Geology & Minerals: Carson National Forest, Canjilon District

Map showing location of the Canjilon District.Echo AmphitheaterThe Canjilon Ranger District (outlined in white on the geologic map) is located in the Colorado Plateau geological province.  Within the Colorado Plateau, sedimentary rocks are piled in flat layers.  This layer-cake stratigraphy is exposed beautifully in canyons that cut the plateau.  In the geologic map below, the colors represent rock units of different ages.  From youngest to oldest:

  • Yellow - Ql - Quaternay landslide and colluvium; Qp - Quaternay piedmont alluvial deposits
  • Gray - Qb - Quaternary basalt and andesite flows and vent deposits
  • Blue-gray - Tps - Tertiary sedimentary rocks
  • Light Olive-green - Kmv - Cretaceous Mesaverde Group; Km - Cretaceous Mancos Shale; Kd - Cretaceous Dakota Sandstone
  • Bright blue - Jm - Jurassic Morrison Group; Jsr - Jurassic San Rafael Group including Entrada Sandstone and Todilto Formation
  • Cyan - Tc - Triassic Chinle Group
  • Purple - Pct - Permian Cutler Formation

Echo Amphitheater

formationsEcho Amphitheater is a natural amphitheater carved from Jurassic sandstone just north of Ghost Ranch.  The interpretative sign to the left points out the contacts of the Todilto Formation, the Entrada Formation, and the Chinle Formation in the cliffs that surround the amphitheater.

During the Late Triassic, the Echo Amphitheater area was probably a broad savanna, with gentle rivers flowing across wide grasslands, depositing hundreds of feet of silt and mud which would later become the Chinle Formation.

Then, during the Early Jurassic, a thick sheet of windblown sand covered the silt and mud of the Chinle Formation.  Today, the resulting Entrada Sandstone is 200 to 265 feet thick near Echo Amphitheater and contains large, sweeping cross-beds and eolian ripple marks.  Near the top of the Entrada Sandstone, the bedding is more horizontal.  This may be the result of the rise of shallow lakes or high groundwater levels.

By the late Jurassic, a large restricted lake had developed in the Chama basin region.  A very thin layer of limestone was deposited early in the lake's history, followed by about 30 feet of gypsum, deposited as the water evaporated and became supersaturated.  The limestone and gypsum make up the Todilto Formation, the gray layer that tops the cliffs on the north side of the amphitheater.

Echo Amphitheater

Photos by Benjamin Kemp