While at Nisqually he wrote in his journal about nearby Mount Rainier and his desire to see it more closely. In August 1833 he arranged a "botanizing excursion" to the mountain, with Lachalet, a Nisqually, and Nuckalkat, a Puyallup, as guides. Three other Native Americans joined the party. They traveled through the thick forests, following the general course of the Puyallup River to the Mowich River and into what is today the northwest part of Mount Rainier National Park. Wanting to reach the snow level, Tolmie chose the nearest snowy peak and climbed it with Lachalet and Nuckalkat. Tolmie Peak is named for this event, although it is not known exactly which peak was summited. During the expedition, Dr. Tolmie discovered a new species of plant which is now known as Tolmie's saxifrage ("Micranthes tolmiei"). Due to this trip, Dr. Tolmie was the first European to explore the Puyallup River valley and Mount Rainier.