Geology

Utah’s highway 18, between the Chuckwalla Trail Head and Enterprise, is a traverse through 170 million years of world-class geology.  The highway crosses the transition from classic Colorado Plateau Geology (think Zion and Grand Canyon geology) to the incredible Basin and Range province of Utah, Nevada, and California (Think Death Valley to the Great Salt Lake Basin).  The transition between the two zones is geologically rich and geologists from around the world are drawn here just to see the many examples of world-class geology.  Highlights include the entire section of the Jurassic Navajo Sandstone with it’s perfectly preserved colossal sand dunes, the Pine Valley laccolith which is the world's largest and best example of this style of igneous intrusion (and associated gigantic gravity slide block features caused by very shallow emplacement), and the interplay of more recent volcanism and topography resulting in some of the very best examples of inverted topography anywhere.  The Jurassic Navajo Sandstone, which is the most prominent geologic feature between the Chuckwalla Trail head and Diamond Valley, are ancient, gigantic sand dunes, from a Sahara sized desert, with structures so well preserved that geologists can very accurately measure the wind patterns from the Jurassic Period.  The Lower Miocene (20.5 million years before present) Pine Valley laccolith is associated with a truly gargantuan regional explosive volcanic episode that blanketed the area with very-large, very-hot ignimbrite flows.  These deposits are well-preserved from Vejo north to Enterprise along the Highway 18 corridor.  The more recent Pleistocene volcanic episode (up to about 600,000 years before present) displayed along HWY 18 from Chuckwalla to Central resulted in dozens of prominent cinder cones, lava flows, and classic inverted topography (for which the St. George area is world-renowned). Great examples of this geologically very-recent event are the Diamond Valley and Vejo cinder cones, which both caused highway builders to make large curves around the cinder cones and the closely associated, and very prominent, Pleistocene (Ice Age) basalt flows which cover the landscape between the Pine Valley Mountains and Dammeron Valley. The area around Veyo preserves the Gunlock-Reef Reservoir-Grand Wash Fault Zone which is regarded as the start of the Basin and Range Province and has been actively and continuously studied for 50 years by geologists.  The Enterprise end of HWY 18 is squarely in the Basin and Range Province with its classic mountain ranges bounding large and relatively flat desert valleys.  Quite literally hundreds of geologic maps, papers, dissertations and reports have been published about the rocks which can be found along Utah State Highway 18.  The very best of these maps and summaries are produced by the Utah Geological Survey, tying together the decades of work done here by geologists from around the world.