Numerous sills (red arrows) intruding Cretaceous shale in Hesperus Mountain. Note the vertical columnar jointing in the sills. So many sills intruded that they expanded the volume of the shale in the mountain by 40%. Credit: Dr. David Gonzales
|View sill formation video. Credit: Tanya Atwater|
Sills form by magma squeezing between, and parallel to, the layers of the hostrock, rather than cross-cut existing layers of strata as dikes do.
Colorado has sills of many different sizes and ages intruded into an astonishing variety of surrounding rocks. One of the largest is Archuleta Mesa in southern Colorado near the New Mexico border.
Here are some spectacular examples – you may enlarge the images to view details by clicking on them and explore the area around the features in 3D using Google Earth:
|This 800-foot-high, west face of Mount Evans in the Mosquito Range near Mosquito Pass (on border between Park and Lake Counties) exposes a thick sill intruding Lower Paleozoic sedimentary rocks five miles east of Leadville. View the area in Google Earth.Click on image to enlarge.Viewer’s Note: This Mount Evans is not the samepeak as the Mount Evans in Clear Creek County that is part of the Front Range’s spectacular skyline,known for the highest one-way road in North America. Colorado has many duplicate place names due to the exploration by many different cultures and organizations over the past 150 years.|
|Red arrows point to sills at Jaque Peak. View the area in Google Earth.Click on image to enlarge.|
|This 1800-foot-high, west face of Mount Sherman exposes three Tertiary sills (red arrows) intruding Lower Paleozoic sedimentary rocks. View the area in Google Earth.Click on image to enlarge.|
|Close-up of lower, Tertiary sill in Mount Sherman. Note how the sill splits and a layer of sedimentary rock is trapped within the sill. Note the vertical, columnar jointing in the sill and horizontal layering in the Lower Paleozoic strata.Click on image to enlarge.|
|Tertiary, Pando Porphyry sill intruding parallel to Pennsylvanian age sands and shales of the Minturn formation. View area in Google Earth.Click on image to enlarge.|
|Closeup of the Pando Porphyry sill. Note that the columnar jointing in the sill is at right angles to the strata in the Minturn formation.Click on image to enlarge.|
|This view of Gothic Mountain shows a sill (red arrow) intruding Cretaceous shales below the flat floor of a laccolith. View in Google Earth.Click on image to enlarge.|
|Closeup of Gothic Mountain sill intruding Cretaceous shale strata. Note the columnar jointing in the Tertiary sill.Click on image to enlarge.|
|Tertiaryandesite sill ina Quarry southwest of Lyons. The sill intrudes the upper Paleozoic Fountain formation. Note the vertical columnar jointing in the sill. Primary use of andesite in this sill is for commercial aggregate.View area in Google Earth.Click on image to enlarge.|
|Four thin sills (brown) intruding black shales north of Segundo and west of Trinidad. More than one geologist has initially been fooled into thinking these sills are sandstone lenses. They look a lot like the sandstones in the area from a distance. View Segundo, Colorado area inGoogle Earth.Click on image to enlarge.|