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Aninlier is an area of olderrocks surrounded by younger rocks. Inliers are typically formed by theerosion of overlying younger rocks to reveal a limitedexposure of the older underlying rocks.Faulting orfolding may also contribute to the observedoutcrop pattern. A classic example fromGreat Britain is that of the inlier of foldedOrdovician andSilurian rocks atHorton in Ribblesdale inNorth Yorkshire which are surrounded by the younger flat-lyingCarboniferous Limestone. The location has long been visited bygeology students and experts. Another example fromSouth Wales is theUsk Inlier inMonmouthshire where Silurian age rocks are upfolded amidstOld Red Sandstone rocks ofDevonian age.
A similar outcrop pattern which results from movement on athrust fault followed by erosion may be termed awindow.
Conversely anoutlier 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 ofTriassicsandstone around theStaffordshire town ofLeek. This is isolated from the very much larger area of Triassic rocks which characterise theEnglish Midlands andCheshire Basin, to the south and west respectively, by the surroundingCarboniferous sandstones andmudstones. Similarly in theBlack Mountains of SouthWales, the summit area ofPen Cerrig-calch is composed from a suite of Carboniferous age sandstones andlimestone. This outcrop is isolated from the main extent of these rocks (which form theSouth Wales Coalfield and its margins some miles to the south) by theDevonianOld Red Sandstone on which it rests by and by which it is surrounded.[1]
Grenvillian inliers located within theAppalachian orogeny include thePine Mountain Belt in Georgia, theFrench BroadMassif along the Tennessee and North Carolina border,Sauratown Mountain Massif in North Carolina, theShenandoah Massif in Virginia, the Baltimore Gneiss Domes in Maryland, the Honey Brook Upland in Pennsylvania, and theReading 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 onCape Breton Island, Nova Scotia, the Steel Mountain Terrane on Newfoundland, and theLong Range Inlier on Newfoundland.[2][3]
In West Africa, theKenieba inlier borders southwesternMali and easternSenegal, and associatedBirimian gold is found inKalana andSabodala respectively.[4]
A similar isolated arrangement of rocks produced for example by movement on a thrust fault followed by erosion may be termed aklippe.[5]