Derek Mottershead

The influence of marine salts, aspect and microbes in the weathering of sandstone in two historic structures

Abstract Sandstone weathering rates are compared on two historic structures, one coastal and one inland. Weathering is accelerated at the coastal site by a factor of 1.59, although this value may underestimate the true difference. Intra-site coastal patterns show that aspect creates a threefold variation in weathering rate. Abundant micro-organism growth protects rock, whereas patchy colonisation leads to increased weathering. Weathering processes and rates cannot properly be interpreted without considering the effects of biological factors and agents, even in environments dominated by marine salts. Conservation of the built heritage requires consideration of aspect and the full range of weathering mechanisms.

Weathering of coastal defensive structures in southwest England: a 500 year stone durability trial

Historic structures can be viewed as exposure trials of the stone of which they are constructed. As such, they represent a geomorphological weathering experiment. Several structures of Henrician (sixteenth century) and greater age on the coast of southwest England have been exposed to coastal salt weathering for 500–600 years. Long-term weathering rates on five different rock groups are derived from careful study of weathering depths and forms. There is significant variation in weathering rate between five major rock groups. Rank ordering of weathering rate values reveals a durability order of these rock groups, which is confirmed by local juxtapositions. Controls on rock durability in the coastal weathering environment include both mechanical and mineralogical characteristics. Specific density, and combined quartz and muscovite content, are positively related to durability; high feldspar and chlorite content are associated with low durability. Copyright

Extreme wave events in the central Mediterranean:geomorphic evidence of tsunami on the Maltese Islands

Field evidence from the Maltese Islands is presented of extreme wave activity in the central Mediterranean Sea. An extensive range of extreme wave signatures, both erosional and depositional, is here presented for the first time and indicates a wave attack from the NE. Existing models of runup and boulder detachment imply that the extreme wave signatures lie beyond the capabilities of storm waves. These considerations, taken together with the range of evidence available, point toward tsunami as the agency responsible, which is consistent with the evidence from Mediterranean marginal coasts opposed to the Maltese Islands. Evidence from existing boulder detachment, tsunami runup and wave velocity models suggests that tsunami with shoreline wave height of up to ∼4 m, and with local velocities of >10 ms–1 would have been required in order to form the signatures observed.

Identification and quantification of weathering by plant roots

Small weathering forms were produced on polished marble tablets by the etching action of plant roots. Imagery of the forms was obtained with a high quality photogrammetric scanner. Automated classification methods, used in image processing, proved inadequate for identifying etched areas, but observer defined mapping within a GIS was able to identify and map the extent and intensity of etching across a range of scales. Root etching patterns were not apparently influenced by forms produced during tablet preparation. Root etching occurs across grains and at grain boundaries, and appears to be more intense at the junction of root networks.

The cart ruts of Malta: an applied geomorphology approach

The mysterious rock-cut cart ruts of Malta are here examined by geomorphologists. They find that the ruts could be caused by two-wheeled carts with a gauge of 1.40m carrying moderate loads. In wet weather the carts would gradually cut into the limestone and reach their ground clearance of 0.675m, causing the carriers to try another route - so there are plenty of them

Bedrock slope evolution in saltrock terrain = Hangentwicklung in Salzgestein Landschaften = Evolution des versants du soubassement sur roches salines

Saltrock is a highly soluble terrain-forming material and, over short periods of time under predominantly solutional erosion processes, shows readily measurable change. It provides an analogue for landform evolution on less rapidly soluble materials such as limestone and gypsum. Saltrock erosion in a Mediterranean environment is observed with erosion pins on both solid outcrop and rockfall debris. Erosion of the salt terrains is very closely related to rainfall events. Surface lowering rates of up to 20 mm per 100 mm rainfall are observed, representing an annualised rate of ca 100 mm a−1 (= 10 m per century or 100 m per millennium). Clear relationships, which are described by bivariate polynomial equations, exist between slope angle and erosion rate. Slope evolution is then modelled with these equations, to reveal good correspondence with the small-scale landforms present at the field sites.

Tsunami landfalls in the Maltese archipelago: reconciling the historical record with geomorphological evidence

Abstract The Maltese Islands lie in the middle of the tsunamigenic Mediterranean domain, around whose margins and islands evidence of historical tsunami landfall has been increasingly recognized in recent years. Critical review of historical evidence of events in 1693 and 1908 indicates extremely modest tsunami impacts. In marked contrast, though, recently discovered geomorphological evidence summarized herein suggests that Maltas coastlines have been overwashed up to elevations of >20 m above sea level by an exceptional event. A new perspective is provided by a review of the central Mediterranean context within which the Maltese evidence is located. Recent advances in understanding the Holocene sequence forming the floor of the Mediterranean Sea present a new stratigraphic and temporal framework within which to elucidate tsunami history. Within 100 km of Malta, terrestrial stratigraphy on Sicily also provides supporting evidence of tsunami impact. Review of these advances suggests that the exceptional event required to emplace the most extreme sedimentary and geomorphological signatures on and around Malta is likely to have had a far-field origin. The currently available circumstantial evidence points strongly towards a probability that the AD 365 earthquake and tsunami were responsible. This, in turn, enables critical reassessment of the exposure of Malta to tsunami hazard.

The changing Maltese soil environment: evidence from the ancient cart-tracks at San Pawl tat-tarġa, Naxxar

Abstract The historic cart ruts of Malta are incised into the underlying bedrock topography. Anomalous relationships between their routeways and the uneven terrain beneath suggest that they originated on a land surface different from that of today. An exposure close to a cart-rut location near Naxxar reveals evidence of limestone terrain development and its role in the evolution of the cart-rut patterns. Specifically, it reveals that cart trackways were most probably superimposed from a soil cover onto an underlying bedrock surface topographically different from the former soil surface. A model is developed demonstrating likely relationships between human activity, soil erosion and trackway evolution leading to the incision of the trackways into the bedrock.