26 LAEAMIE BASIN, WYOMING. 
JURASSIC SYSTEM. 
SUNDANCE FORMATION. 
Occurrence and relations. — The Sundance formation is extensively 
developed in the northern half of the area to which this report 
relates, but it thins out and disappears somewhere west of McGill. 
It is exposed in the uplifts east and north of Medicine Bow, in the 
Freezeout Hills, and in the ridge of which Bootheel Butte is one of 
the summits. 
The Sundance formation in this region presents the features which 
are characteristic of it in the greater part of Wyoming and in the 
Black Hills uplift. The lower beds are predominantly sandy; the 
upper part consists largely of green shales with hard layers carrying 
large numbers of fossils. The most extensive exposure is in the 
anticline east of Medicine Bow, where the formation is 119 feet 
thick^and the strata are as follows: 
Section of Sundance formation in Como Ridge, 6 miles east of Medicine Bow, Wyo. 
Morrison shale. Feet. 
Limestone and shale 15 
Massive disintegrated sandstone, in part shaly 10 
Drab shale with a few lumps of brown limestone 30 
Buff shaly sandstone 5 
Yellow sandy shale with belemnites 42 
Buff shaly sandstone 5 
Yellow shale (lying on Chugwater red beds) 12 
119 
In the ridge 2 miles north of Medicine Bow the lower half of the 
formation is a massive buff sandstone with 8 feet of red shale about 
100 feet above its base and a 2-foot bed of red shale a short distance 
higher. 
Although there is a long time hiatus between the Sundance and 
Chugwater formations the unconformity between them is not marked 
by discordance of dip or conspicuous erosion, and in places it is 
difficult to draw the line between them. This is probably due to 
the derivation of the material in the upper formation from the one 
below. The upper limits of the Sundance are similarly ill defined. 
A section in the eastern side of the Freezeout Hills near Dyer's 
ranch, given by W. N. Logan, a is as follows: 
o Kansas Univ. Quart., vol. 9, 1900, pp. Ill, 113. 
