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Dear colleagues, Working on deformed Proterozoic conglomerates, I measured sharp point maxima for the orientation of long axes of elongate metarhyolitic pebbles. In the '80s, when the art of strain analysis emerged, structural geologists went there, measured hundreds of pebbles and represented them in a Flinn diagram in the Journal of Structural Geology; apparently, their a priori assumption was 'ductile deformation'. However, that assumption can easily be discarded from field evidence: Some elongate pebbles are oriented at about right angles to the preferred direction of the majority of them, and, most convincing, internal sedimentary bedding in pebbles, whenever present, is entirely undeformed, irrespective of its orientation. Also, the metamorphic temperature (from chlorite composition, ~300 C) was far too low for ductile deformation of these lithologies. We have evidence that the elongate pebble shapes result from (sub)glacial abrasion, in a lodgement till, but the (mostly clast-supported) conglomerates were most likely deposited as debris flows in a glacially-influenced alluvial fan. However, clast fabrics in debris flows do often show preferred orientations, but nowhere as 'sharp' as seen here, to the best of my knowledge. The conglomerates and enclosing formations (slates, volcaniclastics) suffered from only a single phase of folding + shearing. Incidentally (?), the point maximum of the long pebble axes lies very close to the steeply plunging axis of these shear folds. Hence my question: Could mechanical rotation during (shear) folding have reoriented the pebbles into the preferred orientation they have at present? Any shared experience or hints from existing literature would be most welcome. Frank ----------------------------------------------------------------- Frank F. Beunk Geology & Geochemistry Cluster, Department of Earth Sciences Faculty of Earth and Life Sciences (FALW), room H-351 De Boelelaan 1085 NL-1081 HV Amsterdam the Netherlands T +31-(0)205987371 Mobile +31-(0)648084510