88 GUIDEBOOK OF THE WESTERN UNITED STATES 
Marathon uplift, a great dome in the earth’s crust, by which all the 
strata have been uplifted 4,500 feet or more in an area about 100 miles 
in diameter. The east side of this dome is crossed between Longfellow 
and Tesnus, where the canyon walls expose successively lower parts 
of the Comanche series and finally the underlying rocks of Paleozoic 
age. This is caused by the westward rise of the strata toward the 
crest of the Marathon dome. 
Near Tesnus the strata of the Comanche series are cut off by 
erosion and present high, westward-facing escarpments which rise 
far above and afford fine views of the Marathon Basin lying below” 
them to the west. In places, also, they constitute more or less isolated 
outlying buttes and mesas, west of the main escarpment. The most 
prominent part of the escarpment near the railroad is House Mountain, 
which lies north of the tracks a few miles west of Tesnus. This 
broad cuesta (elevation 5,460 feet) has a steep face on the west and 
rises 1,500 feet above the plains at its base. ® 
deposited. These differences are due 
to differences in local conditions, and 
it is probable that the sediments in the 
Marathon region were deposited closer 
to an old shore line than those in the El 
Paso and Van Horn regions 
The Pennsylvanian rocks also give 
evidence of a shore line near the Mara- 
thon region in Paleozoic time. They 
thick and consist almost 
the highest formation, and the lower 
formations are characterized chiefly by 
the remains of land plants. It is 
probable that the sand and clay of the 
Pennsylvanian were washed down from 
high lands to the southeast and were 
deposited in a series of deltas along the 
shore line. e three older Pennsyl- 
vanian formations are regarded as of 
Pottsville age, and the fossil shells in 
the upper formation are like those of 
the well-known upper Pennsylvanian 
(post-Pottsville) strata of Kansas and 
central Texas. 
® The upper part of House Mountain 
consists of Comanche limestone dipping 
gently to the east, and the lower slopes 
consist of russet-brown sandstone and 
shale of the Tesnus formation eee 
syivanien), which. h dips about ° SE 
a great angular unconformity, which is 
clearly revealed on the escarpment as 
shown in Plate 13, A. Some of the 
relations here and farther west are 
shown in Figure 10. The mass of older ~~ 
rocks is beveled to a nearly perfect plain 
on which the Comanche strata lie. 
The Paleozoic rocks in the vicinity 
of House Mountain acquired their 
steep inclination as a result of folding ~ 
during the later part of the Paleozoit 
era, which involved the entire area of 
Paleozoic rocks in the Marathon Basin 
and probably far beyond. The folds 
have a northeastward trend. At the 
time of the folding there was faulting 
Some of the faults dip at low 
angles to the southeast and are planes 
along which blocks of the Paleozoic 
these Paleozoic rocks are like those in 
the Ouachita and Appalachian Moun- 
tains and were formed at about the 
same time. After Paleozoic time the 
folded rocks of the Marathon Basin 
and in later Mesozoic time the Lower 
Cretaceous sediments were deposited 
on their upturned saaes. The 
tion of the us codtemutis and 
the development of the Marathon up- 
