A maverick sandstone that calls a granite home

A Tava sandstone dike standing free of the surrounding rock, which eroded away here.

C. Siddoway

Igneous rocks are rebels. Sedimentary rocks follow straight-forward rules—they are deposited in horizontal layers, with the oldest sediments on the bottom. Igneous rocks can do what they want. Molten rock can eat away at other rocks below ground, opening up a cozy space to cool and solidify. It can also come flying—or oozing—out of a volcano, quickly crystallizing on the surface. Or it may squirt through crevices like fractures or boundaries between sedimentary layers, inserting itself as a sheet in any number of orientations. Where these walls of igneous rock cut across rock layers, they are called “dikes.”

Every now and then, when conditions are just right, sediments get to play this game, too. When they’re over-pressurized, water-soaked sands can sometimes get injected into fractures to form “clastic dikes”. Most often, these clastic dikes invade sediments or sedimentary rocks. Only very, very rarely, does sand get to turn the tables on those igneous hooligans, forming dikes of sandstone within igneous rocks.

In Colorado’s Front Range, near Colorado Springs, you can find that strange inversion. Along the Utes Pass Fault, the Tava sandstone forms dikes and similar formations within the billion-year-old Pikes Peak Granite, as well as some even older crystalline rocks to the south. Sheets of sandstone up to six meters thick cut through the rocks, which would confuse the heck out of any young geology students an instructor was mean enough to bring out there.