Gatekeeper Media presents drone footage of Boulder Field at Hickory Run State Park located in Lake Harmony, PA. Why did an 18-acre field of rocks form in the Poconos? The salient feature of the Pocono’s Hickory Run State Park is an 18-acre bed of rocks known as Boulder Field. In 1967, the Hickory Run Boulder Field was designated by the National Park Service as a natural national landmark. Most of the explanations for Boulder Field suggest it was formed during the last ice age that started 20,000 years ago and lasted about 5,000 years. At that time, a glacier that advanced southward from Canada, covering the United States from the Pacific to the Atlantic shores, came to rest less than a mile from Boulder Field.