I’m flying down the hill, on a mountain-biking high, when boom! My tires skid and I tumble back to reality in a twisted heap of limbs and metal. “Nothing like taking a soil sample,” Dave Taylor, my tour guide, jokes as he pulls me onto my feet. I take stock of my injuries — some reddening bruises, a scraped knee — but all in all, worth the exhilaration of soaring through Virginia’s George Washington National Forest at speeds perhaps slightly inadvisable for a beginning mountain biker.