image: Arch-Rattlesnake-Canyon

The most striking arches in Colorado are in massive sandstones on the Colorado Plateau. In Rattlesnake Canyon (pictured above), west of Grand Junction, there are thirty-five arches carved in Mesozoic sandstones, including the one above. There are also arches in parts of central Colorado formed in limestone, granite, and sandstone.

image: cave-bacon

Colorado’s caves range from impressive limestone caverns to modest clay, shelter, and ice caves. Speleologists continue to catalogue Colorado’s more than 265 caves.There is evidence of early human habitation in some caves, and rare animal fossils in others.Two commercial caves, Glenwood Caverns near Glenwood Springs and Cave of the Winds near Manitou Springs, have been thrilling visitors since the 1880’s. Both are dissolved from carbonate rock that is hundreds of millions of years old. Pictured above, cave “bacon” in Glenwood Caverns.

image: Garfield-mesaChimney-Rock
image: Toadstool_at_Grand-Staircase_Escalante_badlandsimage: Pawnee-Buttesimage: teepees

Plateaus, Mesas, Buttes, Chimneys, Teepees, and Hoodoos
These landforms have one thing in common, they are all composed of nearly horizontal layers that are capped by a resistant rock layer, usually sandstone or volcanic rock, which protects more easily erodible strata beneath. These features form a continuum from plateaus (measured in thousands of square miles) down through hoodoos (measured in thousands of square inches). Pictured from top left to bottom right: Mount Garfield,Chimney Rock, Hoodoos near Monument, Pawnee Buttes, and the conical hills called the Teepees east of Pueblo.

image: Meanders in the East River near Gothic.

River Valleys
River valleys in Colorado vary widely in their depth and shape from the narrow, sheer walls of the Black Canon of the Gunnison to the broad valley of the Colorado River in the Grand Valley near Grand Junction. Pictured above: Meanders in the East River near Gothic.

image: Dakota hogback

Hogbacks and Flatirons
Whereas mesas form resistant, flat-lying strata, hogbacks form where sedimentary strata are turned up on end by a large basement uplift. The actual hogback ridge forms where resistant strata are surrounded by erodible rock that leaves the resistant layer relatively isolated as a hogback.Flatirons are formed by the same process but most commonly rest against the mountain front. Pictured above: Dakota hogback South of Morrison.

image: Fan-along-Eagle River

Quaternary Alluvial Fans
Alluvial fans form where debris-laden mountain streams emerge from steep, narrow canyons onto wide valley floors. Indeed, in engineering geology they are commonly referred to as debris fans in order to emphasize the debris and mudflow events that can dominate fan deposition and that are potentially very lethal and damaging. Pictured above:Alluvial fan on the Eagle River.

image: CU-Glenwood-Terraces

Quaternary Terraces and Pediments
A terrace is a broad, nearly flat landform bounded by steep slopes on either side. Terraces can generally be linked to a specific canyon mouth at the mountain front. Quaternary Colorado terraces are associated with glacial cycles and are capped by surficial deposits of coarse sands and gravels. Pediments are surfaces that have been planed off by lateral erosion and then mantled with a veneer of alluvium. Pictured above: Terraces near Glenwood Springs.

image: cinders in Dotsero crater

Quaternary Volcanoes
The oldest known Quaternary volcanism in the state occurred about 1.5 million years ago in the Roaring Fork River Valley, ten miles north-northwest of Aspen. Magma rose along the Crystal River fault and broke out on the side of the valley producing a cinder cone and small flows. More recent eruptions have been recorded near the junction of the Colorado and Eagle rivers where volcanic flows overlie quite modern topography. The most recent volcanism known in Colorado appears near the town of Dotsero, radiometrically dated as 4,150 years old. The layers to the right are composed of basaltic cinders that built the cone near Dotsero. Pictured above:Layers of cinders at the Dotsero volcano.

image: Quaternary-fau

Quaternary Faulting
Faults have broken Quaternary deposits in many basins in Colorado. According to a recent inventory by the Colorado Geological Survey, there are at least ninety faults in Colorado that moved during the Quaternary Period. Pictured above: map showing distribution of Quaternary faults.