At first, heavy rains confined them to the west side of Keystone Canyon where they cut brush and graded trail. Heiden apparently did not go high on the ridge like Cleave, but looked for a lower route. However, finding a route suitable for horses through the canyon proved so difficult, they had to make 3 camps in 2-1/2 miles.

Once on the other side, they found traveling across the four miles of flats relatively easy. Heiden named this area "Dutch Flats" in honor of the three Dutchman who had helped them earlier in the summer.
 

Capt. Abercrombie, who was an excellent fund raiser, promptly named the pass for Senator Frank Thomson of Pennsylvania who supported the military's efforts to find an ice-free trail from coastal to interior Alaska. However, Schrader misspelled Thomson's name on his 1898 map inserting a "p", so the pass has been known as Thompson Pass ever since.

Thus, by the end of the summer of 1898, Capt. Abercrombie was able to report that the Copper River Exploring Expedition had found an ice-free route across the coastal mountains and a probable trail to Copper Center. Mission accomplished.