Demons Bluff is falling down

From March 2021 the beach below Demons Bluff (near Anglesea in Victoria, Australia) has been closed due to the significant risk of cliff collapse. Landslips are unpredictable and can occur without warning. The size of material that falls onto the beach can be very large, with rocks and debris a potentially fatal hazard. A crack about 10 centimeters wide has grown to 70 centimeters in recent years, and that part of the cliff “is about to disappear”. The crack runs about 60 meters along the cliff. A large chunk of cliff collapsed at Demons Bluff in 2019.

A journalist has reported, “The cliff line at Demons Bluff has eroded over tens of millions of years, creating the sandy beach below and the massive red rock face”. How do they know that? How long would it take to create this coastline using the assumptions of uniformitarian geology/geomorphology?

Demons Bluff

Demons Bluff is a sheer, 30 to 50 m high, eroding cliff, fronted by a narrow, 1.5 km long beach that is awash at high tide. The cliffs are prone to extensive, linear failure and there is the risk of localized slumping and toppling of blocks and slabs from high in the cliff face. Rock falls commonly cover parts of the beach, which can only be reached on foot along the base of the cliffs. The mean height of the cliffs at Demons Bluff is 28 meters, with a range of 6-89 meters (Bezore, 2019).

Demons Bluff is comprised of soft sedimentary rocks (siltstones and sandstones). Due to the softness of the rock, these cliffs are easily eroded, and have experienced a number of slips and collapses, small and large. Schmidt hammer readings using compressive strength as a proxy for rock hardness give 22 for Demons Bluff, which is slightly harder than Port Campbell limestone at 17 (Bezore, 2019).

Shore platform

“Coastal erosion takes place with destructive waves. These destructive waves are very high in energy and are most powerful in stormy conditions. The swash is when a wave washes up onto the shoreline and the backwash is when the water from a wave retreats back into the sea. Destructive waves have stronger backwashes than swashes. This strong backwash pulls material away from the shoreline and into the sea resulting in erosion” (GSI, 2021).

“Sea cliffs are steep faces of rock and soil that are formed by destructive waves. Waves crashing against the coastline erode until a notch is formed. The erosion of this notch undercuts the ground above it until it becomes unstable and collapses. This process repeats itself and the sea cliff will continue to retreat and a wave-cut platform will be formed in the sea just beneath the cliffs. This wave-cut platform will form at the low-tide level and is evidence of where the cliff face once stood before erosion caused the cliff face to retreat” (GSI, 2021).

Wave action on rock coasts can form a shore platform whose landward edge is a sea cliff. The shore platform at Anglesea is about 80m wide (DELWP, 2019). And the largest shore platform along the southern coastline of Victoria is about 150m (DELWP, 2019). So, to be conservative, we will assume a shore platform width of 150m.

Shoreline retreat

The pattern of shoreline retreat on rocky coasts through time is complex with periods of rapid erosion affecting different sections of coast at different times due to localized cliff collapse or landslides.

Since 1962 the shoreline at Demons Bluff has retreated by an average of ~6 m, although localized erosion of up to 13 m has occurred (NCCC, 2020). This is an average of 10 cm/year. The shoreline change was determined from aerial photographs. The shoreline was defined as the seaward edge of vegetation or the base of the cliff, whichever was applicable.

Shoreline retreat via cliff collapse or landslide is a common occurrence when the rocks are soft. In January 2017, a man died after a cliff collapsed onto a beach with a rumble-like thunder at Thorpeness, Suffolk, UK. The man and his dog were walking close to the base of the cliffs because the tide was high. The cliff collapsed in an area hit by very high tides on the preceding days. Further cliff falls occurred in this area in 2021. And cliff falls are a natural and unpredictable occurrence along the East Devon coast because the rock from which the cliffs are formed is soft and therefore prone to rock falls and landslides, which can happen at any time, although heavy rainfall can trigger incidences.

How long did it take to erode?

How long would it take to create this coastline of a shore platform and sea cliffs at Demons Bluff? A mean shoreline retreat of 10 cm/year moves across a 150 m shore platform in 1,500 years. This is only a few thousand years, which is much less that the “tens of millions of years” quoted above.

Before discovering the measurement of shoreline retreat at Anglesea, I made the following estimation. When I researched this topic in October 2012, I found that the shoreline retreat for soft sedimentary rocks was typically 10-30 cm/year. To be conservative I chose 10 cm/year. Then because Anglesea is further from the continental shelf than sites further westwards (160km at Anglesea compared to 70km at Port Campbell), I assumed that this could halve the impact of the waves on the shoreline to be erosion of 5 cm/year. A mean shoreline retreat of 5 cm/year moves across a 150 m shore platform in 3,000 years. This estimate is also much less that the “tens of millions of years” quoted above.

Discussion

The two main explanations of the earth’s geological history (Biblical and Uniformitarian) are described in Appendix 1. According to the uniformitarian time scale, the sea level rose after the last ice age to reach its current level about 7,000 years ago (Table 1). Whereas, according to the biblical time scale, the sea level rose after the ice age to reach its current level about 3,800 years ago, which is 54% of the uniformitarian value (Walker, 2015).

