Historical Geology Understanding Our Planet\'s Past

known for its widespread terrestrial redbeds, named so because sediment

grains were cemented with red iron oxide. Their appearance around 1

billion years ago indicates that the atmosphere had substantial levels of

oxygen at this time.

The weathering of primary rocks produced solutions of calcium car-

bonate,magnesium carbonate, calcium sulfate, and sodium chloride, which

subsequently precipitated into limestone, dolomite, gypsum, and halite. Higher

Figure 41Sawatch
sandstone resting on
Precambrian granite, Ute
Pass, El Paso County,
Colorado.
(Photo by N. H. Darton,
courtesy USGS)

Historical Geology