Cache Creek has a very unique geologic history. Flowing from Clear Lake to Woodland, through Cache Creek Canyon and the Capay Valley, it runs through many different rock strata. As people travel along Cache Creek, individuals can find many different rocks and rock types. Along its’ 87 mile length, individuals can find igneous rocks, sedimentary rocks of both continental and marine origin, as well as metamorphic rocks.
At the head of Cache Creek, individuals will find many igneous or volcanic rocks. These rocks are from the eruption of Mount Konocti, on the west shore of Lower Clear Lake, 10,000 years ago. When it erupted, it left a lava flow around Clearlake and the surrounding area. Igneous rocks are characterized by their crystalline appearance. Examples of rocks that can be found on this part of the Creek include basalt and rhyolite.
As Cache Creek flows downhill, the Creek transitions through different layers of marine and continental sedimentary rocks. These layers were created through the formation of the Coast Ranges as the Pacific Plate subducted under the North American Plate. Sedimentary rocks can be identified by their grainy appearance. In sedimentary rocks, these ‘grains’ are often glued together by a surrounding cement.
On this part of the Creek, people will start to find metamorphic rocks intermixed with other sedimentary rocks. Metamorphic rocks, rocks that form under extreme heat and/or pressure, often show the appearance of having been compressed or twisted.
Further downstream, Cache Creek runs through different layers of sedimentary rocks. These layers were created through the formation of the Central Valley as the Coast Ranges and Sierra Nevada eroded to lower elevations. Examples of rocks that can be found on this part of the Creek include sandstone, shale, and gravel deposits.
Rocks from each of these geologic strata can be found anywhere along the Creek. Rocks found closer to Clear Lake can easily erode and travel downstream to the Creek’s mouth. Because of this, it’s easy to get a geologic picture of the Creek’s history while individuals stand at the Creek’s edge. While individuals are at the Creek, here are some questions to ponder:
Questions to Ponder:
1. What part of Cache Creek (geologically speaking, as described above) does it look like you’re on? What type or types of rocks do you see along the creek that make you think this way?
2. Do you notice any rocks around you that seem out of place based on where you are along the Creek? If so, where do you think these rocks came from? Brainstorm about their geologic past.
Due to its’ rich geologic diversity, Cache Creek is a popular site for people to mine for gravel. This has severe affects on both the creek channel and the surrounding habitat of Cache Creek. Because of the mining, much of the Creek became bare and the surrounding habitat was greatly reduced.
As people started to recognize the negative affects mining was having on the Creek, new regulations were put into place. Instead of mining in the creek channel, miners were now limited to mining on the Creek’s edges. Over time, Cache Creek has begun to recover.
As individuals explore the area, here are some other questions to ponder:
3. Do you think this part of the creek (or any area surrounding this part of the creek) was ever a site for a gravel mine? Why or why not? Do you notice anything around you that shows evidence of past mining? If so, what?
4. Even if mining didn’t take place at a particular location, mining can greatly affect areas both up and down the creek. Even if you don’t see evidence of past mining, how might mining have affected this part of the creek?
5. After mining becomes limited, the affected site often has an opportunity to recover. Could this be a site that has been restored from past mining? Do you notice anything around you that supports this theory? If so, what?
Thank you for your interest in this Earth Cache. I hope you enjoyed your visit. As with all Earth Caches, please don't include your answers to the questions in your log. Instead, please email your answers to me when you log this cache.
Other Notes:
Cache Creek Nature Preserve Hours:
Monday – Friday
8:00AM – 4:00PM
and
Sundays
10:00AM – 2:00PM