We’ll meet in the Library South parking lot (C) at 12 p.m.
We will travel southwest across Sonoma County toward the coast just north of Tomales Bay. We’ll be traveling through rolling hills presently in use as dairy farms. Notice evidence of erosion through small mass movements, slope wash, and gullying. Think about hillslope processes of erosion and deposition. As we approach the coast you will notice that the type of rock exposed in outcrops here and there in the fields changes its character and appearance.
Does the change in lithology of the bedrock and outcrops change the general appearance or shape of the landforms? In general, do you think that human activity, land use, affects the landscape, including rates of erosion and deposition? What do you see? Can you observe differences from one field or farm to another?
Our first stop will be near the top of a hill along side one of these farms where we can see some distance. We’ll be looking at the upstream end of a creek that eventually runs into the Pacific Ocean north of Dylan Beach. We’ll get to see this creek or its tributaries at several points along our route.
Our second stop will be just outside of the village of Tomales, where once again we will observe the creek up close as it passes through an extensive wetland and cow pasture. Here we can observe examples of stable and/or unstable channel processes, sediment load, etc.
What sort of sediment load does this channel carry? What are the major controls on velocity in this channel? Does human activity affect the presence and form of this channel? How might it be different if it weren't a cow pasture? Is this a stable stream channel? Why or why not?
Our third stop will be on the road between Tomales and Dylan Beach. We’ll look at gully processes and mass movements, in relation to both natural slope processes and human land use activity.
Make a sketch of this gully and its surroundings for your notes. Is this gully active? What environmental factors probably influence the erosion or lack of erosion on this hillslope? What are the driving forces and resisting forces?
At our fourth stop, we’ll be parked at the top of a large outcrop of ancient sandstone that has been carved by weathering and erosion, possibly due to wave action as well as fluvial processes.
What evidence do you see of weathering here? Since this outcrop is at least several hundred feet above the modern sea level, how could wave action be explained as one of the erosive processes here?
Across the road from the outcrop we will also examine an example of a residual soil forming on the sandstone.
How does a residual soil look different from a soil forming in alluvium? Do you think a residual soil such as this would take more or less time to develop that one formed in fluvially or wind deposited sediment? Why?
Stop Five: We will take a short drive down to Dylan Beach where we’ll be able to view the north end of Tomales Bay up close, and also take a short break at the café or grocery store for bathroom/snacks, etc..
What processes have formed Tomales Bay? Why is it here and what does it mark?
Driving back toward Tomales, we’ll turn north (left) onto Valley Ford Road. As we drive downslope, look to your right to view more sandstone outcrops that have been worn and weathered. Here are examples of honeycomb weathering, or tafoni. We’ll stop here briefly if we have time (stop six).
As we drive north through the hills, note more interesting examples of hillslope processes, and also the abundance of alien vegetation (broom, eucalyptus, pampas grass) along with the native trees. Stop seven will be first at a bridge crossing a meandering estero (San Antonio) to observe fluvial processes within the channel, and then a quick stop from a nearby hill overlooking the channel. Also observe at the outcrop more examples of tafoni in the (Wilson Grove Fm) sandstone roadcut. This roadcut is also known for producing interesting marine fossils. This stream channel represents the downstream end of the same or similar creek we observed at our first stop.
How is the channel different here, and why? Think about erosion and deposition, sediment load, discharge, and effects of land use over time. This channel has experienced considerable instability over the past 100 years or so, according to a local resident, due to overgrazing by cattle on upstream lands. Do you see any evidence of this? What sort of sediment load is dominant in this stream? What can you see?
Our eighth and final stop will be along this same road
where it cuts into a steep slope adjacent to a small stream channel. We’ll
take a look at an active mass movement.
What do you think may have triggered instability on this slope? How well developed are the soils exposed in the scarp? What do you see? What sort of mass movement is this? What is your evidence? Is this slope stabilizing or still moving?
We’ll continue on to Valley Ford, passing through an estero valley before reaching the highway.
How is this estero different from the first one we
crossed? How might tectonics have played a role in the formation of this
valley?