There are two basic tasks which fall to the geologist. We are the historians of the earth. We look at the earth an ask how did things come to be as they are. We are used to looking at the earth through the lens of deep time. We also look at the modern landscape and ask what processes are at work and how will the earth evolve as a result.
The bedrock into which our coast is carved from approximately
the end of Almar St (West Cliff) and Ano Nuevo is a sedimentary
formation known as the Santa Cruz Mudstone. The environment of
deposition for this formation was outer continental shelf to upper
continental slope. The age of this formation is mid-late Miocene
(Miocene was 26-7 million years ago). The sedimentary particles which
make up the mudstone are a combination of fine-grained silts and clays,
which are brought down to the sea by streams, and the siliceous
shells (called frustules) of numerous one-celled marine plants called
diatoms. Subjected to pressure and heat (the earth's internal heat)
by the burial process, some of the silica (a type of glass) dissolves
and is later reprecipitated as a cement which binds the particles
together. The ultimate result of this process is that the
unconsolidated sediments are lithified or turned to stone.
The characteristics peculiar to the Santa Cruz Mudstone are
important to the way in which the coastal landscape evolves. The
silica cement which binds the Mudstone is fairly strong, making it
fairly hard for a sedimentary rock. (One can, more-or-less, hear a
glassy sound when two pieces of the rock are tapped together) On the
other hand, the formation is highly fractured and crisscrossed by
numerous small faults.
The Mudstone is also layered with the strata
dipping slightly seaward. The layers were deposited during different
episodes, so there are differences in composition from layer to layer
which affects the resistance to erosion of the layers.
If you look at the hills north of Santa Cruz, you will notice that
there are a series of flat-lying areas separated by steep bluffs. In
fact, most of Santa Cruz is located on a similar broad, flat plain.
These landforms are known as marine terraces. They are, in fact,
ancient sea floors which were planed flat by waves and then uplifted
by the internal forces of the earth. The steep bluffs which front
these terraces are former sea cliffs, just as the terrace on which
West Cliff Drive is located is fronted by a sea cliff.
This remarkable sequence of stranded ancient sea floors is the
signature of two grand but unrelated processes. The first is the
motions of the EarthÕs tectonic plates. We are presently located on
the eastern edge of the Pacific Plate. If we drive over the hill to
San Jose, we would cross over to the North American Plate. At present,
the Pacific Plate is moving north and the North American Plate. The
rate of separation is about 5 centimeters per year. The boundary
between these two great plates in this area is known as the San Andreas
Fault. There is a bend in the San Andreas centered around the Loma
Prieta area. As a result of this bend, some of the Pacific Plate is
being forced up and over part of the North American Plate. This uplift
is still ongoing and over geologic time it has created the Santa Cruz
Mountains. The average rate of uplift is about 0.3 millimeters per
year.
The other grand process has to do with fluctuations in the
earthÕs orbit around the sun. These fluctuations, called Milankovich
Cycles, give rise to alternating periods of warmth and coolness in the
global climate. During cool periods, large portions of the continents
undergo glaciation. These glaciers can be quite thick (as much as a
mile) and lock up a significant part of the earthÕs surface waters. As
a result of cycles of glaciation and deglaciation, sea level rises and
falls. As the shoreline advances and retreats across the continental
shelf, the waves cut what is known as a marine platform into the
bedrock of the shelf. Such a platform is presently being formed
offshore from Natural Bridges State Park by the scouring action of
thousands and thousands of waves.
In sum, it is the combination of the rise and fall of sea level and of the constant uplift of the land which has created the flight of marine terraces we see today. The ancient seafloor which comprises the lowest Santa Cruz terrace and upon which the visitorÕs center is built is approximately 100,000 years old.
Coastal dunes are similar in many respects to the sand dunes
one finds in the desert but with some significant differences. The
main difference is the presence of water in the environment. Coastal
dunes have a tendency to become vegetated. The presence vegetation
allows the dunes to become stabilized (i.e. to stop migrating). As a
result of this stability, people have a tendency to build things on the
dunes or besides the dunes. A serious problem in many coastal dune
areas is the destabilization of the dunes by dune buggies, people
walking on the vegetation, or other anthropogenic impacts on the
vegetation. When the dunes become destabilized the begin to move. In
areas such as the Oregon coast where the dune fields are 50 miles long
and the dunes as much as 200 feet high, this can cause some obvious
problems.
