Hamra National Park is located in an area of great geological interest that is referred to as the Loos-Hamra channel.
The bedrock is made up red and reddish-grey porphyric granite in the southern part of the park and of pale granite-granodiorite in the northern part. In the Svartåmyran swamp there is a 500-metre-wide band of basic, metamorphic sedimentary rock with the appearance of gneiss.
There are also elements of volcanic rock, metallic vulcanite or porphyry. Along the Svartå river the bedrock is exposed for long stretches. There is a narrow band of crystallized limestone here as well as a band of metamorphic quartz feldspar porphyry of volcanic origin