The Sharp Top sediments are diamictites, a conglomorate rock with stones of different sizes, suggesting they were formed by glaciers scraping together a debris pile that later consolidated into rock. The diamictites are exposed today in the Mechums River formation, southwest of Charlottesville near Batesville.
How could the earth totally freeze into a snowball? One theory, still controversial, is that the continents drifted into a position along the Equator. The reflection from the light rock (there were no plants on the land 750 million years ago...) and the pattern of ocean currents cooled the earth. Then erosion of the continents caused sediments to absorb so much carbon dioxide from the atmosphere that the "greenhouse effect" was dramatically reduced. The atmosphere, with little carbon dioxide, no longer trapped the heat from the sun. The earth cooled dramatically, and all the oceans froze completely.
At least, that's the theory...
Ice may have covered not just Virginia but the entire planet, perhaps comparable to the way ice covers Europa (a moon of Jupiter) today. For roughly 10 million years, life in the oceans was suppressed. More carbon dioxide was emitted into the atmosphere by volcanoes, and no CO2 was removed by photosynthesis. Eventually, the greehouse effect was triggered by the extra carbon dioxide, the earth warmed, and the global ice sheets melted - forming the distinctive sedimentary deposits (diamictites) in Albemarle County.
There may have been a series of dramatic extinctions, then re-blossomings of life after several freezing and warming cycles. The Cambrian "explosion" of life 580 million years ago may have brought an end to the cycles, in part due to the evolution of... bottom-dwelling worms. These animals have stirred/recycled the carbon in ocean sediments for the last 580 million years, perhaps maintaining the essential percentage of carbon dioxide and preventing another reduction in the ability of the atmosphere to retain heat from the sun.
NOTE: Patrick Michaels, once the State Climatologist of Virginia, considers much of the current concerns regarding global warming as overblown. Rejecting statements from the Federal government (especially the Environmental Protection Agency), he reports that the climatological records for the last century in Virginia do not demonstrate that the change in carbon dioxide in the atmosphere is creating any measurable greenhouse effect. Based on his interpretation of the data1, "the statewide average temperature and rainfall haven't changed a lick in the last 100 years. "
Whether or not the earth was a completely frozen popsicle, it is clear that colder climate changed the composition of the vegetation in Virginia in the most recent Ice Age . As shown by analysis of the pollen in the bottom of bogs in the Appalachians, the species we currently consider to be "northern" such as spruce and fir trees were common in Virginia 20,000 years ago. At Great Falls on the Potomac, the forest was composed of spruce and jack pine trees 16,000 years ago - comparable to forests along the Canadian border today. Pollen from oaks and hemlocks appears in quantity 12,000 years ago, and by 8,000 years ago the forests were similar to the oak-hickory forest of today.2