[Illustration: A. _Algal heads in the Park Shale Member of the Gros
Ventre Formation. These calcareous mounds were built by algae
growing in a shallow sea in Cambrian time. They are now exposed on
the divide between North and South Leigh Creeks, nearly 2 miles
above sea level!_]
[Illustration: B. _Bed of “edgewise” conglomerate in the Gallatin
Limestone. Angular plates of solidified lime-ooze were torn from the
sea bottom by storm waves, swept into depressions, and then buried
in lime mud. These fragments, seen in cross section, make the
strange design on the rock. Thin limestone beds below are
undisturbed. National Park Service photo by W. E. Dilley._]
The Bighorn Dolomite of Ordovician age forms ragged hard massive
light-gray to white cliffs 100 to 200 feet high (figs. 32 and 33).
_Dolomite_ is a calcium-magnesium carbonate, but the original sediment
probably was a calcium carbonate mud that was altered by magnesium-rich
sea water shortly after deposition. Corals and other marine animals were
abundant in the clear warm seas at this time.
Dolomite in the Darby Formation of Devonian age differs greatly from the
Bighorn Dolomite; that in the Darby is dark-brown to almost black, has
an oily smell, and contains layers of black, pink, and yellow mudstone
and thin sandstone. The sea bottom during deposition of these rocks was
foul and frequently the water was turbid. Abundant fossil fragments
indicate fishes were common for the first time. Exposures of the Darby
Formation are recognizable by their distinctive dull-yellow thin-layered
slopes between the prominent gray massive cliffs of formations below and
above.
The Madison Limestone of Mississippian age is 1,000 feet thick and is
exposed in spectacular vertical cliffs along canyons in the north, west,
and south parts of the Tetons. It is noted for the abundant remains of
beautifully preserved marine organisms (fig. 39). The fossils and the
relatively pure blue-gray limestone in which they are embedded indicate
deposition in warm tranquil seas. The beautiful Ice Cave on the west
side of the Tetons and all other major caves in the region were
dissolved out of this rock by underground water.
The Pennsylvanian System is represented by the Amsden Formation and the
Tensleep Sandstone. Cliffs of the Tensleep Sandstone can be seen along
the Gros Ventre River at the east edge of the park. The Amsden, below
the Tensleep, consists of red and green shale, sandstone, and thin
limestone. The shale is especially weak and slippery when exposed to
weathering and saturated with water. These are the strata that make up
the glide plane of the Lower Gros Ventre Slide (fig. 5) east of the
park.
The Phosphoria Formation and its equivalents of Permian age are unlike
any other Paleozoic rocks because of their extraordinary content of
uncommon elements. The formation consists of sandy dolomite, widespread
black phosphate beds and black shale that is unusually rich not only in
phosphorus, but also in vanadium, uranium, chromium, zinc, selenium,
molybdenum, cobalt, and silver. The formation is mined extensively in
nearby parts of Idaho and in Wyoming for phosphatic fertilizer, for the
chemical element phosphorus, and for some of the metals that can be
derived from the rocks as byproducts. These elements and compounds are
not everywhere concentrated enough to be of economic interest, but their
dollar value is, in a regional sense, comparable to that of some of the
world’s greatest mineral deposits.
Figure 39. _A glimpse of the sea floor during deposition of the
Madison Limestone 330 million years ago, showing the remains of
brachiopods, corals, and other forms of life that inhabited the
shallow warm water._
[Illustration: A. _Slab in which fossils are somewhat broken and
scattered. Scale slightly reduced. National Park Service photo by W.
E. Dilley and R. A. Mebane._]
[Illustration: B. _Slab in which fossils are remarkably complete.
Silver dollar gives scale. Specimen is in University of Wyoming
Geological Museum._]