Morton continued to serve in the House and as chairman of the RNC until 1971, when he was nominated to be Secretary of the Interior by President Nixon. He was unanimously confirmed by the United States Senate Subcommittee on the Interior, though some concerns regarding Morton's record of environmental protection were raised by Phillip Berry, then-President of the Sierra Club. Morton's predecessor, Walter Joseph Hickel, had been chosen over Morton to fill the job in 1969, but was fired by Nixon in late 1970 for criticizing White House policy. Morton was the only person from the east coast to serve as Interior Secretary in the 20th century.  Upon assuming the office of Secretary of the Interior, Morton promised he would seek to "purify the environment". However, as his tenure progressed, he was gradually isolated from the proceedings of the White House and lost several major divisions to other departments. During the 1973 oil crisis, for example, the oil and natural gas divisions of the department were transferred to the Office of Emergency Preparedness. Furthermore, though Morton was the one who announced the construction of the Trans-Alaska Pipeline System, speculation was put forward that he did not play a major role in determining its route. On August 1, 1974, Morton approved Anza-Borrego State Park for inclusion in the national registry of natural landmarks.  As he was leaving his post in 1975, further criticism was directed at Morton for his overseeing the partial dismantling of the Interior Department. He was also criticized for lacking the vigor necessary to serve as head of a cabinet department, and for failing to see through the proposed creation of a new, stronger cabinet department that would have replaced the Interior.

Answer this question "what was his greatest accomplishment?" by extracting the answer from the text above.
Morton was the one who announced the construction of the Trans-Alaska Pipeline System,