My blog posts typically focus on birds that inhabit the San Bernardino Mountains and the adjacent mountain ranges. Today I’m going to change it up and talk about the mountains instead. Keep in mind, my background is biology, not geology, so I don’t claim to be an expert on these mountains in southern California. But I think you’ll find what I’ll share in this blog to be pretty interesting.
First, why a blog about these mountains by somebody whose focus is primarily birds? Collectively called the Transverse Ranges, they have a fascinating history. Look at a map of California and notice that the Sierra Madre to the north, and the Peninsular Ranges to the south are all oriented north-south. But the Transverse Ranges are oriented east-west. Why is this? For the past 20 million years or so, what is now called the Transverse Ranges have been rotating in a clockwise direction, and they continue to do so even today!
Let’s dive a little deeper into the origin and formation of these “spinning” mountains. The Transverse Ranges stretch eastward from Point Conception at their western end to the San Jacinto Mountains in western Riverside County near Palm Springs. They consist of numerous mountains, including the Santa Ynez Mountains, the Santa Monica Mountains, the Topatopa Mountains, the Verdugo Mountains, the San Gabriel Mountains, the San Bernardino Mountains, and the San Jacinto Mountains.
These mountains started forming about 5 million year ago due to compression along the San Andreas Fault as the Pacific Tectonic Plate (the portion of the Earth’s crust that makes up the Pacific seafloor) moved northwest along the North American Tectonic Plate (the crust underlying the continent of North America).
But the full story of the Transverse Ranges actually started about 20 million year ago, long before the present-day mountains were formed. At that time, the Pacific Plate had just started coming in contact with the North American Plate. Due to a system of geologic faults just east of what is now San Diego, a “block” of the North American Plate broke off and began to pivot and rotate in a clockwise direction because of the force exerted upon it by the movement of the Pacific Plate.
Between 20 and 5 million years ago, this block was pushed about 100 – 150 miles north and slightly west (roughly to the current position of the Transverse Ranges) and rotated about 60 degrees at its western end. About five million years ago, the forces of compression by the Pacific Plate’s movement against the North American Plate caused folding and lifting of the Earth’s crust to create the mountains we see today.
During this time period, the rotational forces continued and the mountains kept rotating another 50 degrees or so on their western end, resulting in the east-west orientation we see today. Because the Pacific Plate continues to move in a northwestern direction, the mountains are still rotating at a rate of approximately 5.5 degrees every million years. So if I’m patient, in another 10 million years or so, my southern view will become a western view!
Due to the temperature regimes at various elevations, the underlying geology, and the stark differences in moisture and temperature on the southern and northern slopes, these mountains have a wide variety of habitats. These habitats are home to an incredible diversity of plants and animals, especially birds. In fact, according to the US Forest Service, nearly 300 species of birds have been recorded in the San Bernardino National Forest.
So the next time you visit these mountains, take them for a "spin” and be sure to bring along your binoculars.
Comments