Ninth International Conference on Permafrost ... - IARC Research

Block Fields, Block Slopes, and Rock Glaciers: A Polygenetic Block Accumulationon the Schafstein (Rhoen Mountains, Germany)Ch. OppPhilipps-University Marburg, GermanyIntroductionDifferent forms of block accumulation are widespread inCentral European Mid-Mountains. In most of the cases, theyare described as block slopes (German: Blockhalden) andblock fields (German: Blockmeer) and other German terms;in rare cases they are described as rock glaciers (Zurawek2002).The biggest known extension of block accumulation locatedin Germany is the basaltic hill of the Schafstein, in the centralpart of the Rhoen Mountains. This block accumulation wasdescribed in the 1960s by Mensching (1960), who calledit Blockmeer, and in the 1990s by Halfmann (1991), whocalled it Blockhalde.The first step of this study was to collect informationfrom literature about the features of different forms of blockaccumulation (cf. Table 1).ResultsWith the help of a self-developed mapping method,different features (cf. Table 1) of the Schafstein blockaccumulation were surveyed, for instance, block forms onthe surface, weathering features, etc. The structure and thethickness of the block accumulation were measured withthe help of refraction seismic techniques. For measuringair temperature below and between the blocks, dataloggerswere used. Rock and soil samples were taken to determinethe mineralogical composition with regard to weatheringdifferences and transport mechanisms.The small-scale survey of the block forms shows that thecentral part of the Schafstein block accumulation has edgedblocks at the upper part and shaped blocks at the bottom ofthe block accumulation. A special feature at the bottom of theblock accumulation is a characteristic wall and depressionstructure, with more than 30° inclination between the walls(cf. Fig. 1), caused by different tensions of different icecement-saturated parts of the block accumulation. Refractionseismic measurements have proved that the central part ofthe block accumulation has a thickness of about 30 and 40 m.During summer time, when air temperature was about 30°C,we measured -1°C air temperature in between the blockFigure 1. Wall and depression structure in the central part of theblock accumulation, the fossil rock glacier.Table 1. Features of different forms of block accumulation (block field, block slope, and rock glacier), compiled after Däuble (2004, changed)from different literature sources.Feature Block field Block slope Rock glacierGeneral feature: accumulation of blocks with only a few or no amount of fine particles, with only rare or no vegetation coverSize no details some 10 m up to some 100 m 60...1500 m long, 60...3000 m broadlong, >20 m broadThickness no general details; mostly >1 m no general details;10...100 msome metersSlope inclination 20° seldom more than 10°; border slopes35°Relief locationon plateaus and slopes; not below a bare rockon slopesnecessarily closely connected tothe source of blocksBlock size no general details, different at least head size, >2 m mostly 0.6...1 mBlock form edged or shaped edged no detailsBlock formationin situ chemical and physicalweathering with transportin situ chemical and physicalweathering with transportphysical weathering,primarily by frostBlock transport solifluction, sliding falling, sliding cohesion transport of the ice cementSurface structure no details no details longitudinal and transverse bulges,depressionsNecessary climate tropic or periglacial periglacial, arid, semi-arid periglacial235