From Prehistoric times, the area around here was wetlands. It was called “Bolsa” by the native Americans, “Laguna” by the Spanish settlers and “Swamp” by the Americans. Clearly much has changed over the years.
Until the 1880s, wetlands (known as Tulare Lake) covered much of modern-day Pleasanton. This area was drained to allow for farming and eventually the suburban environment that you now see.
The hydrogeology of this area encompasses the Tassajara Formation with Quaternary fill material overlying it. Contacts of these two formations are often indistinguishable due to similarities of physical characteristics. The prism of sediments identified as valley fill materials contains from eight to ten separate zones of sand and gravel separated by zones of silt and clay.
This area was (and still is) drained by the “Arroyo de Laguna” which in turn feeds Alameda Creek out to San Francisco Bay. Now that more and more of the open spaces have been developed, the additional drainage has caused severe erosion to the Arroyo. Efforts have begun to stabilize the stream and to reverse much of the damage that has occurred over the years.
To log this cache, please stand on the bridge and answer the following questions: 1. GC Number and Name of the Cache 2. Names of the cachers 3. Look at the nearby bank and count the number of sedimentary layers that you can identify. 4. How many of these layers do you believe are from the Tassajara Formation and how many from the Quaternary fill? 5. How does development cause damage to the watershed and arroyos? How can we change this? 6. Not required, but a picture of your party at the posted coordinates is greatly appreciated.