This collection documents the life of Lieutenant Joseph Clough Morrill and the 3rd Regiment of the California Infantry Volunteers during the Civil War. The collection dates between 1825 and 1903, although the bulk of the documents are from 1862 to 1866. It consists of both personal letters and photographs as well as several official military documents.

The material is arranged into six series:

Series I: Old West Military Papers dates between 1861 and 1866. It consists of approximately 650 military-related documents, mostly handwritten, and many with printed headings from Company B, 3rd California Infantry in the Civil War and immediately after the war when they were still fighting Native Americans in Utah Territory and Nevada Territory. Many are signed and written by Morrill. Included are Circulars from the Commissary Departments of the West and in Missouri, handwritten and printed orders, quarterly and monthly returns of clothing, ammunition, etc., camp and garrison equipment, clothing receipt rolls, inspection reports, marriage certificate (Morrill married a couple in Utah Territory in 1864), resignations, court-martial charges, an 1866 letter requesting a pension for a man who lost an arm during the war, ratings of officers (including Morrill) and other company materials.

Series II: Old West Mining Papers dates between 1881 and 1886. It consists of approximately 275 documents relating to the Gardner and Morrill Developing (later Mining) Company. The papers mostly consist of reports and pay documents. Many are printed with advertisement headings including railroad. Most are from Salt Lake City, Utah Territory.

Series III: Oversize Military Records includes large format military papers and records, similar to that of Series I, dating from 1862-1866.

Series IV: Utah Territory and Colorado Territory Areas dates between 1864 and 1866. It consists of 28 official documents relating to actions in the Civil War in the United States' western territories during and shortly after the war. They detail clothing and provisions, the movement of the battalion between Colorado and Utah, and the names and signatures of soldiers and civilians both in and working for the 3rd California Infantry.

Series V: Fort Bragg, California and Fort Union, California Areas dates between 1862 and 1863. It consists of nine official documents relating to actions in the Civil War in the California area at the beginning of the war. They detail provisions, equipment, and forage for their horses and oxen, as well as the names of people employed at Fort Bragg.

Series VI: Other Personal Papers and Correspondence, dates between 1825 and 1903. It consists of approximately 25 documents relating to Morrill including pension papers, GAR papers, pre-war papers from New Hampshire, and Loyal Legion booklet, program, and letters to Morrill. There is one letter regarding his school in San Francisco and one in 1853 describing the city of San Francisco. There are also various certificates including one signed by Alex Ramsey, governor of Minnesota and U.S. Senator, a signature of Thomas Arthur Lloyd, governor of Utah from 1889-93, two post-war cartes-de-visite of Morrill (circa 1873), and a cabinet photo of him taken in New Hampshire circa 1885. There is also a copy of a tribute to him on his 75th birthday in 1902 and his death notice in 1903.