Schematic Cross-Secion of the Sandstone Bedrock at Chaco.
URBAN GENESIS AT CHACO: Case Study of the Origin of Civilizations. Chapters: 1 2 3 4 5 6
Schematic Cross-Secion of the Sandstone Bedrock at Chaco.
The walls of Chaco Canyon are made of the Cliff House sandstone, which consists of massive, unstratified layers separated by thin few layers of harder, stratified sandstone. The Cliff House formation dips gently into the earth toward the north, at about one degree in the vicinity of Chaco Canyon. The exposed edge of the layer is eroding back and retreating to the north, forming a nearly continuous line of south-facing cliffs, Chacra Mesa, which runs across the center of the Chaco Plateau. Occasionally a resistant block endures as the surrounding layer retreats to the north, leaving freestanding buttes or mesas just south of the line of cliffs.
Chaco Wash is a seasonal stream that flows westward down into the center of the basin. Chaco Canyon is formed where that stream intersects the receding front of the Cliff House layers. The canyon is about nine miles long and one mile wide; its floor is almost completely flat and level as shown in the schematic section. The northern and eastern rim of the canyon are the edge of the retreating Cliff House layer, and on the south and west the canyon is enclosed by two isolated mesas of that same bedrock formation. The cliffs that define the canyon are only one hundred feet high; but since they are formed from unstratified sandstone, they are usually vertical. As a result the canyon is almost completely visually enclosed. The distant horizon is only visible through South Gap and Fajada Gap, at the middle and eastern end of the canyon. The resulting form of Chaco Canyon is therefore a bounded, readily identifiable space.
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