Question

The following passage is adapted from an essay on twentieth-century paleontology:

In August 1909, while conducting geological fieldwork in the Canadian Rockies near Field, British Columbia, American paleontologist Charles Doolittle Walcott made a remarkable discovery on the slopes of Fossil Ridge. While traversing a trail between Mount Wapta and Mount Field, Walcott's pack horse stumbled on a slab of dark shale. Splitting the stone, Walcott uncovered exceptionally preserved fossils of soft-bodied organisms from the Middle Cambrian period, dating back roughly 508 million years. Unlike typical fossil sites, which preserve only hard mineralized structures such as shells and bones, the Burgess Shale retained delicate internal organs, appendages, and soft tissue outlines compressed between thin layers of sediment.

During subsequent field seasons between 1910 and 1924, Walcott returned repeatedly to the quarry that now bears his name. Assisted by his family, he extracted over 65,000 specimens, meticulously documenting each find in his field notebooks. Walcott categorized these ancient organisms—such as the bizarre five-eyed Opabinia and the spiny Hallucigenia—within existing modern taxonomic groups, classifying them primarily as early primitive ancestors of living marine annelids and crustaceans. Although later paleontologists in the 1970s, led by Harry Whittington, re-examined the specimens and argued that many represented entirely novel, extinct phyla unrelated to modern lineages, Walcott’s initial recovery and painstaking preservation of the quarry's fossils established the site as one of the most significant paleontological discoveries of the twentieth century.

According to the passage, Walcott first encountered the fossil-bearing slab of shale when which of the following occurred?