High on the rocky slopes above Field is a layer of sedimentary rock known as the Burgess Shale, which is famous worldwide, for it has unraveled the mysteries of a major stage of evolution. Encased in the shale, the fossils here are of marine invertebrates around 530 million years old. Generally fossils are the remains of vertebrates, but at this site some freak event—probably a mudslide—suddenly buried thousands of spineless animals (invertebrates), preserving them by keeping out the oxygen that would have decayed their delicate bodies.

Protected by UNESCO as a World Heritage Site, the two research areas are open only to those accompanied by a licensed guide. The Burgess Shale Geoscience Foundation (250/343-6006 or 800/343-3006, www.burgess-shale.bc.ca) guides trips to both sites between July and mid-September.

The access to Walcott’s Quarry is along a strenuous 10-kilometer (6.2-mile) trail that gains 760 meters (2,500 feet) in elevation. Trips leave Friday–Monday at 8 a.m. from the trading post at the Field intersection, returning around 6:30 p.m.; $100 per person.

Trips to the more easily reached Mount Stephen Fossil Beds depart Saturday and Sunday at 8:30 a.m., returning at around 4:30 p.m.; $75 per person. The trail to the Mount Stephen beds gains 520 meters (1,710 feet) of elevation in three kilometers (1.9 miles). The trails to both sites are unrelenting in their elevation gain—you must be fit to hike them. Reservations are a must.