Human history in North Cascades National Park and the surrounding region begins 8–10,000 years ago, after the end of the last glacial period. Paleo-Indians slowly advanced from Puget Sound into the interior mountain region as the glacial ice retreated. Archaeological evidence from other sites hundreds of miles away from the park indicate that Hozomeen chert, a type of rock well-suited to the fabrication of implements, was mined from near Hozomeen Mountain, just east of the park border, for the last 8,400 years. Tools such as microblades made from Hozomeen chert are part of the archaeological record throughout the Skagit River Valley, west of the park and in regions to the east. Prehistoric micro blades 9,600 years old have been discovered at Cascade Pass, a mountain pass that connects the western lowlands to the interior regions of the park and the Stehekin River Valley. The microblades are part of an archaeological assemblage that includes five distinct cultural periods, indicating that people were traveling into the mountains nearly 10,000 years ago. As well as the archaeological excavation at Cascade Pass, there are another 260 prehistoric sites that have been identified in the park.