The folds in Colorado’s rocks, are intriguing to geologists and lay viewers alike. For geologists who have studied Colorado rocks, the variety is surprising. The state has metamorphic folds, basement-cored folds, salt-cored folds, monoclines, syndepositional folds, anticlines, synclines, domes, basins, refolded folds, evaporite-flowage folds, collapse folds, disharmonic folds, and forced folds.
Anticline: foldswith limbs that dip outward away from the hinge of the bend; convex upward
Syncline: folds with limbs that dip inward toward the hinge of the bend; concave upward
Monoclines (right) are folds that have only one limb with horizontal beds on either side of the steep limb. Monoclines range in size from rather small, north of Fort Collins, to huge monoclines associated with the Uncompahgre Plateau and White River Plateau basement blocks. Photo at right shows a well exposed monocline on the northwest end of the Uncompahgre Plateau.
Basement-Cored Folds were formed during the Laramide mountain building event, when igneous and metamorphic rocks broke into large blocks throughout Colorado. These broken blocks pushed up into the overlying sedimentary rocks, forcing them to fold over the edges of the Precambrian blocks. The shape of the block determined the shape of the folds in the overlying sedimentary strata. Where the blocks moved up without any rotation, the overlying sedimentary rocks formed monoclines. Photo above-left shows an anticline four miles east of Lyons on Rabbit Mountain. The fold reflects the geometry of the basement block that moved up into the overlying sedimentary strata and created a fold in those strata.
Salt anticlines result when salt deposits flow upwards, folding the overlying sedimentary rocks into anticlines. Long, linear anticlines with cores of salt are found in the Eagle and Carbondale areas of central Colorado. Salt flowed upward into the cores of these anticlines for millions of years, creating angular unconformities in the sediments being deposited on the flanks of the growing folds. When the salt reached the surface and eroded, it dissolved much quicker than the overlying rocks and formed large, linear valleys over the crest of the anticlines. This rapid dissolution of the salt cores caused the rocks near them to collapse, creating the inward-dipping layers that define a syncline. Photo to right is a diagram of a the Paradox Valley salt anticline. The far end of the valley is the collapsed syncline.
When previously folded rocks were again subjected to heat and pressure, they were refolded and became refolded folds. Early geologists studying Precambrian structures found many clues indicating two periods of folding. Small, tight folds that were folded once during an earlier period of folding, then folded again and tightened bya second period of folding.
To the left, anticline and synclines in metamorphic rocks near Blackhawk. Within these large folds are many small, tight folds that were formed during an earlier period of folding and were refolded by the second period, shown in the photo to the right.