The Harz is a so called Mittelgebirge, a low mountain range or hill country, 180 km long and 30 km wide. This is the German term for all areas which are mountainous but not Alpine. The highest peak is the Brocken, 1,142 m asl, a granite massive formed by a huge intrusion of lava.
The Harz is rather steep at its borders and has a high plateau which is hilly. Around Clausthal-Zellerfeld it is called Oberharz (Upper Harz) and lies between 600 m and 800 m asl. The Unterharz (Lower Harz) is the eastern part and lies between 500m asl near Rübeland to less than 400 m asl north of Sangerhausen. The hinterland of the Harz is below 300 m asl.
The Harz runs northwest to southeast, a direction which was - in geology - named after the Harz and called hercynian. Many important geologic structures in Germany follow either this direction, or the nearly orthogonal northeast southwest direction which is called variscian. These adverbs are used for characterizing the striking of layers or faults. If you have a look at the geologic map, you can see the geographic alignment, and the connected faults at the northern and southern rim, being hercynian. But the internal structure runs variscian.
The Harz was formed by an uplift, which affected the whole area of the Harz. The uplift started during the lower Cretaceous (140-97Ma) and stopped in the upper Cretaceous (97-67Ma). The first movements are called Early Kimmerian Phase (140Ma). Most of the uplift happened during the Subhercyne Phase (83 Ma). In this phase the limestone layers of the northern hinterland were tilted upright or upside down by the movement.
The forces at the northern rim were much bigger, so the northern lifting was higher and the impact at the fault more intensive. The Harz is very steep at its northern rim. At the southern rim the overlying rocks ly flat on the crystalline and run out without being overturned or even tilted.
The crystalline rocks of the Harz can be found in the hinterland, at a depth of about 4,000 m below surface. Once the rocks had a cap of younger rocks, like in the neighborhood, at least until the uplift started. Then the lifted Harz formed an island in the Cretaceous sea and the rocks were subject to weathering. All younger sediments down to the upper Perm were eroded.
From the early Middle Ages, the Harz was famous for his ore deposits. The mining of the Rammelsberg made the city of Goslar famous. The mined ores were sulfidic lead (Pb) and zinc (Zn) ores, which contained many other metals in low percentage. But those rare and valuable metals, like gold and silver, made the mine so important. Coinage of the metals took place in Goslar, and so the mine provided a continuous flow of money into the caskets of their sovereign.
A second kind of deposits is found in the Oberharz (high Harz) near Clausthal-Zellerfeld. Vaults with ore, striking hercynic (northeast to southwest). This are hydrothermal deposits, formed by water circulating through the rock with high temperature (about 380 °C). This water was able to dissolve the extremely small amounts of metal in the rocks and transport them to the vaults, where they were redeposited.
The Harz consists of rather old rocks which are heavily alterated and faulted by old orogenies. Then there are numerous important deposits of valuable ores, which show different origins. And this in a rather boring hinterland of young, horizontal Triassic and very young glacial sediments. This made the Harz a predestined target for intensive and fruitful geologic work.
The area around Goslar is often called "klassische oder goldene Quadratmeile der Geologie" (classic or golden square mile of geology) by geologists. This means, that on a small area an enormous amount of different and important geologic features can be found. This proverb was not - as often said - coined once by Johann Wolfgang von Goethe, but he repeated it and made it more popular. The square mile is obviously not one of the modern miles, it is most likely a 7.5 km long Prussian Mile.
The Harz has an extraordinary versatility of its geology on a rather small area. From the Silurian slates to the upside down lacustrine limestones from the Trias, spectacular plutonites like the Brocken massif and metamorphic rock, pillow lava and various ore deposits. Several geologic terms were named after local specialties. The term hercynic is derived from the Latin form of the name Harz. The greywacke, originally Grauwacke is a term used by the local miners for worthless rocks, derived from the grey colour and pudding like structure (Wacke). The ultramaffic rock Harzburgit was named after its locus typicus Bad Harzburg at the northern rim of the Harz, near Goslar.
The Harz has several small limestone and gypsum areas with different and difficult geology. Small areas of lower Devonian limestones, former corall reefs, are found at Bad Grund and Herzberg. Around Rübeland is a larger area of Devonian limestones, which are used for making cement. But most rocks of the Harz are not soluble, like the granite of the Brocken massif, lower Carbon slates, greywacke and quartzite.
The Gypsum Karst at the southern rim of the Harz is described on its own page.