Moss Rock Preserve is within the Cahaba Ridges District of the Tennessee Section of the Valley and Ridge Province. The Preserve is bisected by Hurricane Branch which flows northeast through preserve. This small creek divides two big mountains.
To the south of the creek is Pine Mountain, an elongated ridge trending from the southwest to the northeast. The southwestern tip is a few miles away from the preserve at Brock Gap, which is very near the border of Jefferson and Shelby County. The northeastern end is near the preserve where the mountain meets Patton Creek.
The slopes of Pine Mountain in the preserve – which includes the boulder field – are comprised of the Pine Sandstone, a member of the Pottsville geologic formation. This is a light gray, quartoze sandstone that is almost pure quartz. Shale is also present between layers of the sandstone but, as it weather so quickly, it is rarely found at the surface.
Hurricane Creek cuts through a slightly different set of rocks known as the Pottsville Formation Undifferentiated. This formation contains shale and coal, plus a lithic sandstone – one whose grains are a mixture of quartz and other rock types.
Step across Hurricane Creek and you’re on Shades Mountain. This – the first large ridge to the south of Red Mountain – extends from Bluff Park to Irondale. Within Moss Rock Preserve one can study the Shades Sandstone, yet another member of the Pottsville Formation. Up close, the Shades and other sandstones at the preserve look very similar – they are classified separately based on age, geography, and other sedimentary rocks with which they are associated.
When did the Pottsville Formation originate? Most the rocks that formed during the Pennsylvanian Period in Alabama belong to this formation. That means these rocks are about 300 million years old!