The actual boundaries of the Ruhr vary slightly depending on the source, but a good working definition is to define the Lippe and Ruhr as its northern and southern boundaries respectively, the Rhine as its western boundary, and the town of Hamm as the eastern limit. In the Middle Ages, local power was vested primarily in the counts (Grafen) of Berg, Mark and Cleves. The left bank of the Rhine was held by the Archbishop of Cologne. The Hellweg was an important trade route crossing the region from Duisburg to Dortmund and beyond as far as the rivers Weser and Elbe. The most important towns of the region were concentrated along the Hellweg. As a result of the Congress of Vienna, the entire area came under the control of Prussia (the state had already gained possessions there). This event was almost concomitant with developments which would eventually make the region one of the most important industrial areas in the world. In 1946, the state of North Rhine-Westphalia came into being centred on the Ruhr. Nowadays, its hitherto important coal and steel industries have drastically declined and the region is in a state of re-adjustment. Carboniferous period During the Carboniferous Period, in the Paleozoic era, which began 360 million years ago and ended 300 million years ago, layers of slate, coal and sandstone were formed. 400 to 300 million years ago, new mountains were uplifted during the Variscan mountain-building period. During the Silesian period, layers were deposited which became seams of coal over a period lasting millions of years. During this period there was a constant shifting between marshy conditions and overflowing seas such that the depositing of plant material and sediment from the sea resulted in the current situation of coal layers separated by stone layers. The main representatives of flora in the coal marshes were of the genus Lepidodendron and genus Sigillaria, tree-like plants, which belong to the plant classification Lycopodiophyta. Members of both genus reached heights of up to 40 meters with a trunk diameter of over a meter. Cretaceous Period In the Cretaceous, 135 million to about 66 million years ago, the region was submerged under a tropical ocean. In its waters lived ammonites. On the floor of the sea, a thick layer of marl formed. The sediments covered the layers of carbon and contained also the shells of giant ammonites. Quaternary The Ice Age brought changes