[Illustration: A. _In Early Cambrian time an arm of the Pacific
Ocean occupied a deep trough in Idaho, Nevada, and part of Utah. The
land to the east was a broad gently rolling plain of Precambrian
rocks drained by sluggish westward-flowing streams. The site of the
Teton Range was part of this plain. Slow subsidence of the land
caused the sea to move eastward during Middle Cambrian time flooding
the Precambrian plain._]
[Illustration: B. _By Late Cambrian time the sea had drowned all of
Montana and most of Wyoming. The Flathead Sandstone and Gros Ventre
Formation were deposited as the sea advanced. The Gallatin Limestone
was being deposited when the shoreline was in about the position
shown in this drawing._]
[Illustration: C. _In Early Ordovician time uplift of the land
caused the sea to retreat back into the trough, exposing the
Cambrian deposits to erosion. Cambrian deposits were partly stripped
off of some areas. The Bighorn Dolomite was deposited during the
next advance of the sea in Middle and Late Ordovician time._]
[Illustration: Figure 36. _Conglomeratic basal bed of Flathead
Sandstone and underlying Precambrian granite gneiss; contact is
indicated by a dark horizontal line about 1 foot below hammer. This
contact is all that is left to mark a 2-billion year gap in the rock
record of earth history. The locality is on the crest of the Teton
Range 1 mile northwest of Lake Solitude._]
Once again the shoreline crept eastward, the seas cleared, and the
Gallatin Limestone was deposited. The Gallatin, like the Death Canyon
Limestone Member, was laid down for the most part in quiet, clear water,
probably at depths of 100 to 200 feet. However, a few beds of “edgewise”
conglomerate indicate the occurrence of sporadic storms. At this time,
the sea covered all of Idaho and Montana and most of Wyoming (fig. 35B)
and extended eastward across the Dakotas to connect with shallow seas
that covered the eastern United States. Soon after this maximum stage
was reached slow uplift caused the sea to retreat gradually westward.
The site of the Teton Range emerged above the waves, where, as far as is
now known, it may have been exposed to erosion for nearly 70 million
years (fig. 35C).
The above historical summary of geologic events in Cambrian time is
recorded in the Cambrian formations. This is an example of the
reconstructions, based on the sedimentary rock record, that have been
made of the Paleozoic systems in this area.