An inlier is an area of older rocks surrounded by younger rocks. Inliers are typically formed by the

erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...

of overlying younger rocks to reveal a limited exposure of the older underlying rocks.

Faulting In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

or folding may also contribute to the observed

outcrop An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth and other terrestrial planets. Features Outcrops do not cover the majority of the Earth's land surface because in most p ...

pattern. A classic example from

Great Britain Great Britain is an island in the North Atlantic Ocean off the north-west coast of continental Europe, consisting of the countries England, Scotland, and Wales. With an area of , it is the largest of the British Isles, the List of European ...

is that of the inlier of folded

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

and

Silurian The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...

rocks at Horton in Ribblesdale in

North Yorkshire North Yorkshire is a Ceremonial counties of England, ceremonial county in Northern England.The Unitary authorities of England, unitary authority areas of City of York, York and North Yorkshire (district), North Yorkshire are in Yorkshire and t ...

which are surrounded by the younger flat-lying Carboniferous Limestone. The location has long been visited by

geology Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth ...

students and experts. Another example from

South Wales South Wales ( ) is a Regions of Wales, loosely defined region of Wales bordered by England to the east and mid Wales to the north. Generally considered to include the Historic counties of Wales, historic counties of Glamorgan and Monmouthshire ( ...

is the Usk Inlier in

Monmouthshire Monmouthshire ( ; ) is a Principal areas of Wales, county in the South East Wales, south east of Wales. It borders Powys to the north; the English counties of Herefordshire and Gloucestershire to the north and east; the Severn Estuary to the s ...

where Silurian age rocks are upfolded amidst

Old Red Sandstone Old Red Sandstone, abbreviated ORS, is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the eastern seaboard of North America. It ...

rocks of

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

age. A similar outcrop pattern which results from movement on a

thrust fault A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. I ...

followed by erosion may be termed a

window A window is an opening in a wall, door, roof, or vehicle that allows the exchange of light and may also allow the passage of sound and sometimes air. Modern windows are usually glazed or covered in some other transparent or translucent ma ...

. 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

Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

around the

Staffordshire Staffordshire (; postal abbreviation ''Staffs''.) is a Ceremonial counties of England, ceremonial county in the West Midlands (region), West Midlands of England. It borders Cheshire to the north-west, Derbyshire and Leicestershire to the east, ...

town of

Leek A leek is a vegetable, a cultivar of ''Allium ampeloprasum'', the broadleaf wild leek (synonym (taxonomy), syn. ''Allium porrum''). The edible part of the plant is a bundle of Leaf sheath, leaf sheaths that is sometimes erroneously called a "s ...

. This is isolated from the very much larger area of Triassic rocks which characterise the

English Midlands The Midlands is the central region of England, to the south of Northern England, to the north of southern England, to the east of Wales, and to the west of the North Sea. The Midlands comprises the ceremonial counties of Derbyshire, Herefordshi ...

and Cheshire Basin, to the south and west respectively, by the surrounding

Carboniferous The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...

sandstones and

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...

s. Similarly in the Black Mountains of South

Wales Wales ( ) is a Countries of the United Kingdom, country that is part of the United Kingdom. It is bordered by the Irish Sea to the north and west, England to the England–Wales border, east, the Bristol Channel to the south, and the Celtic ...

, the summit area of

Pen Cerrig-calch Pen Cerrig-calch is a spur (mountain), subsidiary summit of Waun Fach in the Black Mountains, Wales, Black Mountains in the Brecon Beacons National Park in southern Powys, Wales. Its summit, at a height of 701 m (2,300 ft), is marked b ...

is composed from a suite of Carboniferous age sandstones and

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

. This outcrop is isolated from the main extent of these rocks (which form the

South Wales Coalfield The South Wales Coalfield () extends across Pembrokeshire, Carmarthenshire, Swansea, Neath Port Talbot, Bridgend, Rhondda Cynon Taf, Merthyr Tydfil, Caerphilly, Blaenau Gwent and Torfaen. It is rich in coal deposits, especially in the South Wales ...

and its margins some miles to the south) by the

on which it rests by and by which it is surrounded. Grenvillian inliers located within the Appalachian orogeny include the Pine Mountain Belt in Georgia, the French Broad

Massif A massif () is a principal mountain mass, such as a compact portion of a mountain range, containing one or more summits (e.g. France's Massif Central). In mountaineering literature, ''massif'' is frequently used to denote the main mass of an ...

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 Cape Breton Island (, formerly '; or '; ) is a rugged and irregularly shaped island on the Atlantic coast of North America and part of the province of Nova Scotia, Canada. The island accounts for 18.7% of Nova Scotia's total area. Although ...

, Nova Scotia, the Steel Mountain Terrane on Newfoundland, and the Long Range Inlier on Newfoundland. In West Africa, the Kenieba inlier borders southwestern

Mali Mali, officially the Republic of Mali, is a landlocked country in West Africa. It is the List of African countries by area, eighth-largest country in Africa, with an area of over . The country is bordered to the north by Algeria, to the east b ...

and eastern

Senegal Senegal, officially the Republic of Senegal, is the westernmost country in West Africa, situated on the Atlantic Ocean coastline. It borders Mauritania to Mauritania–Senegal border, the north, Mali to Mali–Senegal border, the east, Guinea t ...

, and associated Birimian gold is found in Kalana and Sabodala respectively. A similar isolated arrangement of rocks produced for example by movement on a thrust fault followed by erosion may be termed a klippe.Whitten D.G.A. & Brooks J.R.V. 1972, ''The Penguin Dictionary of Geology'', London, England

References

{{reflist Petrology Rock formations