Geology & Geodiversity

The Carboniferous Limestone bedrock of the area was deposited during the time period known as the Dinantian, 360 million years ago (Ma) to 325 Ma. Deposition of the rocks of the AONB occurred over some 15 million years up to approximately 330 Ma, when the region was located near the equator. At this time the area was dominated by shallow tropical seas, with varying sea levels and changing sedimentary conditions (giving periods of clear water then episodes of sediment-laden waters).

There are three principal limestone formations underlying the Arnside and Silverdale AONB, and they unify its character and shape its landscape. The three formations are in decreasing age: the Dalton Beds, the Park Limestone and the Urswick Limestone (Lower and Upper). Additionally the overlying and younger Gleaston Formation is exposed in a few places. A pattern of repeated deposition within the differing lithologies demonstrates a marked cyclicity with repeating rock sequences being apparent. The three blocks exhibit different lithologies and appearances: the Dalton Beds are interbedded limestones, sandstones and mudrocks, the Park Limestone is thinly bedded and the Urswick is relatively massively bedded.

Additionally there are recurrent beds of mudstones and shales. The most extensive is the Woodbine shale that outcrops in a number of locations around the AONB and is close to the boundary between the Upper and Lower Urswick limestones. These differing lithologies and the structural movements within them have ensured the development of a wide variety of landforms and contributed to the formation of many different karst landscape features. Strong regional faulting and associated folding within the limestone split the local area into a series of steep sided upland blocks, such as Arnside Knott, that are separated by low-lying basins. This pattern of landform, which is so characteristic of the AONB, has greatly influenced the way in which the AONB landscape has been settled, used and developed.

Limestone Features

The most important landscape features and natural resources of the AONB are those associated with the Carboniferous Limestone outcrops. A large number of typical karst landscape features can be identified in and around the AONB. Many of the most characteristic features of the area, such as the limestone pavements, result from the chemical weathering by solution along bedding planes and joints of the Carboniferous Limestone bedrock.

Importantly in addition to recent karst features, formed during the Quaternary geological period, there is an excellent example of a palaeokarst landform, exposed in Trowbarrow LNR. Palaeokarst surfaces represent episodes of sub-aerial exposure of newly formed limestone beds that have been exposed to weathering and soil deposition, either through tectonic uplift or falling sea level. Surface features on the exposure in Trowbarrow show evidence of palaeo-soils and plant root systems as well as karst features.

Geodiversity

What does the word Geodiversity mean? It’s a fairly new word, and like biodiversity, which came into existence as a shortened form of Biological Diversity, Geodiversity is short for Geological and Geomorphological Diversity…. (little wonder that we needed a short expression for the concept). 

Geodiversity may be defined as the variety of rocks, fossils, minerals, natural processes, landforms and soils that underlie and determine the character of our landscape and environment. Geodiversity is the foundation of the environment we live in. It provides the building blocks of life, forms the source of much of the world’s wealth, is an important factor in our cultural identity and plays a vital role in our future development. It links people, landscape, biodiversity and culture, and is a vital natural resource.

An appreciation of geodiversity is also needed to have a full understanding of an area’s biodiversity as it has a fundamental influence on the location and type of habitats that form and therefore on what species are found. It also has an important impact on the economic activities and history of settlement in any given place. It is fundamental to how our natural environment works and, consequently, to the way we live and work.

Whilst the AONB is geologically limited, being almost entirely Carboniferous limestone in composition, there are a large number of important and diverse features and attributes within the limestone terrain that give the area an unexpected range of geological interest. It is a nationally and regionally important geodiversity resource with features that are of great scientific value and general interest, including sites that are regarded by geologists as among the most important limestone localities in northern England.

The AONB’s geology and geomorphology show many different characteristics associated with a complex geological history. There are four sites of Special Scientific Interest (SSSIs) which are of particular geological importance: Trowbarrow Quarry, Gait Barrows, Hawes Water and Hale Moss Caves and approx 15% of the AONB is covered by Limestone Pavement Orders, over a total of 16 sites.

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