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 water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is d ...

of overlying younger rocks to reveal a limited exposure of the older underlying rocks. Faulting 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. Features Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficia ...

pattern. A classic example from

Great Britain Great Britain is an island in the North Atlantic Ocean off the northwest coast of continental Europe. With an area of , it is the largest of the British Isles, the largest European island and the ninth-largest island in the world. It ...

is that of the inlier of folded

Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. T ...

and

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 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 shortest period of the Paleoz ...

rocks at Horton in Ribblesdale in

North Yorkshire North Yorkshire is the largest ceremonial county (lieutenancy area) in England, covering an area of . Around 40% of the county is covered by national parks, including most of the Yorkshire Dales and the North York Moors. It is one of four co ...

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 Earth and other Astronomical object, astronomical objects, the features or rock (geology), rocks of which it is composed, and the processes by which they change over time. Modern geology ...

students and experts. Another example from

South Wales South Wales ( cy, De Cymru) is a 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 Glamorgan and Monmouthshire, south Wales extends westwards ...

is the

Usk Inlier The Usk Inlier is a domed outcrop of rock strata of Silurian age in Monmouthshire in south-eastern Wales. It is located in the countryside between the towns of Caerleon and Pontypool and the village of Raglan. The longer axis of the dome or ' pe ...

Monmouthshire Monmouthshire ( cy, Sir Fynwy) is a county in the south-east of Wales. The name derives from the historic county of the same name; the modern county covers the eastern three-fifths of the historic county. The largest town is Abergavenny, wit ...

where Silurian age rocks are upfolded amidst

Old Red Sandstone The Old Red Sandstone 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 northeastern seaboard of North America. It also exte ...

rocks of

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, wh ...

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. If ...

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 mat ...

. 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 ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest per ...

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicat ...

around the

Staffordshire Staffordshire (; postal abbreviation Staffs.) is a landlocked county in the West Midlands region of England. It borders Cheshire to the northwest, Derbyshire and Leicestershire to the east, Warwickshire to the southeast, the West Midlands C ...

town of

Leek The leek is a vegetable, a cultivar of '' Allium ampeloprasum'', the broadleaf wild leek ( syn. ''Allium porrum''). The edible part of the plant is a bundle of leaf sheaths that is sometimes erroneously called a stem or stalk. The genus '' Al ...

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

English Midlands The Midlands (also referred to as Central England) are a part of England that broadly correspond to the Kingdom of Mercia of the Early Middle Ages, bordered by Wales, Northern England and Southern England. The Midlands were important in the ...

and

Cheshire Basin The Cheshire Basin is a late Palaeozoic and Mesozoic sedimentary basin extending under most of the county of Cheshire in northwest England. It extends northwards into the Manchester area and south into Shropshire. The basin possesses something of t ...

, to the south and west respectively, by the surrounding

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carboniferou ...

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 (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology.' ...

s. Similarly in the Black Mountains of South

Wales Wales ( cy, Cymru ) is a country that is part of the United Kingdom. It is bordered by England to the east, the Irish Sea to the north and west, the Celtic Sea to the south west and the Bristol Channel to the south. It had a population in ...

, the summit area of Pen Cerrig-calch is composed from a suite of Carboniferous age sandstones and

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms w ...

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

South Wales Coalfield The South Wales Coalfield ( cy, Maes glo De Cymru) 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, espe ...

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 The Alleghanian orogeny or Appalachian orogeny is one of the geological mountain-forming events that formed the Appalachian Mountains and Allegheny Mountains. The term and spelling Alleghany orogeny was originally proposed by H.P. Woodward in 195 ...

include the Pine Mountain Belt in Georgia, the

French Broad The French Broad River is a river in the U.S. states of North Carolina and Tennessee. It flows from near the town of Rosman in Transylvania County, North Carolina, into Tennessee, where its confluence with the Holston River at Knoxville forms ...

Massif In geology, a massif ( or ) is a section of a planet's crust that is demarcated by faults or flexures. In the movement of the crust, a massif tends to retain its internal structure while being displaced as a whole. The term also refers to a ...

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 (french: link=no, île du Cap-Breton, formerly '; gd, Ceap Breatainn or '; mic, Unamaꞌki) is an island on the Atlantic coast of North America and part of the province of Nova Scotia, Canada. The island accounts for 18. ...

, 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,, , ff, 𞤈𞤫𞤲𞥆𞤣𞤢𞥄𞤲𞤣𞤭 𞤃𞤢𞥄𞤤𞤭, Renndaandi Maali, italics=no, ar, جمهورية مالي, Jumhūriyyāt Mālī is a landlocked country in West Africa. Ma ...

and eastern

Senegal Senegal,; Wolof: ''Senegaal''; Pulaar: 𞤅𞤫𞤲𞤫𞤺𞤢𞥄𞤤𞤭 (Senegaali); Arabic: السنغال ''As-Sinighal'') officially the Republic of Senegal,; Wolof: ''Réewum Senegaal''; Pulaar : 𞤈𞤫𞤲𞤣𞤢𞥄𞤲𞤣𞤭 ...

, 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