92 GUIDEBOOK OF THE WESTERN UNITED STATES 
through error to the mountains to the north, which had before been 
known as the Sierra Comanche. The Glass Mountains form the 
northern and northwestern rim of the Marathon Basin and have the 
form of a cuesta or sloping mesa which trends northeast. The 
southward-facing escarpments of these mountains rise 1,000 to 2,500 
feet above the general level of the Marathon Basin and in the western 
part present a high broken crest of bold cliffs of dolomite, which 
attain an elevation of 6,500 feet. 
The Glass Mountains are carved from Permian dolomites, lime- 
stones, and shales, tilted northwest. These beds are overlain uncon- 
formably by the Lower Cretaneous strata and lie unconformably on 
the Pennsylvanian strata. 
Six miles southwest of Marathon is old Fort Pefia Colorada, which 
was located near a gap in a novaculite ridge, where there is a spring 
and one of the few streams of running water in the region. This was 
a station on both the military road east and west and the Comanche 
trail that led southward from Fort Stockton to the Rio Grande. At 
one time it was garrisoned with soldiers to protect travelers from 
Indians and outlaws. 
8 The ect table, by P. B. King, shows the Permian formations of the 
Glass Mountain 
Thick- 
Formation ness Character 
(feet) 
B praca bh. 700 fos momen of limestone fragments, with some red beds and lime- 
és <a ppowrth: ward In part Losigcema probably of reef origin, bene heres 
apitan limestone. 2 part into thin-bedded limestone and westward in 
into sandy limestone. 
Sandstone and siliceous acess a Aca igi cus beds of limestone. Changes 
Word formation. 1, 200 | “eastward into cherty limes 
Leonard and Hess for- 2, 000 Sandstone and siliceous shale (Leonard), which change eastward 
mations. into thin-bedded limestone (Hess). Basal conglomerate. 
: Shale and limestone; rests on strata. of Pennsylvanian age, locally 
Wolfcamp formation. 700 | * with great unconformity. v Age 
These formations show complex ing foraminifers, s sponges, corals, cri- 
ch 
the strike of the mountains. In the | pods, od bei a great number 
eastern half of the range nearly all the | of brachiopods in which the Productus 
strata are limestone; in the ™m | group greatly predominate. The brach- 
. ‘ ypes. 
stone and shale. These changes in | such as Leptodus and Richthofenia. A 
shown in Figure 11. feature of the fossils in the Glass Moun- 
