Terschelling is approximately 30 kilometers long and the width varies from 2 to 5 kilometers. The first human settlements on the dunes, elevations and raised mounds in the salt marshes were formed around the year 800 AD. This is still clearly visible; the villages of Midsland, Landerum, Formerum and Lies are built on elevated parts of the surrounding polder area. The difference in height is less noticeable in the villages of Hoorn and Oosterend. Settlements that were lower, in the strip between Midsland and West-Terschelling, have since disappeared from the Terschelling landscape: Stattum, Schittrum and Stortum. The shape and size of Terschelling, just like the other Wadden islands, is still subject to change.

On the lee side of the islands, initially nothing more than bare, drifting sand heaps, over the years soil from rivers was deposited. There were muddy soils, clay-like salt marshes on peat layers from which later the meadows were formed and the polder was created. This created livelihoods for humans, agriculture and livestock, protected by dikes. The first embankment around the Terschellinger polder dates back to around 1000 AD. The dyke was improved in the 19th century and raised to Delta height in three phases between 1962 and 1968 after the 1953 flood disaster. The sea and the birds brought seeds and later helped by man via helmet pots, afforestation, agriculture, grazing etc. Terschelling (as well as the other Wadden islands) has grown into a nature reserve of very great value. The western point of Terschelling used to be a separate sandbank, called the Noordsvaarder, but was attached to the island around 1860 by silting up a rift (a channel through which ebb and flood water flows). At the time, the eastern point also consisted of various loose sandbanks that grew together and became one whole with the island in the 19th century. It is now a more than 4,400 hectare nature reserve, the Boschplaat.