|Geologic History in our Backyard
Whether sculpted by bulldozers or the forces of plate tectonics and erosion, the surface of the Earth is always changing. Sometimes, as in the excavation for the Museum's new wing, we have the opportunity to see evidence of geologic events which is usually hidden under the ground beneath our feet.
Museum Curator of Paleontology Dr. Tom Deméré interprets what is now visible in the pit: "What we have is layer cake geology exposed in the excavation. A lower light colored sequence of soft fine-grained sandstones is overlaid by a reddish unit of weakly cemented coarse-grained sandstones and cobble conglomerates. These sedimentary rock layers record portions of our local geologic history from about 500 thousand to 3 million years ago. Some portions of this time interval are missing here from the geologic record due to uplift and erosion. Fossils found in the older rock unit clearly indicate marine conditions, with deposition of fine sand on an ancient continental shelf. Those fossils are now in our research collections."
The layers of sandstone, now visible behind the Museum, reveal that a very different environment was here in years past. Two to three million years ago, the location of Balboa Park was underwater. The Museum's location was well beyond the surf zone, as indicated by the fine sediments and species of fossil mollusks found here. (See A Tale of Two Excavations: Fossil Discoveries in the Museum Pit)
With the Pacific Plate sliding past the North American Plate, the west coast was uplifted and the shoreline receded. The top layer of sandstone, reddish, coarse-grained, and loaded with cobbles, reveals a history of sandy beaches and a coastal river flood plain.
Lindavista and San Diego Formations visible in the excavation for the new wing of the museum.
See larger drawings of the geologic formations of
In the top layer of coarse-grained sandstone, rounded reddish cobbles provide evidence of another, much older, part of our local geologic history -- one that extends back almost 50 million years. The cobbles are called rhyolite, and they are from Sonora, Mexico. How did they get here?
Drawing of the geologic formations by Brad Riney 1999