Cieszyn is located within large geological unit – Outer Carpathian Mountains. The deeper substratum is built of Carboniferous rocks which occur at a depth of about 700 m in the Cieszyn area. They are overlined by Cretaceous-Tertiary rocks. Carpathian Mountains are built of flysch rocks, which show a specific process of sedimentation consisting in alternating deposition of sand layers and shale layers. Carpathian flysch consists of several series which become detached from the older stiff substratum, folded in several orogenic phases and thrust onto one another in the form of several nappes. The nappes then became faulted by the Carpathian Mountain foreground. Flysch sediments are locally covered by relatively thin Quaternary sediments – this includes the Cieszyn area).
      This part of the Silesian Plateau shows nappe tectonics. The nappes are aligned from the south towards the north with a small declination to the north-east causing faulting, which resulted in the development of a series of north-east strike-slope faults. In tectonic terms the following units are distinguished in Cieszyn Silesia:
• Podśląska Nappe,
• Silesian Nappe which is divided into:
- Cieszyńska Nappe
- Godulska Nappe
• Przedmagurska Flake
• Magurska Nappe
      The area of Cieszyn is mainly located on the Cieszyńska Nappe, which is built of Lower and Upper Cieszyn Shale, alternated with Cieszyn Limestone. These are all lower Cretaceous rocks.
      Lower Cieszyn Shale is strongly clayey, thick-fissile, with colouring from yellow-grey to black. It is overlain by platy-occurring limestone with moulds of organic origin.
 

Fig. l. Schematic cross-section of the nappes of flysch Carpathian Mountains between the Wisła and the Soła: l – Podśląska Nappe 2 – Cieszyńska Nappe. 3 – Godulska Nappe. 4 – Przedmagurska Flake. 5 – Magurska Nappe. 6 - Miocene. 7 – Upper-Silesian Coal Basin. 8 – Cieszyn-Andrychów Ridge (in R. Unrug (Ed.). Przewodnik geologiczny - Karpaty fliszowe między Olzą a Dunajcem, 1979).

      The limestone (of total depth 150 m) shows whitish, grey, often brown colours, alternated with yellow shales. Because limestone is more resistant to the weathering processes, it builds the hills’ ridges. Upper Cieszyn Shales with a depth of 300 m developed as shale sandstone and they contain insertions of sandstone or limestone. Their colour is rather dark, sometimes steel-grey. In Upper Shales there are dikes of interesting magmatic rocks called teschinite.

Teschenite originated as a result of magma cooling inside the Earth and the slow development of minerals in the form of crystals. They were described for the first time by Ludwik Kohenggeger, the author of the first map of Cieszyn Silesia published in 1891. Teschenite has many varieties which differ one from another in their chemico-mineralogical composition. The main components of teschenite includes amphiboles, pyroxenes, plagioclases, feldspars, biotites and olivines. These are different silicates of magnesium, iron, calcite and sodium, and also aluminosilicate of sodium and calcium which occur in different magmatic rocks. It is variously coloured (from white to yellow, grey, green, brown, red-brown and black) and forms grains, sticks, needles, lamellas, flakes and other shapes.
      The colour of teschenite is quite changeable, from white-grey, grey-green to quite black, and the texture of these rocks depends on the predominating one or two rock-forming minerals. In thicker dikes usually coarse grains are visible with a predomination of light plagioclases (the so-called leucocratic variation). In fine-grained teschenite dark components predominate (melanocratic variation). Dark minerals are not very resistant to weathering and therefore teschenite subjected to weathering processes are similar to sandstone and it easily disintegrates. The thickness of teschenite dikes varies from several centimetres to several tens of metres. Their length reaches several hundred metres. Teschenite became exposed due to denudation (washing off) of the overlying sedimentary layers.
      In the Cieszyn Plateau, teschenite occurs in the Cieszyn area (in Boguszowice and Kalembice), between Puńców and Mnisztwo, in Rudowo, Zamarskie, and in the area of Iskrzyczyn, Wiślica and Grodziec. Sometimes it occurs as small rocks or exposures and it is also possible to find it in old worked-out quarries.
      At present teschenite does not have any economic use, but several decades ago they were exploited and used as road building material in many localities in Cieszyn Plateau. Despite the fact that it is an interesting and rare rock, so far only the quarry on Goruszce Hill in Grodziec Śląski was protected as a site of natural heritage (established in 1958) and in 2002 a small exposure near ul. Kręta in Cieszyn was established as documentation site.

      In Upper Cieszyn Shale (and also in many other flysch shale) quite abundant insertions of iron ore occur. They form ironstone nodules or layers of clayey ironstone. They were intensively exploited for a long time (mainly in the 18th and 19th centuries) in the area of Ustroń, Cisownica, Leszna Górna and Puńców.
      In the north-eastern and south-eastern parts of Cieszyn, Cretaceous sediments are exposed on the surface. The exposed sediments are subjected to intensive weathering and erosion processes, which result in clayey soil. In other parts of the town Cretaceous sediments are covered by a thin layer of Quaternary sediments or clay.
      Quaternary sediments represent the most recent geological formation of Cieszyn Silesia. They are developed as deposits of river accumulation. These sediments locally contain erratics, pebbles, gravel, silt, clay, loam and loess. Carpathian gravel consisting of Godula and Ligota Sandstone occur in the Cieszyn area. The depth of gravel and sand-gravel sediments shows local changeability. In Cieszyn it can reach 4 m in the region of the Olza.
      Among natural resources which occur in the area of Cieszyn, the only important mineral in economic terms is the Krasna-Bielowiec deposit of building sand. This deposit is located within the hill between the Krośnianka, Krosna and Bilowiec (also called Bielowiec) streams in the eastern part of the town. This deposit is exploited under the terms of a mining licence.