The shore and the entrance to the canal are strengthened by huge dikes.
Neuwerk, containing some marshland protected by dikes, has two lighthouses and a lifeboat station.
A few hornblendeschists are metamorphosed gabbros; others have developed from dikes or sills of lamprophyre.
Most of the island is occupied by the band of the old rocks, which include mica, glaucophane and sericite-schists and slates; there are small intrusions of granite, and numerous dikes and masses of basic eruptive rocks.
All the volcanic rocks of these islands are submarine stratified tuffs which are penetrated here and there by andesite or diabase dikes.