Three things are needed to produce coastal dunes: 1) a plentiful supply of sand (this was probably provided by the last sea level rise), 2) a place to store the sand (provided at Natural Bridges by the streams which cut down through the marine terrace), and 3) strong and steady winds (here we have the Westerlies which tend to arise in the afternoon for most of the year and can blow at speeds of 20 to 20 miles per hour.
A close examination of the dune sand reveals that it is typically finer than the sand found on the adjacent beach. Dunes are formed by the action of the wind, not by the action of waves as is the beach. When the wind blows strongly, the sand grains on the beach are entrained by the wind and transported into the dune. There is a limit to the maximum size grain that the wind can transport. As a result, the wind tends to winnow the finer grains from the beach. In the long-term, the removal of sand from the beach to the dunes can constitute a net loss of beach sand from the coastal sand supply system.
Due to the relatively small size of the drainage basins and to
the seasonal rainfall patterns, most of our coastal streams do not have
high discharge except during storms. In fact, for most of the year,
stream flow is fed by ground water seeping into the stream beds. As a
result of this low flow regime, the waves will construct a sand bar
across the mouths of the streams causing water to pond behind the beach.
With stream discharge low, the streams do not have much competence,
that is, they can usually transport only small particles. When these
fine-grained particles reach the ponded area, they settle out and mud
flats begin to accumulate. These mudflats are in turn colonized by
vegetation and a biologically-rich ecosystem eventually evolves.
Periodically, large winter rainstorms cause the streams to swell and
the bar is breached. This flushes or cleanses the lagoon. The waves
can also breach the bar by overwashing and eroding it. If the amount
of water behind the bar is large, then release of this water can be
very forceful and create a danger to persons in the area.
Although terrestrial geologic processes do operate at the shoreline, the thing that distinguishes coastal geology is the dominant role played by waves as a geologic agent. Most of the waves you see are generated by the wind. In terms of the energy expended against the shoreline, the most significant waves are generated by storm winds. Waves tend to come in two distinct varieties. One type of wave is know as a sea. These are waves which are produced by local storms. They tend to be very chaotic and include a broad range of wave heights. The other type of waves are know as swell. These are waves produced by distant storms. Although they can be very large, they tend to be long and smooth and very consistent in size and period. Swell waves carry the energy of large storms over incredible distances. Some of the swells which expend their energy on our coast come from huge storms off of Antarctica, others from the Gulf of Alaska or near Japan.
We could hardly talk about the geology of the coastal zone
without talking about beaches. A beach is a collection of
unconsolidated material deposited against the coastline by waves.
Beaches can be made of a variety of materials. There are coral
beaches, black beaches, green beaches, and even pink beaches. Each
color is due to the predominance of different minerals in the beach
deposits. Many beaches around the world, our own beaches included,
are formed predominantly by grains of quartz and feldspar. These
minerals are very common in continental crust, for example, in rocks
like granite. Over time, these rocks, exposed at the earths surface,
weather and break down into their constituent mineral grains. These
grains, along with other rock fragments, are washed by runoff into the
streams where they are eventually carried to the sea. Once at the
shoreline, the coarser grains are distributed along the shore by
waves. The finer material is washed out to sea where it eventually
settles to the bottom in quieter, deep-water environments.
In addition to being geologically interesting, beaches are
important for other reasons. One is as a recreational resource.
Beaches are one of those environments which people seem to gravitate
to naturally. I don't think I have to belabor the point. Another
reason is that beaches act as buffers to attack of the coast by storm
waves. In many cases, it may take several storms before enough sand
has been removed from the beach to allow the waves to seriously batter
the cliffs or dunes. The presence of beaches and their width play a
very important role in determining rates of coastal erosion.
When waves break at an angle to the beach, an important phenomenon occurs. This phenomenon is called littoral drift. The breaking of the waves at an angle results in some of the waves' momentum being directed along the shore and this along-shore component of momentum generates something known as a longshore current. Most people are familiar with longshore currents even if they are not familiar with the name. Any one who has gone swimming at the beach only to emerge some distance downcoast from where he or she left the towel and beach blanket has been transported by a longshore current.
