Definition
An escarpment forms as an effect of faulting or erosion and separates two relatively level areas of differing elevations. The surface of the steep slope is called a scarp face.
Scarps are generally formed by one of two processes:
Differential erosion of sedimentary rocks
Most commonly, an escarpment is a transition from one series of sedimentary rocks to another series of a different age and composition.
Vertical movement of the Earth’s crust along a geological fault
Escarpments are also frequently formed by faults. When a fault displaces the ground surface so that one side is higher than the other, a fault scarp is created. This can occur in dip-slip faults, or when a strike-slip fault brings a piece of high ground adjacent to an area of lower ground. A good example of a fault scarp is on the Wellington fault, most visible in Harcourt Park.
More loosely, the term scarp describes the zone between coastal lowlands and continental plateaus which have a marked, abrupt change in elevation caused by coastal erosion at the base of the plateau.
Geology and rock condition
The hills and coastal slope from Pukerua Bay to Paekakariki are formed of ‘greywacke’ bedrock – hard, interbedded sandstone and argillite (siltstone). The greywacke rocks have been folded and faulted by several phases of deformation, and as a result are sheared and closely jointed (100-500mm spacing). Bedding (the layers of sedimentary rock) is variable throughout the region, but north of Pukerua Bay it generally dips steeply to the east and northeast into the coastal slope. There are extensive bedrock exposures on the rock platform along the coast foreshore and in road cuts along SH1 and the railway line.
The near-surface rock of the lower slopes along and immediately above the highway and railway line is slightly weathered to unweathered, but is generally closely jointed and fractured. On the upper slopes the rock is moderately to highly weathered and is overlain by a thin (200-400mm) surface layer of soil, loess (wind deposited silt) and colluvium (loose, unconsolidated sediments deposited at the base of a hillslope) ~3-6m thick, which exhibits widespread superficial soil creep and incipient slumping in places.
Geological structure
The main geological structures in the region are active faults and old faults. Active faults are faults that have moved in the last c125,000 years and are likely to move again in the foreseeable future causing a large earthquake (~>= M7) and possible ground surface rupture. The two major faults in the area are the Pukerua Fault that passes through Pukerua Bay and is inferred to continue offshore, and the Ohariu Fault that extends from Porirua Harbour through the hills to the north, and at the northern end Te Puka Stream on the western side of the new Transmission Gully Motorway (TGM).
Both Ohariu and Pukerua Bay faults are capable of generating earthquakes of M 7.2-7.5 at intervals of about 2,000 to 5,000 years, resulting in shaking intensities of MM9-MM10 in the Kapiti area (the same as the Wellington fault).
Most of the other major geological structures in the area are old faults which do not show recent movements. Preferential erosion along zones of crushed and sheared rock has formed topographic lineaments by the alignment of gullies and linear stream courses.
Slope instability
The steep coastal slope between Pukerua Bay and Paekakariki has a recent history of landsliding and slope instability problems, and is mapped as having high to very high earthquake-induced landslide (EIL) susceptibility.
The coastal slope has not undergone MM9 or MM10 shaking in the last 160 years, over which time large sections of its toe have been undercut and destabilised by cuts for railway and SH1 construction and quarrying, leaving it potentially more vulnerable to slope failures.
The Pukerua Bay to Paekakariki section of the North Island Main Trunk railway has been prone to instability problems since completion in 1886, with slope failure processes apparently accelerated by cuts for railway construction, and also removal of native bush cover. A number of slope failures have occurred along this section of the railway line in the last 60 years. There have been several local rock and debris falls from steep cuts and tunnel portals, e.g. at the southern portal of No 6 tunnel, where a rock fall shelter was constructed. A significant rock fall also occurred at the north portal of No 7 tunnel in 1999, disrupting trains and SH1 traffic for several days while debris was cleared and a protective fence erected.
Most of these failures were relatively small rock and scree falls which occurred during or after heavy rainfall. However, some slope failures have also occurred during dry weather, without any triggering event or failure history. A rock and debris fall on 19 November 1980 that derailed a south-bound train is one example of such an event.
Earthquake induced slope hazard study
In the mid-90s, the Wellington Regional Council commissioned a study on earthquake induced slope hazards. The study shows that the coastal route between Pukerua Bay and Paekakariki (including the Paekakariki township and railway station) has a high to very high susceptibility to slope failures in earthquakes, due to the very steep and high slopes as well as the presence of the geomorphic features that appear to be the scars of large prehistoric landslides which may have been caused by large earthquakes.
In comparison, the northern section of the Transmission Gully route has only moderate to locally high susceptibility to slope failures.
Getting up close and personal
In 2016, the Paekakariki Escarpment portion of the Te Araroa walkway was opened. The track is 10km long and runs high above SH1 and the railway, reaching up to 220m. Yeetrees has created a power trail along this walkway for your extra enjoyment. Walking this track is not a requirement for completing this earthcache.
Earthcache tasks
To get credit for this earthcache, email the answers to the following questions:
- At WP1, from your GPS, what is the altitude? (DO NOT cross the road! Stay on the seaward side)
- At WP2, look due south. The gully was formed by stream erosion. But why did this occur at this particular location?
- At WP3, from your GPS, what is the altitude?
- What is the distance and bearing between WP1 and WP3?
- Take 10m from the distance from question 4 (for the width of the road) and calculate the angle of the slope between WP1 and WP3.
- In the event of a MM10 intensity earthquake on the Ohariu fault, how much material is expected to fall onto the coastal road, and how long will it take to clear and reopen the route?
- (Optionally) Take a photo of yourself with your GPS, showing part of the escarpment at WP3 and post it with your online log.
Logging requirements
To log this cache you must visit the waypoints and undertake the above earthcache tasks. Once you have answered the above questions, email them to me. You can log your find straight away, do not include your answers in the log. I'll contact you if there are any issues. Dubious finds or logs without emailed answers will be deleted.