We have seen that based on observations, it is estimated that it took 1,500 years (or 3,000 years) to produce the current shoreline at Demons Bluff. This period of time is closer to the biblical timescale (of 3,800 years) than the geologic time scale (of 7,000 years). And a similar finding applies to the sea cliffs of Dover (Appendix 2).

So calculations of how long it took to create the coastline at Demons Bluff in Victoria, Australia, give results that are closer to the biblical time scale than the uniformitarian time scale. This indicates that the biblical time scale is more realistic than the uniformitarian one.

Conclusions

This is further evidence that rock coastlines are eroding faster than we think. Demons Bluff is eroding faster than is assumed under the uniformitarian time scale. Calculations of how long it took to create the coastline at Demons Bluff, gives results that are closer to the biblical time scale than the uniformitarian time scale. This indicates that the biblical time scale is more realistic than the uniformitarian one.

The uniformitarian (geologic) time scale can confuse journalists. They can think that landforms take millions of years to form, whereas today erosional features like rock shorelines only take a few thousands of years to form.

Appendix 1: Different explanations of geological history

The earth’s geological and climatological history has determined the current geomorphological and geological structure of the earth. At present there are two main explanations of the earth’s geological and climatological history. One (Biblical) is based on recorded history in the Bible and the other (Uniformitarian) is based on assuming that the present (processes today) is the key to the past (ancient processes) and on the hypothetical geologic time scale. The biblical explanation allows for catastrophic events (such as the flood of Noah), whereas the Uniformitarian explanation minimizes the role of catastrophic events (because it has a preference for gradual events). The biblical explanation mainly involves rapid processes over short periods of time, whereas the Uniformitarian explanation mainly involves slow processes over long periods of time.

Biblical explanation – The global flood about 4.5k years ago and associated tectonic activity and volcanism produced enormously thick sequences of sedimentary rocks over a short period of time. Fossils represent the order of burial during the flood. The flood was followed by an ice age during which there were volcanic and glacial impacts.

Uniformitarian explanation – Sea levels changed gradually over millions of years to produce the enormously thick sequences of sedimentary rocks that record the long history of geological time. Fossils represent the evolution of species. There were multiple ice ages over millions of years. The most recent ice age ended about 10k years ago.

Some dates associated with these two explanations are shown in Table 1. Their timescales deviate further apart from each other as one goes backward in time. Over a time period of 4.5k years the age calibration factor (between the two explanations) decreases from 1.0 to 0.0017. And by the beginning of the flood the age calibration factor is 0.0000056. So the two time scales deviate greatly during the flood. As the uniformitarian dates are said to be supported by radiometric dating, this shows that the assumptions used in radiometric dating are invalid during the flood. Although radiometric dating may be useful for relative dating in this time period, it’s erroneous for absolute dating. This indicates that indirect measurement methods (like radiometric dating) can be unreliable when they are not calibrated against real measurements (like historical records).

Table 1. Comparison between historical dates according to the Biblical and uniformitarian time scales.

Appendix 2: The sea cliffs of Dover

One of the most well-known sea cliffs are the white cliffs of Dover, which are part of chalk cliffs on the shores of Kent, Sussex, Hampshire and Dorset in Southern England. How long would it take to create the coastline of shore platforms and sea cliffs at Dover?

The widest shore platform that I could find near the chalk cliffs of Dover is 540m at low tide at Foreness Point in Kent (So, 1965). This is said to be the most extensive intertidal chalk shore platform in Britain. Foreness Point is in the Isle of Thanet. The mean historical cliff recession rate for the Isle of Thanet has been measured (via maps and photographs) to be about 30cm/year (May, 1971; Dornbusch, 2005). And measurements over the past 150 years using historical maps and aerial photographs showed a cliff retreat rate along the East Sussex coast of 22-45 cm/year (interquartile range), with a median of 36 cm/year (Hurst et al, 2016).

A shoreline retreat of 30 cm/year moves across a 540 m shore platform in 1,800 years. This is only a few thousand years, like it was for Demons Bluff.

References

Bezore R, 2019, “The morphology and evolution of rock coasts over eustatic cycles in temperate, wave dominated environments”, PhD Thesis, University of Melbourne.

DELWP, 2019, “Monitoring sandy coasts in south west Victoria. Coastal management guidelines”, The State of Victoria Department of Environment, Land, Water and Planning.

Dornbusch U, 2005, Retreat of chalk cliffs and downwearing of shore platforms in the eastern channel during the last century, University of Sussex, p.5

GSI, 2021, “Coastal erosion”, Geological survey Ireland.

Hurst M D, Rood D H, Ellis M A, Anderson R S, Dornbusch U, 2016, “Recent acceleration in coastal cliff retreat rates on the south coast of Great Britain”, Earth, Atmospheric and Planetary Sciences, 113, 13336-13341.

May V, 1971, The retreat of Chalk Cliffs, Geographical Journal, 137, 203-206.

NCCC, 2020, “Understanding shoreline change on the Victorian coast: Anglesea, Great Ocean Road”, National Centre for Coasts and Climate, University of Melbourne.

So C L, 1965, “Coastal Platforms of the Isle of Thanet, Kent”, Trans. of the Institute of British Geographers, 37, 147-156.

Walker T, 2015, “A preliminary age calibration for the post-glacial-maximum period”, J Creation, 29, 1, 6-8.

Written, October 2021

Also see: London Bridge is falling down