The Entrada Formation



At its appropriate position in the stratigraphic column the Entrada sandstone is a persistent unit in the Upper Jurassic of southwestern Utah. In Southwest Utah it is recognizable in the topography as a red, white-blotched slope between gray cliff-making rocks below and white massive beds above. The boundaries of the formation are plainly marked by strong contrast in style of sedimentation ; the weakly cemented Entrada sandstone lies immediately on the compact limestone of the Carmel formation and immediately below thick resistant gypsum beds of the Curtis formation. The strata in the Entrada sandstone are apparently accordant in attitude with those in the Carmel formation, but generally are separated from those in the Curtis by a plane of unconformity, thought to indicate but a short lapse of time.

The Entrada sandstone consists chiefly of fine-grained sandstone in regular beds less than a foot thick, generally dark red, inconspicuously banded with white. It includes thin sheets and small aggregates of gypsum, many lenticular beds of gypsiferous shale, some calcareous shales, and small amounts of conglomerate made up of red and green pellets of clay and subangular fragments of quartz. Among the grains visible under the microscope are bits of selenite, halite, biotite, feldspar, chert, magnetite, and quartzite. The cementing material is gypsum and, in much smaller degree, lime. From its top to its bottom the sandstone seems uniform; variation in bedding, texture, and composition are within narrow limits. The formation is remarkably friable; it disintegrates so readily that, except on freshly scoured canyon walls and in recent road cuts, its features are largely masked.

As a topographic feature, the Entrada sandstone of Southwest Utah closely resembles that in the neighboring regions, but east of Paria River Valley it is characteristically a single stratum or group of thick strata of massive cross-bedded sandstone 100 to 500 feet thick a huge mass superficially much like the Navajo sandstone. Westward of Paria Valley the thick, quartzose, lime-cemented, cliif-making sandstones are gradually replaced by thin gypsiferous beds such as those exposed in Shurtz Creek, Coal Creek, and Stephens Canyons.

-from Geology of Eastern Iron County, Utah by Herbert E. Gregory


The Entrada sandstone is the most conspicuous formation of the San Rafael group. West of the Colorado River, where its thickness is measured in hundreds of feet, it resembles the Navajo sandstone in massiveness, structure, and boldness of sculpture. In the San Juan country the formation maintains the characteristic composition and erosion features, though diminished in thickness to less than 100 feet. Exposures in Dry Canyon, in the walls of Cottonwood Wash, and southward along Butler Wash show a conspicuous band of brick-red or orange-red rock that contrasts strongly with the dark-red and green-gray Morrison above and the tan-colored Navajo. Essentially the Entrada is a fine, even-grained sandstone with subordinate amounts of bright-red shale. In some places it is a single massive, strongly cross-bedded ledge; in other places a series of thick-bedded sandstones or sandstones interbedded with shales, within which. are local unconformities. The rock consists chiefly of round. and angular grains of quartz, a little feldspar, and a few flakes of biotite. The cement is commonly calcite and ferric iron, in proportions that determine the color tone. Because the cement is weak and the grains small and nearly uniform in size, the Entrada where massive weathers into low domes and round-edged ledges. Where lenses of relatively resistant materials make up part of the bed, erosion produces forms like spools, hourglasses, and "rock babies." The "Beehives" along Dry Canyon, the "Goblet of Venus" on the Blanding-Kigalia road, and the "hoodoos" at: the head of Buck Canyon, west of Bluff, are representative features. In most sections the beds at the base of the Entrada grade downward through a series of red sandstones to the Carmel beneath. In other sections an erosional unconformity separates the two formations and doubtless accounts for the difference in thickness of the Carmel at -places not far apart. In Dry Wash only discontinuous patches of the Carmel remain, and in places the Entrada rests directly on the eroded surface of the Navajo.

-from The San Juan Country - A Geographic and Reconnaissance of Southeastern Utah by Herbert E. Gregory