The Arches is located within the Humber Zone which is the most western of the 4 zones and is next to the Dunnage Zone.

Techtonostratigrapic Zones (East to West)

1. Avalon Zone

2. Gander Zone

3. Dunnage Zone

4. Humber Zone

All the rocks within the park are a part of the Humber Arm Supergroup which includes the Curling Group, Northern Head Group and the Cow Head Group. A southeasterly high angle fault is also located in The Arches Provincial Park. 

The nearby Long Range Mountains are comprised of crystalline basement rock and are much older than the surrounding lowlands, the area is of Appalachian orogen. The mountains are considered a massif but are comprised of several inliers where older rocks are surrounded by younger ones and were reactivated during Appalachain deformation.


The rocks associated with the inlier are a part of the Humber Zone and are considered the foreland belt of Appalachian orogen. The highest point of the mountains is bounded in the south, west, north and locally east by proterozoic/paleozoic volcanic sedimentary cover rocks. The southern portion of the Long Range inlier is split into several different tectonic divisions:

. High-Grade Long Range Gneiss Complex

. Foliated Plutonic Rocks, Mostly Grenvillian in age

. Mafic Dikes (Long Range Dike Swarm)

. Thin remnants of Neoproterozoic to Early Paleozoic cover sequence 

. Early Silurian gabbroic intrusions (430 Ma Taylor Brook gabbro)

. Minor felsic dikes, sills, and porphyries

. 541/250 million-year-old cover sequence quartzite-marble-dolomite 

Overthrusting started in the Middle Ordovician and was possibly accentuated in the Devonian period, the thrusting came to an end during Appalachain deformation events. Compression and folding are greatest to the west of the overthrust and lies at the base of the Long Range Mountains but dies out further towards the western and eastern sides of the range. 

The area is influenced by the drainage of rivers from the Long Range Mountains, streams pass through several deep glacial lakes at the base of the mountain front. Those Rivers, Portland creek and Parson's Pond reach the Gulf Of St. Lawrence and take eroded sediment to the bay. 

The Arches


The surrounding area around The Arches but not visible in the park is the Lower Head Formation, it is placed within the Middle Ordovician and is comprised of greywacke which is dark-coarse grained quartz and contains more than 15% clay interbedded with grey/green conglomerate and shale. The sandstone in the Lower Head Formation is medium-to thick-bedded with a tectonic contact above the formation.

The formation was rapidly deposited during syn-depositional deformation, the erosion of the underlying section of the Cow Head Group which created a small unconformity. The Lower Head was deposited by massive slope failures and considered turbidite deposition, this could have occurred as sediments on the continental shelf slide to the ocean floor. 


The rock formation that makes up The Arches is a part of the Cow Head Group and is a mix of three different formations, the younger Lower Head Formation, Shallow Bay Formation and the Green Point Formation. The lower portion of the Shallow Bay and Green Point Formations are located in the Furongain Period while the upper zone is placed within the Lower Ordovician.  

The group is a succession of conglomerate, shale, limestone and sandstone. The limestone was deposited in shallow water while the intervening conglomerate layers were placed in deeper waters along with mudstone.

The Arches is comprised of conglomerate, limestone, mudstones and shales which have formed in the lower continental slope of Laurentia (The North American Craton which is a stable interior of a continent comprised of crystalline basement rock). The limestone platforms are formed from the death and decay of living organisms as their skeletal remains build up a reef.


Limestone broke from the carbonate platform at the rim of the continental margin and tumbled down the slope where it stayed which is why house-sized blocks of limestone are sometimes visible in the formation.


The Shallow Bay Formation is comprised of 4 different members:

. Downes Point Member:

Consists of a series of conglomerates 100 meters thick that range in age from Middle/Late Cambrian.

. Tuckers Cove Member:

60 Meters of interbedded quartzite calcarenite conglomerate and minor ribbon limestone (abrupt lateral changes in rock types), sandstone, siltstone and shale which are Middle/Late Cambrian in age.


. Stearing Island Member:

Series of mega-conglomerate 80 meters thick with minor interbedded limestone, quartzose calcarenite and shale which was deposited in the Upper Cambrian and Lower Ordovician or Upper Tremadoc.