In addition to transporting swimmers, longshore currents also transport beach sand. It is this motion of beach sand which is known as littoral drift. The magnitude of littoral drift can be quite high. When Santa Cruz Harbor was first built, neither the net long-term direction nor the magnitude of littoral drift were known. At the time, the beach at Seabright possessed an extremely narrow beach and the sea cliffs there were among the faster eroding in the county. When the jetties were constructed in 1962, the west jetty began to act as a barrier to littoral drift and sand accumulated up against it. This accretion of sand continued for approximately the next six years. By keeping track of the accumulation rate, we were able to determine what the long-term littoral drift direction and magnitude were for northern Monterey Bay.
As it turns out, littoral drift proceeds from north to south in northern Monterey Bay. The rate of littoral drift is on the average a whopping quarter of a million cubic yards of sand per year. If you were to bundle all of this sand into individual cubic yard bundles and lay them end-to-end, a year's supply would stretch from Santa Cruz to San Luis Obispo.
Sea cliffs represent the leading edge of the marine platform
cutting process. By a combination of hydraulic impact and abrasion, the
waves are constantly wearing away the rocks. The process is a
perfectly natural one and the question is not whether the cliffs are
eroding, but how fast.
The rate of seacliff retreat is dependent on how much energy the waves have and on the resistance of the rock material. In general, igneous rocks like granite are very resistant. This is the type of rock you findalong much of the Monterey Peninsula. Sedimentary rocks, like sandstone, tend to be much less resistant, although there is a great deal of variation. The rock formation of which these sea cliffs are composed is called the Santa Cruz Mudstone. It is fairly resistant as sedimentary rocks go (due to its silica cement) with yearly erosion rates on the average of a few inches per year. The Purisima formation, however, which outcrops between the lighthouse and beach drive inAptos, is much less resistant and in places is eroding as much as a foot per year. The difference in resistance between these two sedimentary rock formations is made even more evident by the fact that the Purisima has a much higher erosionrate even though it outcrops inside the bay, which offers a significant amount of shelter from incoming waves.
One realization which is often overlooked is that there is a
genuine connection between geology and biology. A perfect example is
provided by the tide pools at Natural Bridges. The intertidal
environment here is a wealth of biologic diversity. These tide pools
would not be here, however, if it weren't for the presence of an
appropriate physical environment. These flat-lying ledges you see
before you and on which the tide pools form are known as shore
platforms. They are wave-cut features with a world-wide distribution.
At present, the exact mechanism of their formation remains a bit of
a mystery. It may be that the mechanism depends significantly on the
type of rock involved. The formation of shore platforms at Natural
Bridges appears to be due to erosion along thin, less-resistant beds
which lie within the reach of the waves. Other explanations which have
been offered include weathering of the rock in the splash zone above the
mean high tide level. Apparently cycles of wetting the rock then
drying it out completely weaken the strength of the rock.
Along the stretch of coast where the Santa Cruz Mudstone
outcrops, much of the coastal erosion occurs along zones of weakness
created by faults and fractures. Instead of a homogeneous retreat of
the cliff line, one has erosion by dissection. As one of these zones
of weakness is attacked by the waves, a surge channel begins to form.
One of the most striking features of coastal geomorphology
occurs where variations in resistance within the rock formation leads
to the formations of small promontories. Where waves attack the
flanks of the promontories and there are zones of weakness to be found
(such as faults), then a hole may be worn through the sides and a
natural bridge formed. In fact, two or three holes can be formed and
a series of natural bridges can be formed. The bridges at Natural
Bridges were once a series of three bridges. Now only one remains.
What eventually happens is that the hole or arch expands so much that
the weight of the overlying rock can no longer be supported and the
bridge collapses. The result is a small headland with an offshore
rock. Such offshore rocks are known as sea stacks.
Natural Bridges State Park has suffered some over the years because of its loss of bridges. The park remains unique, however, in the realm of bridges, but for another reason. In its present configuration, the bridge sequence at Natural Bridges depicts, in spatial order, the time sequence of the birth, life, and death of a natural bridge. First a headland forms, then an arch is worn through along a fault, finally the bridge collapses leaving a sea stack behind. You might say that the bridges have been transformed from a natural wonder to an educational resource.