many rounded pebbles of quartz and feldspar. Some beds of green shale at top. (570) PRECAMBRIAN Granite, gneiss, and pegmatite. [Illustration: Figure 34. _Absolute ages of the formations in Alaska Basin. Shaded parts of the scale show intervals for which there is no record._] STRATIGRAPHIC SCALE ABSOLUTE ENLARGED PIECE OF TIME (Years YARDSTICK SHOWN ON ago) FIGURE 19 2 PALEZOIC PENNSYLVANIAN ? 300 million MISSISSIPPIAN MADISON DEVONIAN DARBY 3 400 million SILURIAN ORDOVICIAN BIGHORN 500 million 4 CAMBRIAN GALLATIN GROS VENTRE FLATHEAD 600 million PRECAMBRIAN 5 Figure 35. _The first invasions of the Paleozoic sea._ [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._]