Continuing my journey around the Marathon Basin, this photograph, taken one mile south of the Highway 90/385 junction, is a good illustration of the basin south of Hwy 90. Many of the hills there are capped by a chert bed known as the Caballos Novaculite, Caballos from Horse Mountain in the basin where the chert is particularly thick and prominent. The term “novaculite” comes from Arkansas where this rock also crops out. There it is used for whetstones. The novalculite is very hard, microcrystalline, chemically inert, and brittle. It doesn’t erode chemically, only mechanically, and so caps hills. Note the scallopped outcrops in mid-picture, called “flatirons” by geologists. Flatirons are are found on the flanks of several hills in the basin.
The mesa on the right horizon is Elephant Mountain, prominent to the west of the basin. The mountain is capped by an enormous nepheline syenite sill, four miles long, two miles wide and 1,200 feet thick, weighing about 3 billion tons. The mountain was named for its shape, which resembles an elephant’s back when viewed from some angles. For more see River Road Vistas.