During the Early Carboniferous period, which was about 330-340 million years ago, there was widespread volcanic activity in the area of the River Clyde Valley. Geological remains of this activity can still be seen today in the lavas of the Kilpatrick Hills, Campsie Fells and Renfrewshire Heights.
Dumbarton Rock was one of the volcanoes which poured out lava during the height of this volcanic activity. All that now remains is the basalt plug at the root of the long-extinct volcano.
For more detailed geological information about the Dumbarton Castle volcanic plug, please click on the image of the Castle above.