Brunkebergsåsen was an esker

Esker

An esker is a long winding ridge of stratified sand and gravel, examples of which occur in glaciated and formerly glaciated regions of Europe and North America...

 that once reached over much of Stockholm

Stockholm

Stockholm is the capital and the largest city of Sweden and constitutes the most populated urban area in Scandinavia. Stockholm is the most populous city in Sweden, with a population of 851,155 in the municipality , 1.37 million in the urban area , and around 2.1 million in the metropolitan area...

's Norrmalm district. Geologically, it is a part of the much larger Stockholmsåsen

Stockholmsåsen

Stockholmsåsen, is a 60 km long esker stretching from southern Uppland to northeastern Södermanland, i Stockholm County, Sweden. The esker extends from Arlanda to Jordbro and Västerhaninge, passing through Stockholm where it is known as Brunkebergsåsen....

. It formed a considerable obstacle to traffic, effectively dividing Norrmalm into a western and an eastern part. Consequently, most of it has been dug away over the centuries to make room for the development of that district. The pedestrian tunnel Brunkebergstunneln and, since the 1910s, the eastern part of Kungsgatan

Kungsgatan, Stockholm

Kungsgatan is a street in central Stockholm.At its western end it is connected to Kungsholmen by Kungsbron bridge, from where it stretches east to Stureplan public square. It is intercepted by the streets Vasagatan, Drottninggatan, and Sveavägen...

 cut through Brunkebergsåsen's southern part. Conspicuous remnants of the esker can bee seen in the vicinity of Johannes kyrka, at Observatorielunden

Observatorielunden

Observatorielunden is a park in Stockholm's Vasastaden district. It stretches over the steep hill Observatoriekullen, one of the last remnants of Brunkebergsåsen, the esker that once reached across much of Norrmalm but was mostly levelled during the development of that district...

, and Vanadislunden

Vanadislunden

Vanadislunden is a big park area in Stockholm, Sweden, located in Vasastaden.The area is named after Vanadis, which is another name for the Norse Goddess Freyja....

.

Further reading

Essay, with several maps, on the city's development in Stockholm City Museum

Stockholm City Museum

The Stockholm City Museum is a museum documenting and exhibiting the history of Stockholm. The museum is housed in Södra Stadshuset at Slussen on Södermalm. The building was completed in 1685...

's publication CITY, Del I Byggnadsinventering 1974-75, pp 9–37 (Swedish)