The Precambrian (~1 billion years old) quartzites, sandstones and shales of the Uinta Mountain Group appear to provide optimal nesting habitat for cliff swallows. Exposed in Green River canyon reaches such as Swallow Canyon, Lodore Canyon and Whirlpool Canyon, the flaming red quartzites and sandstones provide numerous ledges below which canyon swallows build their nests. These ledges are products of a common geological process know as “jointing,” where the tensile strength of a stressed rock is exceeded and the rock pulls apart creating fracture surfaces (joints) oriented perpendicular to the direction of movement. Joints are small-scale fractures unaccompanied by displacement.
Approximately 1 billion years ago, ancient shoreline environments deposited sandy sediments in what is today Northeastern Utah. After being subjected to moderate temperatures and pressures these sediments become the modern-day Uinta Mountain Group. Between 40 and 70 million years ago, tectonic activity along the western coast of North America deformed the rocks in northeastern Utah (including the Uinta Mountain Group), forming a convex geological structure known as an anticline. The creation of this structure helped form the Uinta Mountains. The combination of compressional stresses associated with this tectonic activity and pressure release associated with the incision of the Green River, caused fracturing within the quartzites and sandstones, forming what geologists refer to as “joints.”
Canyon swallows nesting along the Green River appear to strongly prefer exposed joint surfaces for their nesting habitat. These planar surfaces, often expressed as overhanging ledges, may protect the nests from wind and rain. To this amateur ecologist, the abundance of Cliff Swallow nests was greatest along the canyon reaches exhibiting river-level exposures of the Uinta Mountain Formation, suggesting that the swallows prefer the small-scale deformation structures associated with these ancient sedimentary deposits.
Figure 1. Cliff swallow nesting in Swallow Canyon