When tectonic activity ceases or becomes very slow, external weathering and erosional forces gradually become the dominant factors in the sculpturing of the landscape. The influence of geology on the landscape passes to a more detailed level where even minor variations in the lithology and structure of rocks may have an influence on the landforms.
Joint Structures. All rocks develop joints as they consolidate and crack under the stresses set up by cooling or pressure changes. Both intrusive and extrusive igneous rocks develop joints in response to contraction while cooling, well exemplified by basalt. Sedimentary rocks usually develop joints at right angles to their bedding planes. Some jointing, patterns are, systematic (regular) while non-systematic joints are generally curved fractures which cross each other irregularly. Joint directions have a profound influence in guiding the course of erosional processes, which becomes reflected in the alignments in landforms. For instance, in many valley floors, streams follow joint directions, particularly in jointed igneous rocks and on flat-lying sedimentary rocks. It is of considerable significance to landform studies that joint frequency in most rocks apparently decreases with depth and that many joints are formed only when pressure on the rock is relieved by surface erosion, perhaps in association with uplift. This process is known as unloading or the pressure release mechanism. In sedimentary rocks, bedding planes open up and joints are formed at right angles to them. In rocks with no original structures, sheeting may occur, creating joints parallel to the existing land and surface.
The significance of joints in facilitating erosion is therefore complicated by the fact that the joints themselves seem to depend on erosion. This relationship may be self-perpetuating, since there is probably a tendency for distinct joint-orientated landforms, such as domes, canyons and cliffs, to persist. This is an example of feedback in geomorphology, whereby the form (the cliff) controls the process of unloading. which in turn determines the form.
"The world is a book, and those who do not travel read only a page", St. Augustine said. Here at www.travel-university.org we believe that every page must be read and explored. Travel is an avenue of learning that no text or classroom can teach. The world is a living classroom and you the student. We invite you to the www.travel-university.org library where you can read general interest and detail oriented articles.
