646 E. B 'lack welder — Reef of Calcareous Algae 



Art. XLYI. — A Fully Exposed Reef of Calcareous Algce (?) 

 in the Middle Cambrian of the Teton Mountains /* by 

 Eliot Blackwelder. 



Most field geologists have seen in Paleozoic limestones the 

 large hemispherical masses of concentric laminae to which the 

 name Cryptozoon has generally been applied. Usually, how- 

 ever, the structures are exposed only in plane section in the 

 sides of quarries or on flat glaciated surfaces. It seems to be 

 rare to find them so preserved and exhumed as to stand out 

 much as they may be supposed to have appeared when origi- 

 nally formed. This fact is justification for describing an instance 

 in which those conditions are realized. 



On the west slope of the Teton Range in western Wyoming 

 an interstream ridgef has been carved from a sequence of 

 Paleozoic formations. The Cambrian there consists of alter- 

 nating hard limestones and more or less calcareous gray and 

 greenish shales with sandy strata near the bottom. Many of 

 the beds are sparingly fossiliferous, — the faunas indicating 

 Middle and Upper Cambrian horizons. About 400 feet below 

 the base of the Bighorn dolomite, and about 40 feet above the 

 massive bed of limestone which is the chief cliff-making mem- 

 ber of the unnamed formation, between the Flathead quartzite 

 and the Gallatin limestone, masses of limestone having the 

 form of an old-fashioned beehive lie imbedded in a matrix of 

 calcareous green shale, although resting upon massive lime- 

 stone. At the point where the outcrop of the bed crosses the 

 sharp crest of the ridge, which at that place is slightly above 

 timber-line, the shale has been largely removed from the lime- 

 stone bodies. Much of it has been washed away by water 

 from the melting snows in early summer ; and, in addition, 

 the exposed position of the outcrop has given the wind an 

 opportunity to blow away all loose small particles. Although 

 the bed is but 7 feet thick, the gentle dip gives it a relatively 

 broad outcrop and hence the denuding agencies have stripped 

 the shaly matrix from an area of an acre or more. The accom- 

 panying photograph (fig. 1) shows a portion of the ancient 

 reef (?) thus exposed. Farther back on the right, one may see 

 the clean dip slope of the Middle Cambrian limestone referred 

 to above. As shown by the photograph, the limestone masses 

 are of various sizes, averaging 2 to 4 feet in diameter. They 

 are nearly hemispherical in form and all are characterized by 

 numerous tumid projections (see fig. 2). The bodies stand iso- 



* Published by permission of the Director of the U. S. Geological Survey. 



f The exact point is 1*9 miles N. 64° W. of bench mark 10,685 on the 

 Grand Teton Quadrangle of the U. S. Geological Survey. It is on the divide 

 between the two forks of Leigh Creek, which are, however, incorrectly 

 named Leigh and Badger Creeks on the maps mentioned. 



