After leaving the Forest Service in 1928, Marshall worked to complete his studies for a PhD in plant physiology at Johns Hopkins University in Baltimore, Maryland. The following year he made his first trip to Alaska, visiting the upper Koyukuk River and the central Brooks Range, and preparing for an extended stay for study. The scientific objective of the trip was to study tree growth at the northern timberline near the Arctic Divide. For his 15-month sojourn in the small town of Wiseman, Alaska, Marshall rented a one-room cabin next to the only roadhouse in the village. He furnished it with books, records, a phonograph player, and a writing desk. He placed the desk so that he could sit by the cabin's single window and admire the view of the Koyukuk River and the range of steep, snow-covered mountains in the background. His travels engendered in him a great love for the central Brooks Range in the Alaskan wilderness. Marshall was one of the first to explore much of the range, especially the headwaters of the North fork of the Koyukuk River, where he bestowed the name Gates of the Arctic on a pair of mountains, Boreal Mountain (6,375 ft) and Frigid Crags (5,501 ft).