A photo of Ernst Arroyo.

Ernst Arroyo (creek) cuts through a hogback of Boquillas Formation, a layered chalky limestone and shale of Late Cretaceous age. The Boquillas Formation crops out in many areas of the Big Bend region. The formation's limey sediments were originally deposited in shallow marine waters of the Western Interior Seaway, a shallow seaway that stretched from the Gulf of Mexico to Alaska and across the Great Plains and Rocky Mountain region during the Cretaceous Period. The seaway withdrew from the Big Bend Region about 70 to 65 million years ago when the Laramide Orogeny (associated with the uplift of the Rocky Mountains) began to affect the region around Big Bend.