1. Erosional Features (Typical of Active Margins)
Today, we’re continuing to highlight evidence of plate tectonics in the coastal landscape.
Sea cliffs, wave platform, but no pocket beach
West Cliff Drive, Santa Cruz, CA

2. Erosional Features (Typical of Active Margins)
Sea cliffs, sea stack, pocket beach between headlands
Bonny Doon Beach, Central California

3. Bathymetry at Mavericks
At active margins, shelf can be an erosional feature

4. Features associated with sea cliff retreat
Formation of erosional coast features

5. Sea Arch, Kachemak Bay, Alaska

6. Sea Arch – Santa Barbara, CA

7. Variability in Wave Cut Platform
Steep cliffs reflect wave energy

8. Control on Cliff Slope
relative influences of terrestrial vs. marine processes

9. Sea Cliff Retreat – Processes and Timing
Abrasion of a Basal Notch
Thresholded Failure
Comminution Lag - Protective Feedback
Figure from Hapke and Richmond, 2002, Marine Geology
1. At the beginning of the cycle, the toe of the sea cliff is attacked by the abrasive assault of waves – that entrain sediment in their orbital motion or turbulent broken wave bore that rushes up the beach face – the sediment grains are the tools of abrasion.
2. This abrasion excavates a “basal notch”, leaving the overhanging cliff material undermined. Which leads to failure – a thresholded process occurring when driving stresses exceed resisting stresses and often assisted by the presence of groundwater in the cliff material.
3. In the rightmost panel, we see the naturally-derived armor that protects the base of cliff – which is the recently failed material itself. That armor hangs around until sufficient wave energy has been delivered to comminute the failed material to particle sizes fine enough to be removed by littoral transport.
This is indeed an episodic evolution – This figure from Cheryl Hapke and Bruce Richmond’s 2002 Marine Geology Paper shows a failure signature during the 1998 El-Nino winter. You can see the dramatic retreat, and the failed material below. So the evolution is episodic and punctuated, due in part to the fact that the timing of energy delivery is episodic and punctuated – there’s a distribution of tide levels and an inconsistent time series of wave heights.
That results in this pattern of fits and starts to describe sea cliff retreat – I’ve shown in this conceptual plot on the lower right. So we have to be mindful that when sea cliff retreat rates are reported, the bulk of the retreat might be occurring in just a few short-lived events separated by periods of constant cliff position. You can see that frequent small events can produce the same retreat rate as infrequent large events – a recurring theme in geomorphology to be sure.
Episodic Sea Cliff Failure
Retreat History

10. Sea Cliff Retreat Rates in California
~30 cm/yr
by veneer of sand.
Goleta beach, Santa Barbara, CA

11. Sea Cliff Erosion – feedbacks
grain entrainment - abrasion - positive feedback
beach development - wave energy dissipation - negative feedback

12. Assessment of Wave impacts on Sea Cliffs
fluids (R)
morphology (EJ,EDC)
after: Ruggiero et al., 1996, 2001

13. Correlation of Wave Runup and Sea Cliff Retreat
after: Ruggiero et al., 2001
What about influence of lithology?

14. Santa Cruz Mountains and SF Peninsula
Make the point that the “plate boundary”, though depicted as a clean feature is really a bit more complex.
Identify SAF, San Gregorio, and Hayward Faults.
Each accommodates some of the relative motion between the plates.
Think of when I shear my keyboard protector.
Looking obliquely at the SF peninsula, we see this rather formidable topo feature - the SC Mts.
The origin of this feature can be explained by plate tectonics
and those same explanations play strongly into a series of features along the N. shore of Monterey Bay
I call this region the Mountain factory and now I’ll explain why
Now keep that idea of the Mt. factory on the mental desktop because we’ll revisit that concept.
Change pace for a minute - drive down Hwy. 1 to Santa Cruz.
Image courtesy of USGS / PG&E Cooperative Research

15. Monterey Canyon - tectonic offset
Figure from Greene et al., 1991
Tectonic beheading of canyon channels?
Series of faults - more complex than simple “transform” plate boundary.

16. Erosional Geomorphic Features - Marine Terraces
When you impose this sea level fluctuation on a tectonically-active margin that is uplifting, like California - we watched the land rise over a meter in some 90 seconds of terrifying shaking during the Loma Prieta Earthquake. Those two processes (uplift and sea level fluctuation) collaborate to build these spectacular flights of marine terraces - notice the nice flat planar treads of the stairstep separated by the risers which are the paleo sea cliffs - actively forming today at the water’s edge.
Highway 1 near Santa Cruz, California

17. Marine Terrace Generation and Degradation
Animation by Robert S. Anderson

18. Marine Terrace Profiles
Here’s a profile - note the more “diffused” older terrace scarp
concept of diffusion - akin to chemical diffusion -
draw overlapping diffused scarp profiles on blackboard
Figure from Rosenbloom and Anderson, JGR, 1994

19. Another view of the terraces
Another way to destroy the older terraces - fluvial incision
carve away the platforms by widening the river channels - simple geometry
From Anderson et al., 1999, Basin Research

20. Fluvial Dissection oblique viwe of the whole system
including uplift, wave cutting, stream incision, and cliff decay
From Anderson et al., 1999, Basin Research

21. Isostatic Uplift and Landform Preservation
Holocene Beach Ridges - Sweden -
isostatic uplift from deglaciation since late Pleistocene = 2mm/yr