. Factory Cove Member: 

The member consists of a 100-meter thick sequence of ribbon/parted limestone with minor shale punctuated by beds or boulder conglomerate/mega-conglomerate and was deposited in the Late Arenig (Lower Ordovician Epoch 1.6 million years old). 


The Arches formed from the exploitation of joints and fractures within the limestone of the Shallow Bay Formation, certain areas are eroded more than others which is where the arches form. Once the supports have been eroded away the arch than collapses on itself as the supports cant hold the weight of the above rock.

Once a window has opened within the rock formation it expands and eventually forms an arch, to be classed as an arch the opening must be three feet in one direction. Gravity forces layers of loose rock to fall from the ceiling and collapse, then wave action moves the sediment away along with weathering the base of the rocks. 



There was once an ancestral mountain range that was worn down by river, wave and wind action, the mountain range broke down further during the last 1-2 million years as several glacial episodes eroded the area more.


Those glaciers melted around 8,000 years ago and the sea level rose and covered the area, the wave action and seawater fractured the limestone/shale and exploited the rock. Sea Level then dropped to current and erosion by waves continue. 


In the future, The Arches will be reduced to sea stacks or rock pillars this will take several thousands of years to happen but it is in the park's future.  


Geological Index Maps of Newfoundland and Labrador

The Arches, Newfoundland and Labrador, CAN


S. HENRY WILLIAMS AND ROBERT K. STEVENS Summary account of the Lower Ordovician (Arenig) graptolite biostratigraphy of the Cow Head Group, western Newfoundland. Geol. Soc. Denmark, vol. 35, pp. 259- 270, Copenhagen, July 1st, 1987

Svend Stouge, Gabriella Bagnoli and Duncan McIlroy With contributions from J. Maletz, W. Douglas Boyce, Ian Knight and Sebastian Scorrer and D. McIlroy 

Cambrian – Middle Ordovician Platform-Slope Stratigraphy, Palaeontology and Geochemistry of Western Newfoundland Open File NFLD/3325, St. John’s, Newfoundland, June 2017, for The International Subcommission on Ediacaran Stratigraphy (ICES) The International Subcommission on Cambrian Stratigraphy (ISCS), Geological Museum, National History Museum of Denmark, University of Copenhagen,  Department of Earth Sciences, University of Pisa (Italy), Department of Earth Sciences, Memorial University of Newfoundland. Department of Earth Sciences, Freie Universitat, Berlin,  Geological Survey, Newfoundland Department of Natural Resources, St. John's, Newfoundland

Stratigraphy and correlation of the Cambro-Ordovician Cow Head Group, western Newfoundland; James, N P; Stevens, R K. Geological Survey of Canada, Bulletin 366, 1986, 143 pages (4 sheets), Open Access 

Williams, H. and Cawood Geology, Humber Arm Allochthon, Newfoundland MAP_NUMBER1678A

Geological Survey of Canada 1989: Geology, Humber Arm Allochthon, Newfoundland. Map 1678A. Scale: 1:250 000. Geological Survey of Canada. GS# NFLD/1852

Mark Cooper, John Weissenberger, Ian Knight, Doug Hostad, Derek Gillespie, Henry Williams, Elliott Burden, Janet Porter-Chaudhry, Don Rae and Elizabeth Clark Basin Evolution in Western Newfoundland: New Insights from Hydrocarbon Exploration PanCanadian Petroleum, Newfoundland Geological Survey, Hunt Oil, Memorial University and Mobil Oil

Types of Arches

Arches National Park, Utah. National Park Service, USA

Philip Oxley, Hon. F.W.Rowe Minister Of Mines and Resource, C.K Howse Deputy Minister Of Mines, GEOLOGY OF PARSONS POND ST. PAULS AREA, WEST COAST St. John's. Newfoundland, Geological Survey Report No. 5 1953, PROVINCE OF NEWFOUNDLAND, DEPARTMENT OF MINES AND RESOURCES MINES BRANCH.

Charles Schuchert, Carl O. Dunbar Stratigraphy of Western Newfoundland  Geological Society Of America 1934 Memoir 1

Alana M. Hinchey and Ian Knight Geological Surveys Branch, Newfoundland and Labrador Department of Natural Resources The southern Long Range inlier, Newfoundland: evidence for early Appalachian thrusting of Paleozoic metasedimentary rocks within a Proterozoic basement massif Atlantic Geology Volume 47, 2011, AAPG Datapages/Archives