Surface lowering of limestone pavement as determined by cosmogenic ( 36Cl) analysis
Determination of the rate and total amount of limestone pavement surface lowering is a critical issue in developing models of regional landscape change in limestone terrain. Erratic-capped pedestals have frequently been used for this purpose but problems concerning definition and measurement of pedestal height, and the absence of a secure timeframe for erratic emplacement, have resulted in conflicting interpretations. We have used cosmogenic ( 36Cl) to establish the emplacement age of erratic boulders and the total amount of pavement surface lowering at sites in northwest England. Since erratic emplacement at 17.9ka the limestone pavement has been lowered by 22-45cm (average: 33±10cm), assuming lowering was continuous. Although indicating some spatial heterogeneity, the results contrast with earlier reported values based on the measurement of pedestal heights and inferred age for deglaciation. We consider that changes in climate and the character and duration of regolith covers to have been important influences in promoting surface lowering. It is argued that nivation (chemical and mechanical snow-related processes) associated with several cool/cold periods is likely to have played an important role in surface lowering. Complicating factors associated with surface lowering (thickness and longevity of snow and regolith covers) are identified but as yet cannot be quantified.