You may have noticed two iconic flat-topped mountains when looking east from the junction of Hwy 41 and Hwy 145 (“Four Corners”) north of Fresno. These beautiful landmarks known as “Little Table Mountain” and “Table Mountain” have very different geologic stories and are commonly confused, yet they paint a fascinating picture of our region.
Table Mountain consists of many flat-topped mountains that were once connected, including Big Table Mountain and Kennedy Table Mountain. From the Four Corners, if you were to follow Millerton Road east to the town of Friant and continue up past Table Mountain Casino and onto Auberry Road you would initially pass Little Table Mountain on the north side of the San Joaquin river and eventually come to Sierra Foothill Conservancy’s (SFC’s) largest nature preserve, the 2000-acre Ruth McKenzie Table Mountain Preserve. Big Table Mountain makes up the centerpiece of the Preserve which has been owned and managed by SFC since 1998. The Preserve’s name comes from the property’s former owner, Ruth Bea McKenzie, who wanted it to remain in ranching and open space after her passing.
Why are the table mountains flat on top? Over millions of years, a fascinating geologic process known as “inverted topography” took place, where what was once the bottom of a river channel now forms the top of the mesas. Aerial views of the tables show a meandering stream pattern that was formed when an unknown volcano erupted, causing lava to flow down the ancestral San Joaquin River channel. Once the lava cooled, it remained in the channel and solidified. Initially it was all connected, but over time rain eroded the less resistant material on each side of the lava-filled riverbed slowly exposing the table mountains.
Interestingly, the lava flow never made it all the way to Little Table Mountain which is located at the bottom of the ancestral river channel. It is made up of old cemented river gravels of the ancestral San Joaquin River making its appearance slightly different from the larger tables nearby. The gravel conglomerate was deposited by rivers on a gentle slope draining the Ancestral Sierra Range about 40-50 million years ago. Over time, less resistant material surrounding these old deposits also eroded away.
The weathering of Big Table Mountain’s lava rock over the last ten million years formed special soil that is very important to native biotic communities. In spring, rainwater collects in the table’s low spots, forming vernal pools. Since the rock is impermeable, these pools hold water for several weeks or months until it eventually evaporates. The pools provide habitat for rare crustaceans that “come to life” in the presence of water. As the pools dry up, they are filled with rare plants including breathtaking wildflowers known as “meadow foam” – a must-see for wildflower enthusiasts.
SFC’s McKenzie Table Mountain Preserve is special in that it represents an exceptional collection of critical habitat, working rangeland, scenic open space, and a multitude of valuable cultural and historical resources. Because of Sierra Foothill Conservancy’s efforts this special place will be preserved permanently for future generations to enjoy, and so that the habitat and wildlife who depend on it can continue to thrive. SFC offers annual hikes and nature classes on the Preserve so the public can enjoy this natural wonder. Stay tuned for upcoming events as we celebrate our 25th Anniversary and as safety regulations allow. For more information about the McKenzie Preserve and SFC’s protected lands, please visit www.sierrafoothill.org.