Complex sentence: They usually yield close to quarry-quality fossils, and require no digging to access, but the fossils have been exposed to the weather and have been picked up and transported by large vehicles, thus some fossils are scraped and gouged.
Split into simpler sentences: They usually yield close to quarry-quality fossils, and require no digging to access. One problem with these sites is that the fossils have been exposed to the weather and have been picked up and transported by large vehicles, thus some fossils are scraped and gouged.