Driving from Fredericksburg to Enchanted Rock, the first granite that you see is at the Bear Mountain Quarry. The quarry is the reddish area on the hill to the right of the road. Bear Mountain is an isolated granite knob completely surrounded by Cretaceous sedimentary rocks. The hill on the left, which is taller than Bear Mountain, is made up of Cretaceous limestone. The low ground around the hills is made up of easily weathered Cretaceous sandstones [show me the map].
Back in the Cretaceous when the sedimentary rocks were being deposited, Bear Mountain was already a high spot. An island in the sea with sandstones as beaches and limestones being deposited in deeper water. So Bear Mountain was a mountain before there were bears. The granite island situation might have been analogous (on some scale) to what I have heard about parts of Virgin Gorda in the British Virgin Islands.
Bear Mountain Quarry is operated by the Cold Spring Granite Company, although various quarry operators have been working there since 1904. A type of granite called "Radiant Red" when sold for building stone or "Mountain Red" when sold for monument, is produced there. Radiant Red has a rather different appearance than the typical Llano Uplift granite, in both color and texture.
A polished slab of Bear Mountain Granite |
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click on the image for a larger picture of this granite |
Geologist Note: While I don't have access to any of the granite outcrop, I have looked at a thin section from this site. The granite shows low temperature ductile deformation features that are so far unique to this locale, although published descriptions of several granites occurring along the southern margin of the Llano Uplift describe similar features. Note the weak foliation visible in the polished slab, diagonal from lower left to upper right. Unfortunately, there is not a radiometric age date for the Bear Mountain Granite.
