Inliers and outliers (geology)
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An inlier is an area of older rocks surrounded by younger rocks. Inliers are typically formed by the erosion of overlying younger rocks to reveal a limited exposure of the older underlying rocks. Faulting or folding may also contribute to the observed outcrop pattern. A classic example from Great Britain is that of the inlier of folded Ordovician and Silurian rocks at Horton in Ribblesdale in North Yorkshire which are surrounded by the younger flat-lying Carboniferous Limestone. The location has long been visited by geology students and experts. Another example from South Wales is the Usk Inlier in Monmouthshire where Silurian age rocks are upfolded amidst Old Red Sandstone rocks of Devonian age.
A similar outcrop pattern which results from movement on a thrust fault followed by erosion may be termed a window.
Conversely an outlier is an area of younger rock surrounded by older rocks. An outlier is typically formed when sufficient erosion of surrounding rocks has taken place to sever the younger rock's original continuity with a larger mass of the same younger rocks nearby.
One example from Great Britain is the mass of
Grenvillian inliers located within the Appalachian orogeny include the Pine Mountain Belt in Georgia, the French Broad Massif along the Tennessee and North Carolina border, Sauratown Mountain Massif in North Carolina, the Shenandoah Massif in Virginia, the Baltimore Gneiss Domes in Maryland, the Honey Brook Upland in Pennsylvania, and the Reading Prong, which extends from Pennsylvania into New York. Other inliers include the Berkshire Massif, which extends from Massachusetts into Vermont, and the Green Mountain Massif in Vermont. Associated Canadian inliers include the Blair River Inlier on Cape Breton Island, Nova Scotia, the Steel Mountain Terrane on Newfoundland, and the Long Range Inlier on Newfoundland.[2][3]
In West Africa, the
A similar isolated arrangement of rocks produced for example by movement on a thrust fault followed by erosion may be termed a klippe. [5]
References
- ^ British Geological Survey, 2007 Bedrock Geology UK South 5th Edn, 1:625K scale map, Keyworth
- ^ Miller, Brent (1997). Geology, Geochronology, and Tectonic Significance of the Blair River Inlier, Northern Cape Breton Island, Nova Scotia. Halifax: Dalhousie University. p. 260.
- ISBN 0-660-13134-X.
- ISBN 9780045560011.
- ^ Whitten D.G.A. & Brooks J.R.V. 1972, The Penguin Dictionary of Geology, London, England