{"id":2062,"date":"2022-04-19T08:23:34","date_gmt":"2022-04-19T08:23:34","guid":{"rendered":"https:\/\/ehtchampions.org.uk\/ch\/?p=2062"},"modified":"2022-04-19T08:23:34","modified_gmt":"2022-04-19T08:23:34","slug":"the-bilberry-hill-unconformity","status":"publish","type":"post","link":"https:\/\/ehtchampions.org.uk\/ch\/the-bilberry-hill-unconformity\/","title":{"rendered":"The Bilberry Hill Unconformity"},"content":{"rendered":"

In recent years, work by the Lickey Hills Geo-Champions has revealed exposures of an unconformity on Bilberry Hill and Rednal Hill.\u00a0 In both locations a strongly-cemented breccia, dominated by angular quartzite fragments, rests on the eroded surface of the Lickey Quartzite Formation (LQF).\u00a0 On 11 April, a work party on Bilberry Hill explored the limits of the outcrop of this boundary, and sought to gain a better appreciation of its significance.<\/p>\n

Adrian Wyatt’s work revealed a good example of the resistant breccia, sitting unconformably on closely-jointed Lickey Quartzite, which was more readily eroded.<\/div>\n
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Malcolm Coghill’s exposure turned out to be Lickey Quartzite.\u00a0 It was heavily jointed, but the joints were firmly cemented.\u00a0 The significance of this initially eluded me.<\/div>\n
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Hannah Whitten worked on the southern end of the main outcrop.\u00a0 This revealed the very uneven unconformity well above the undercut which marks the junction with the blocky and easily-eroded quartzite below.<\/div>\n
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A cleft runs through the main outcrop: erosion has broken through the breccia layer to reach the more readily eroded heavily-jointed Lickey Quartzite.<\/em><\/div>\n
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In the outcrop photo below, I have darkened the breccia to illustrate the uneven nature of the unconformity.\u00a0 The quartzite immediately beneath the unconformity is flush with the breccia, and therefore is just as resistant to erosion. I would explain this apparent anomaly as follows:<\/div>\n
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Following uplift of the deformed Lickey Quartzite Formation (LQF), erosion produced an uneven surface.\u00a0 Angular fragments of quartzite, in places mixed with sand, accumulated in the hollows.\u00a0 A strongly-seasonal climate led to the development of a silcrete.\u00a0 The cementation process lithified the breccia, but silica was also precipitated along the joints in the LQF.\u00a0 This self-limiting process produced a crust of more-or-less even thickness, ignoring the undulations of the unconformity.\u00a0 Therefore, where we see the rock has been undercut by the erosion of the heavily-jointed LQF, it is not always coincident with the unconformity – it simply marks the base of the silcrete crust.\u00a0 [This is similar to Hutton’s Unconformity on Arran, where a calcrete layer leads many an unwary geologist to misplace the location of the erosion surface.]<\/div>\n
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The east side of the main outcrop.\u00a0 The breccia has been darkened to enhance the visibility of the unconformity.\u00a0 On the left-hand side of the exposure, the pale Lickey Quartzite can be seen to be undercut at the base of the silcrete.<\/em><\/div>\n
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The photo of the polished section of a LQF specimen from Bilberry Hill shows the rock to be a very mature, well-sorted quartz arenite (quartzite).\u00a0 The grains have developed an interlocking mosaic texture because of deep burial (>1km).\u00a0 Feldspars can be seen in various states of breakdown, and cavities have been left where they have been completely removed.\u00a0 Weathering of feldspars can release calcium ions into solution in the soil water.\u00a0 Under the right conditions, these can react with hydrogen carbonate ions to precipitate calcite.\u00a0 Given that we have a potential source of calcium ions in the quartzite, there is a possibility that we are dealing with a calcrete, rather than a silcrete.\u00a0 In testing the breccia with HCl, I have not obtained a positive result for calcite, but it is likely to be present only in very small amounts: grinding a sample into fine fragments will increase the surface area over which a reaction can occur, and may produce a positive result.\u00a0 However, I consider this an unlikely outcome.\u00a0 The most common feldspars in sandstones are potassium-rich, and contain very little calcium.\u00a0 Nevertheless we shouldn’t discount the possibility…yet.<\/div>\n
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Polished surface of the Lickey Quartzite found at Bilberry Hill.<\/em><\/div>\n","protected":false},"excerpt":{"rendered":"

In recent years, work by the Lickey Hills Geo-Champions has revealed exposures of an unconformity on Bilberry Hill and Rednal Hill.\u00a0 In both locations a strongly-cemented breccia, dominated by angular quartzite fragments, rests on the eroded surface of the Lickey …<\/span> Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":91,"featured_media":2063,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[20],"tags":[],"class_list":["post-2062","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-lickeynews"],"_links":{"self":[{"href":"https:\/\/ehtchampions.org.uk\/ch\/wp-json\/wp\/v2\/posts\/2062","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/ehtchampions.org.uk\/ch\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/ehtchampions.org.uk\/ch\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/ehtchampions.org.uk\/ch\/wp-json\/wp\/v2\/users\/91"}],"replies":[{"embeddable":true,"href":"https:\/\/ehtchampions.org.uk\/ch\/wp-json\/wp\/v2\/comments?post=2062"}],"version-history":[{"count":1,"href":"https:\/\/ehtchampions.org.uk\/ch\/wp-json\/wp\/v2\/posts\/2062\/revisions"}],"predecessor-version":[{"id":2066,"href":"https:\/\/ehtchampions.org.uk\/ch\/wp-json\/wp\/v2\/posts\/2062\/revisions\/2066"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/ehtchampions.org.uk\/ch\/wp-json\/wp\/v2\/media\/2063"}],"wp:attachment":[{"href":"https:\/\/ehtchampions.org.uk\/ch\/wp-json\/wp\/v2\/media?parent=2062"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/ehtchampions.org.uk\/ch\/wp-json\/wp\/v2\/categories?post=2062"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/ehtchampions.org.uk\/ch\/wp-json\/wp\/v2\/tags?post=2062"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}