22 IKON OKES OF IRON SPRINGS DISTRICT, UTAH. 
DEVELOPMENT OF TOPOGKAPIIY. 
The outlines of the andesite cores at their contacts with the sedi- 
ments are determined largely by their original forms. Their surface 
forms are in general those of mature topography. While nearly all 
the surfaces are smooth and rounded, differential erosion along ver- 
tical and concentric fissures has developed most irregular and fan- 
tastic forms. (See PL VIII, A, B.) 
Erosion of the sediments has developed monoclinal ridges with 
their steeper edges toward the andesite and dip slopes away from it. 
Especially conspicuous are the ridges of pink and red conglomerates, 
sandstones, and limestones of the Eocene, which may be seen for 
many miles about from points on the laccoliths. In general the relief 
and slopes of the sediments are low. 
The horizontal lava flows, overlying the sedimentary deposits and 
fringing the outer edge of the sedimentary area, form conspicuous 
foothills or mountains, only a little lower than the laccoliths them- 
selves. Erosion has worked down along joints and faults, developing 
a mesa type of topography, and in detail often presenting the same 
erosion forms as the laccolithic andesite (PL VIII, C). 
In a broad way the Carboniferous, Cretaceous, and Eocene sedi- 
ments occupy a valley between mountains formed by the andesite 
laccoliths and the flat-topped lava hills. 
Faults considerably modify the general relations thus outlined. 
Fault scarps are common, some of them ranging up to 1,000 feet in 
height, but as a rule erosion has partially or completely masked them. 
Erosion has frequently worked down along fault planes, with the 
result that many depressions mark faults. Instances of drainage 
along fault lines are seen in both branches of the Queatchupah and 
in several minor valleys to the southeast, in Stoddard Canyon, south- 
east and northeast of Joel Springs Canyon, around Antelope Range, 
Antelope Springs, and westward, in the Eightmile Hills, and west of 
the northern part of The Three Peaks. Instances of canyons follow- 
ing joints were seen abundantly in the western part of The Three 
Peaks, where there are a number of short parallel canyons following 
a nearly east-west slicing of the andesite. 
The lower slopes in general are grade slopes in the heavy mantle of 
debris resulting from rapid erosion and inadequate transportation. 
On the desert this material is spread out in broad, low fans. The 
grade slopes are notched by sharp ravines and canyons. 
The principal events in the development of the present topography 
have been: (1) Intrusion of laccoliths in early Miocene time, followed 
by faulting and erosion, exposing the laccoliths and surrounding belts 
of sediments and developing in the laccolith cores a mature type of 
topography; (2) extrusion of late Miocene lavas, followed by fault- 
