Under Compton Peak, a rock with a disappointing view located in Northern Virginia (USA), hides a marvel : the basalt column in the Catoctin Formation.
The basaltic lavas formed there, about 700 million years, basaltic floods.
Rifting or other kind of tectonic extension cause huge lava flows that flooded the existing landscape, surrounding and isolating the granite hills. The rapid cooling of the exposed lava caused his prismation.
Lava, volcanic ash and sediment material accumulated before the cessation of volcanic activity, were submerged under the advancing of the sea during the Cambrian, in the early stages of the formation of the Appalachian geosyncline.
The columns, to the origin of hexagonal or at least polygonal cuts, have changed over time in cylindrical form ... this is called " spheroidal weathering ", particularly striking when it meets on geometric shapes.