The Cambrian Sawatch Sandstone lies almost horizontally on Precambrian [granite] in Glenwood Canyon. In the foreground is the Colorado River. (Jack Rathbone photo)
Cambrian Period
(500-570 million years ago)
The first fossiliferous rocks in Colorado were deposited during the Cambrian Period, at a time when over much of the world the seas were creeping in across wide, level plains formed during the Lipalian Interval. Colorado was not covered by these seas until quite late in the Cambrian Period. Beach deposits progressively younger in age suggest that the sea invaded from the west, and spread slowly eastward, inundating most of the central part of the state but not the extreme north or south.
The beach deposits, now called the Sawatch Sandstone because they are well exposed in the Sawatch Range, are composed mostly of fine quartz sand. They are colored with glauconite, a green mineral, and [hematite], a dark red mineral, so that the rock has a variegated appearance. The post office at Manitou is built of this red and green rock, and good exposures of it exist in Williams Canyon near Manitou, along U. S. Highway 24 northwest of Manitou, near Red Cliff and Minturn, and in Glenwood Canyon.
The sea which crept over Colorado at this time contained small conical-shelled [mollusks], [brachiopods], and [trilobites]. Their shells can occasionally be found in Cambrian rocks in Williams Canyon and in the Sawatch and Mosquito Ranges. At two localities unusual [fossils] called [graptolites] have been found in thin Upper Cambrian shales overlying the Sawatch Sandstone.
These [fossils] can occasionally be found in Cambrian rocks in central Colorado.
Ordovician Period
(440-500 million years ago)
The sea deepened and widened as the Ordovician Period began, and a series of limestones and dolomites was deposited, either on top of the Sawatch Sandstone or, where the Sawatch had not been deposited, directly on the Precambrian. These rocks are now called the Manitou Formation.
The [fossils] in these rocks are much more varied than those in the Sawatch Sandstone: snails, [echinoderms], sponges, [cephalopods], [brachiopods], and [trilobites] are common. The Ordovician sea must have teemed with life, as many rocks deposited at this time are more than half composed of animal remains. In addition to hard-shelled animals which formed fossils, there were probably abundant soft-bodied animals such as jellyfish and worms, which left no record of their presence.