geological record, but the Permian portion is of especial interest because of its presence in the area; some 95% of the outcrops are of Permian origin.
The Glass Mountains themselves have been chosen as the standard section for the Permian period because of the presence of a continuous layer of Permian sediment 1500 to 2000 m thick. The layers of the Permian in the area, as first proposed in 1939, are the Wolfcampian, the Leonardian, the Guadalupian, and the Ochoan, named for various local features (the first two are named for Wolf Camp, a town in the Glass Mountains, and Leonard Mountain, also in that mountain range). The sediment in the Glass Mountains extends from early Wolfcampian to late Guadalupian time. The strata (layers of rock) have been subdivided into smaller divisions as more research has been done on the stratigraphy of the Glass Mountains. In addition, there are various formations identified in the Glass Mountains.
The strata of the Glass Mountains contain a number of fossils, many of which are from the Permian. One part of the significance of the fossil finds in the Glass Mountains and other parts of the Delaware Basin is the use of their occurrence to classify strata, layers of rock marked by different times of deposition; in fact, the presence of fossils in various strata have been the primary means of classification, along with features of the rock itself. Fossils found in the Glass Mountains, found in marine sediment, are those of marine organisms. The selection of fossils includes algae, sponges, bryozoans, brachiopods, crinoids, conodonts, coral, ostracodes, gastropods, nautiloids, and other early forms of marine life. The Permian fossils of the Glass Mountains are shallow, warm-water marine life, like the kinds expected in a modern-day coral reef; the results are to be expected for what was once the Capitan Reef millions of years ago.
Some of these are suitable for stratigraphic correlation, the classification of strata from different areas into a chronological framework to determine which rock was laid down at similar times, because of various factors, like (1) sufficient numbers of fossils, both in an individual site and in other sites to be correlated, (2) sufficient rate of change in the population through time to permit analysis of evolutionary change through the strata, and (3) enough study of the populations to allow for a knowledgeable analysis.
Of special interest are fusulinids, a kind of foraminifer (a kind of shelled protist), and ammonoids, a kind of mollusk related to the modern-day chambered nautilus. The fossilized shells of these organisms have been used in early stratigraphic correlation. Many of the fossils of Permian life were discovered within the various sediments that compose the layered formations of the Glass Mountains. In fact, another possible origin of the name for the area, the Spanish "Sierra del Vidrio," is the presence of many fossilized shells with a glassy appearance. But there are more fossils in the layers below, and they have helped in the stratigraphical correlation of the strata in the Glass Mountains to those in other areas. The presence of different fossils in different strata make classification based on their location in the rock useful, just as their presence allows the correlation of strata from diverse locations.
In the Wolfcampian series appear characteristic genera of fusulinids and ammonoids, especially Pseudoschwagerina, which appears only within this series, and Properrinites, respectively. Other genera include Schwagerina and other fusulinids and other ammonoids, most of which first appear in the Wolfcampian series. There are other groups of animals, many of which have been named in the list given earlier, but there are also bivalves and a few trilobites.
In the Leonardian series appear the fusulinid Parafusulina, descended from Schwagerina, which was also one of those to appear in the Wolfcampian series. The series also contains other fossils, including those similar to those described earlier, and also contains a number of nautiloids.
The Guadalupian series includes two zones based on a sharp change of fusulinid genera, from Parafusulina to Polydiexodina. There are also some ammonoid zones and many sponges and algae. The Ochoa series has very few fossils, probably because conditions grew much less favorable for life during the last of the Permian, when many marine invertebrates, the kinds preserved in earlier strata, were going extinct.
After some collecting work a few years earlier, serious work on paleontological excavations in the Glass Mountains began in the early twentieth century with the work of J.A. Udden from 1904 onwards and P.B. King in the first descriptions of the stratigraphy of the mountains. P.B. King and R.E. King also added other works to the knowledge of the Glass Mountains, including sections of the stratigraphy, maps, and works on the fauna. Although there has been less work done on the Glass Mountains in the following years (compared to the work done on the nearby Guadalupe Mountains on the northwestern side of the Delaware Basin), there has still been work done in the area, mainly done by G.A. Cooper, R. Grant, and C.A. Ross. The fossils have been discovered primarily by excavation, as fossils have been preserved in the rock, and the surrounding rock can be removed from the fossils. There is continuing interest in the area.
Fossils in the Glass Mountains have been used for determining correlations among sites in North America and in other places. As mentioned, the Permian series of the Glass Mountains is the standard series for correlation with other strata in North America. The Wolfcampian series of the Glass Mountains was chosen as the lowest of four series for correlation comparisons with similar strata in North America, and the Leonardian series has been chosen as the series for correlation comparisons for the upper part of Lower Permian strata in North America. Fauna from the Leonardian formation has been compared to similar Fauna in the Ural Mountains in Russia.
And as mentioned, the fossils in these areas have been important criteria for correlations. Fossil correlations from one area to another can be used to evaluate sediments from similar dates. The Ural Mountains, Russia, also contain similar fossil sites (the Permian was named for the Perm area of the Urals). Other similar sites in the Permian are found in areas across the western United States, including a band from Kansas to Texas. There have also been similar discoveries in other parts of the Americas, including Central and South America and northern North America. The geological knowledge also has some applied use, as economic geology, which is concerned with natural resources like natural gas, oil, and minerals, is interested in geological dating.
In short, what is known from the Permian series in the Glass Mountains has been used for correlations with stratigraphy and fossils elsewhere, and the fossils found in the Glass Mountains have been the primary method of determining these correlations.