Current archaeological evidence points to the Frisian coast or Western Jutland as the possible birthplace of this type of vessel. The transformation of the cog into a true seagoing trader came not only during the time of the intense trade between West and East, but also as a direct answer to the closure of the western entrance to the Limfjord. For centuries, the Limfjord in northern Jutland offered a fairly protected passage between the North Sea and the Baltic. Due to its unusual geographical conditions and strong currents, the passage was constantly filling with sand and was completely blocked by the early 12th century. This change produced new challenges. The larger ships, which could not be pulled across the sand bars, had to sail around the Jutland peninsula and circumnavigate the dangerous Cape Skagen to get to the Baltic. This resulted in major modifications to old ship structures, which can be observed by analyzing the evolution of the earliest cog finds of "Kollerup", "Skagen", and "Kolding". This caused a boom in the number of small cogs, and the need for spacious and seaworthy ships led to the development of the cog as the workhorse of the Hanseatic League. It soon became the main cargo carrier in Atlantic and Baltic waters.