Established by Presidential Proclamation: May 11, 1908 by President Theodore Roosevelt
Abolished as a National Monument: August 24, 1937 by the 75th Congress
Wikipedia:
https://en.wikipedia.org/wiki/Lewis_and_Clark_Caverns
Montana State Parks:
National Parks Traveler
Proclaimed in 1908, Montana's Lewis and Clark Cavern National Monument never could get the monetary support needed to develop it is a viable unit of the National Park System. Congress abolished it in 1937.
National Park Service
The Lewis and Clark Cavern National Monument was established May 11, 1908, to preserve the limestone cave discovered in 1895 by D. A. Morrison, of Whitehall, Mont. It was so named because it overlooks, for a distance of over 50 miles, the trail of Lewis and Clark along the Jefferson River. At the time the monument was established the land reserved was unsurveyed and its location within the section, township, and range therefore not conclusively determined. After the survey of the land another presidential proclamation, dated May 16, 1911, was issued definitely delimiting the reservation. The total area of the monument is 160 acres.
https://www.nps.gov/parkhistory/online_books/glimpses2/glimpses17.htm