Age and origin of blockfields on Scottish mountains

• Published in: Scottish Geographical Journal
• Main Authors: Hopkinson, Charles, Ballantyne, Colin
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2014
• Subjects: Blockfields; Regolith; Clay minerals; Frost weathering; Quaternary; Neogene; Cold-based glacier; Ice Sheet
• Online Access: https://research-portal.st-andrews.ac.uk/en/researchoutput/age-and-origin-of-blockfields-on-scottish-mountains(027ab8fb-b9f7-4ad7-86d5-2e320e9b3aef).html; https://doi.org/10.1080/14702541.2013.855808

Numerous researchers have proposed that blockfields on mid-latitude plateaux are frost-modified relicts formed initially by chemical weathering under warm, humid pre-Quaternary (Neogene) conditions. Others contend that they formed mainly by frost action during the Quaternary. We test these competing hypotheses for blockfields on three mountains in NW Scotland: a matrix-supported diamict blockfield (sandstone), clast-supported diamict blockfield (quartzite) and an openwork blockfield (schist). Clay concentrations in the fine fraction are low (0–6.8%) and both clay:silt ratios and secondary clay minerals (illite, kaolinite and gibbsite) are consistent with prolonged chemical weathering under periglacial conditions. There is negligible depletion of labile elements in the fine fraction relative to the parent rock. Conversely, evidence for surface and near-surface granular disaggregation, an increase in clast angularity with depth, dominantly angular sand grains, in situ detached clasts and fractured clasts above rockhead favour formation by frost weathering under Pleistocene stadial conditions. There is no convincing evidence for blockfield inheritance from Neogene regolith. As the blockfields were buried under cold-based glacier ice during the last glacial maximum, they pre-date ice-sheet build-up at ∼35–30 ka. Measured rates of plateau surface lowering suggest that present blockfield regolith is probably of Late Pleistocene (<135 ka) age.