While the unique geographic location of the Sky Islands is well recognized as a primary factor for the elevated biodiversity of the region, its unique tectonic history is often overlooked. The mixing of tectonic environments is an important supplement to the mixing of flora and faunal regimes in contributing to the biodiversity of the Madrean Archipelago. The Sky Islands region is located near the actively deforming plate margin of the Western United States that has seen active and diverse tectonics spanning more than 300 million years, many aspects of which are preserved in the present-day geology. This tectonic history has played a fundamental role in the development and nature of the topography, bedrock geology, and soil distribution through the region that in turn are important factors for understanding the biodiversity. Consideration of the geologic and tectonic history of the Sky Islands also provides important insights into the “deep time” factors contributing to present-day biodiversity that fall outside the normal realm of human perception.