White Rocks

Location: Boulder County
Size: 105 acres
Designated: 1979
Landowner: Boulder Open Space & Mountain Parks

The White Rocks Natural Area takes its name from the outcroppings of light-colored Fox Hills sandstone exposed by the undercutting action of Boulder Creek.   About 65 million years ago, as the Rocky Mountains were being uplifted, a large inland sea covered the central portion of what is now the United States.  Large sediment-rich rivers emptied into this sea, creating deltas.  The striking and beautiful sandstone cliffs at the Natural Area are a portion of one of these ancient deltas.  The surface of the sandstone displays interesting patterns referred to as turtlebacks. Researchers relate these patterns to fractures caused by the regular wetting and drying of the rocks.

Fairy shrimp dwell in pools that form when rainwater accumulates in the shallow depressions among the turtlebacks.  These crustaceans are uncommon in Colorado.   They complete their life cycle in a few short weeks, laying drought-resistant, encysted eggs which dry into the dirt at the bottom of the pools.  The eggs hatch the following spring when temperature and moisture conditions are right.  Several other interesting invertebrates occur at White Rocks, include the mining bee (Perdita opuntiae), which feeds upon the pollen of prickly pear cactus (Opuntia spp.) and builds its nests in holes which it excavates in the sandstone.  One early naturalist described this insect as "the bee who works in stone".  In addition, the area is home to several rare ant species, including one (Aphaenogaster fulva) that is rarely found west of the Mississippi River.

White Rocks is also home to the only known population of the Black Spleenwort fern (Asplenium adiantum-nigurm) in Colorado. Although quite rare in North America, this plant is also known from Hawaii, Europe, Asia and Africa!   One explanation for this wide distribution is that the fern’s microscopic spores may be carried high into the atmosphere where the jet stream distributes them around the world.  Several eastern deciduous forest relicts can be found in a moist alcove below the black spleenwort, including American groundnut (Apios americana) and Joe Pye weed (Eupatorium maculatum).  The rare grass, fork-tipped three awn (Aristida basirema) also grows in cracks on top of the cliffs.  This species is known from a very few sites in Colorado.

While the cliffs themselves have many interesting features, the context of this Natural Area is also important.  The cliffs are situated along Boulder Creek near many acres of riparian forest and gravel pits which have been reclaimed specifically for wildlife value.  The area is private property, and human access is strictly limited by the landowner.  The variety of habitats coupled with very low levels of human activity has produced a place with an abundance and diversity of life.

This natural area is privately owned, and not open to the public.  For further information contact the Colorado Natural Areas Program