Cenizo Journal Spring 2021 | Page 13

LOS CABALLOS ( Highway 385 , two miles south of the Marathon Border Patrol Station , 1970 )
“ Highly deformed rocks in the Ouachita fold belt , a northeasterly trending range , uplifted about 275 to 290 million years ago . The intricate folding is shown by whitish rock bands — called caballos ( the Spanish word for horses )— exposed on both sides of this highway . The Ouachita fold is comparable in age to the uplift that formed the Appalachians in the eastern part of the United States .
“ The northwesterly trending Del Norte-Santiago range ( southmost extension of the Rocky Mountains ) forms the southwestern skyline . The rocks of this range were deposited in a sea that covered the Ouachita fold belt after erosion had reduced the highlands and a later submergence lowered the area . Santiago Peak ( named for a local man who was killed by Indians and buried beneath the peak ) is the high , flat-topped mountain to the southwest . It was once a mass of molten magma that cooled and hardened underneath the earth ’ s surface and was uncovered by later erosion . The Del Norte-Santiago range , uplifted and folded 40 to 60 million years ago , is not half the age of the Ouachita fold . This is a remarkable fusion of “ old ” and “ young ” mountains — and is unmatched at any other site in North America .”
To truly understand how bizarre the caballo novaculite of Brewster county is , one must consider the vast difference in age
between the mountains . When the Ouachita fold was formed , it preserved fossils from the Devonian period , an era that spanned 419 million years ago to 358 million years ago . These fossils would be fishes ( the Devonian period is known as the age of fish ), plants and terrestrial arthropods ( basically ancient bugs ). The Devonian period is when life began to move from the oceans to the land , diversifying rapidly and changing the composition of Earth ’ s atmosphere .
To give a clearer understanding of the difference in time between the Ouachita formations and the surrounding igneous formations , the Ouachita fold occurred about 30 million years before dinosaurs existed , and the Del Norte-Santiago range came into existence after dinosaurs became extinct . To have geologic formations with such a huge age disparity be visible side-by-side is a geologist ’ s delight .
In this one tiny area , the young and vigorous Rocky Mountains terminate , shaking hands with the very tail-end of the ancient Ouachita fold , which stretches through Oklahoma and Arkansas before meeting the Appalachian Mountains in the northeast of the continent . Two ranges representing vastly different eras and processes come to rest in West Texas , poking their heads out of the ground to be studied and enjoyed .
Look for more Historical Markers in upcoming issues of the Cenizo .
Cenizo Spring 2021

13