P'tit Sault Blockhouse
Edmundston is located at the edge of the New Brunswick "panhandle," in the northeastern section of the Appalachian Mountains at the junction of the Saint John and Madawaska Rivers in the northwestern part of the province. When the land was first being settled, Edmundston was called Petit-Sault.
The P’tit Sault Blockhouse consists of a strategic site on a rocky hillock, where a defensive installation was built in 1841 overlooking the confluence of the Saint John and the Madawaska Rivers. The blockhouse is now a historic site. It was originally destroyed by lightning in 1855, and was subsequently rebuilt in the year 2000 to the original specifications.
Bedrock Geology & Orogeny Events
New Brunswick contains a diversity of bedrock types that range from 1 billion years to 200 million years old. In the Edmunston area the bedrock consists mainly of strongly folded, thick successions of calcareous and siliciclastic turbidites ranging from Late Ordovician to Early Devonian.
The Acadian orogeny is a long-lasting mountain building event which began in the Middle Devonian, reaching its peak in the early Late Devonian. It was active for approximately 50 million years, beginning roughly around 375 million years ago. The Acadian orogeny involved the collision of a series of Avalonian continental fragments with the Laurasian continent. Geographically, the Acadian orogeny extended from the Maritime provinces migrating in a southwesterly direction toward Alabama. However, the Northern Appalachian region, from New England northeastward into Gaspé region, was the most greatly affected region by the collision.
A familiar feature of orogens is the presence of a sedimentary basin, termed a foredeep basin, situated ahead of the orogen. The foredeep basins are filled in the main by the erosional detritus from the adjacent orogen.
Sedimentary basins develop in divergent, intraplate, convergent, transform, hybrid, and miscellaneous settings. Within each setting are several variants, dependent on type(s) of underlying crust, structural position, sediment supply, and inheritance. Subsidence of sedimentary basins results from (1) thinning of crust (2) thickening of mantle lithosphere (3) sedimentary and volcanic loading (4) tectonic loading (5) subcrustal loading (6) asthenospheric flow, and (7) crustal densification. Basins vary greatly in size, life span, and preservation potential, with short-lived basins formed in active tectonic settings, especially on oceanic crust, having low preservation potential, and long-lived basins formed in intraplate settings having the highest preservation potential.
Matapedia Basin
The Matapedia Basin is a large, (mainly) marine, composite (two-part) successor basin that extends through central Maine, northwestern New Brunswick and Gaspé Peninsula. The deep-marine sedimentary sequences that are present here were likely deposited in foredeep basins developed in front of rising nappes during closure of the Iapetus Ocean and the subsequent collision of the Gondwanan and Laurentian continental margins. The Matapédia basin consists of the uppermost Ordovician – lowermost Silurian deep-water, fine-grained carbonate–siliciclastic rocks. The Lower Devonian Temiscouata Formation of Madawaska County, northwestern New Brunswick forms part of a more extensive slate belt that extends from eastern Gaspe Peninsula to Long Island Sound.
The major rocks in the belt are dark grey slates and siltstones that weather dark grey or rusty brown. Both rock types have vague, light grey silty or sandy/ discontinuous parallel laminations or color bands. The rocks are locally calcareous; some are argillaceous limestone, the carbonate content of which weathers readily leaving the rocks with a silty texture.
Sedimentary Basin Complexes of New Brunswick
Thus, the bedrock here at the P’tit Sault Blockhouse originally formed from layer upon layer of fine silt and clay which later folded and transformed into slate. In slate, tiny, flaky clay minerals have recrystallized in alignment, resulting in surfaces along which rock can be easily split, or cleaved. The slates have a strong northeast trending cleavage and are aligned vertically, while the original sedimentary layering is roughly horizontal.
Tilted Beds and Strike and Dip
To measure and describe the geometry of geological layers, geologists apply the concepts of strike and dip.
Strike refers to the line formed by the intersection of a horizontal plane and an inclined surface. This line is called a strike line, and the direction the line points in (either direction, as a line points in two opposite directions) is the strike angle.
Dip is the angle between that horizontal plane and the inclined surface measured perpendicular to the strike line down to the inclined surface.
A useful way to think about strike and dip is to look at the roof of a house. A house’s roof has a ridge along the top, and then sides that slope away from the ridge. The ridge is like a strike line, and the angle that the roof tilts is the dip of the roof.
Strike and dip of a roof. The sloping roof of a building is a useful analogy to illustrate strike and dip. The ridge of the roof defines the strike of the roof. The roof dips away from the ridge with a characteristic angle (the dip angle).
In order to log this earthcache, please send a private message to the cache owner with the answers to the following questions:
*** All sincere efforts to answer the questions and complete the tasks will be accepted.
1. [REQUIRED] In accordance with the updated guidelines from Geocaching Headquarters published in June 2019, photos are now an acceptable logging requirement and WILL BE REQUIRED TO LOG THIS CACHE.
- Post a photo in your log of yourself or a personal item with the Blockhouse in the background to prove you visited the site.
2. What is a "panhandle" in geological terms?
3. How is a "panhandle" different from a peninsula?
4. What is argillaceous?
5. How is the cleavage of the slate aligned?
6. What is the strike and dip of the outcrop?