The Marsdiep is a deep tide-race between Den Helder

Den Helder

Den Helder is a municipality and a city in the Netherlands, in the province of North Holland. Den Helder occupies the northernmost point of the North Holland peninsula...

 and Texel

Texel

Texel is a municipality and an island in the Netherlands, in the province of North Holland. It is the biggest and most populated of the Frisian Islands in the Wadden Sea, and also the westernmost of this archipelago, which extends to Denmark...

 in the Netherlands

Netherlands

The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...

, and running southwards between sandbanks. That gap connects the North Sea

North Sea

In the southwest, beyond the Straits of Dover, the North Sea becomes the English Channel connecting to the Atlantic Ocean. In the east, it connects to the Baltic Sea via the Skagerrak and Kattegat, narrow straits that separate Denmark from Norway and Sweden respectively...

 and the Waddenzee.

Around 1000 AD and before, much of the modern Waddenzee and IJsselmeer

IJsselmeer

IJsselmeer is a shallow artificial lake of 1100 km² in the central Netherlands bordering the provinces of Flevoland, North Holland and Friesland, with an average depth of 5 to 6 m. The IJsselmeer is the largest lake in Western Europe....

 was land and the Marsdiep was an ordinary river. An early form of its name is Maresdeop, a name probably related to modern Dutch moerasdiep ("swamp deep"). During the All Saints' Flood (1170)

All Saints' Flood (1170)

The All Saints' Flood of 1170 was a catastrophic flood in the Netherlands that took place in 1170. Large parts of the Northern Netherlands, and Holland territories were overflowed. This is the flood that created the Zuiderzee...

 the sea broke through the original dune barrier and created the channel. For more information see Wieringen

Wieringen

Wieringen is a municipality in the Netherlands, in the province of North Holland. It consists of a former island, also named Wieringen, and there are plans to make Wieringen an island again by widening the Amsteldiepkanaal into a lake called the Wieringerrandmeer.-Population centres :The...

.

The Marsdiep was in the age of sail

Age of Sail

The Age of Sail was the period in which international trade and naval warfare were dominated by sailing ships, lasting from the 16th to the mid 19th century...

in English called the Texel, in distinction from the island of Texel.