460 GEORGE ROGERS MANSFIELD 
southwest, but northeast of the mountain the dips of the limestone 
are northeast. Thus the structure of the limestone is a syncli- 
norium, having the general form of an inverted fan fold. The 
intrusion at Sugarloaf Mountain is a thickened sill or incipient 
laccolith, arching with the strata in the northwestern extension of 
the mountain but eroded on the southwest limb beneath the summit 
and southeastward. The fold which forms the mountain is one of 
the subordinate folds of the synclinorium rather than a major 
structural feature supported by a relatively large intrusive body, 
as postulated by St. John. The geologic structure section AA’ 
illustrates the features described above. Its line forms an angle 
of about 30° with that of St. John’s section. 
An example of what is believed to be the stump of an upright 
eroded fan fold is found in the western part of the Crow Creek 
quadrangle, see Figure 9.- Snowdrift Mountain, part of one of the 
most persistent anticlines of the region, is flanked on either side 
by synclines which are in general inclined eastward. The Webster 
syncline on the east is markedly unsymmetrical, the west limb 
being steep and locally overturned eastward, but the east limb 
has a gentle westerly dip. The Georgetown syncline along the 
west side of Snowdrift Mountain is deeper and the limbs are steeper. 
The east limb is locally vertical or even overturned. Thus the 
intervening Snowdrift anticline is with little doubt an eroded fan 
fold. The structure sections along the lines SS’ and TT’ illustrate 
the features cited. At the line SS’ the axis of the fan fold is some- 
what inclined eastward and the Webster syncline is broken by a 
local thrust. Although the Snowdrift anticline does not every- 
where show a tendency toward fan folding it is closely folded 
throughout most of its length and here and there exhibits that 
tendency, as shown on the west flank of Pelican Ridge, see structure 
section GG’, Figure 10B. The Dry Valley anticline, west of the 
Snowdrift anticline, see section SS’ Figure 9B, locally has similar 
tendencies. Other instances which may not be figured here are 
illustrated in the forthcoming detailed report. 
The Meadow Creek graben.—-Perhaps the most striking effect 
of normal faulting in the region is the production of horst and 
graben structure in the northwestern part, see Figure 1c. The 
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