Letters, mostly received by Thomas P. Roberts (TPR) in the course of his professional career, along with some family exchanges. Early letters dated from 1857 to 1863 are from family members, including TPR's mother, uncle, brother, and father, William Milnor Roberts. These letters pertain to family matters and personal reflections on the war and its causes. Of particular interest is the letter from TPR's brother, John B. G. Roberts, who attended a funeral in Washington, D. C. in 1861 and mentions meeting Abraham Lincoln. Postwar letters from 1866 to 1879 are mostly from William Milnor Roberts to various parties concerning engineering projects, including the survey for the intended route of the Northern Pacific Railroad. Letters dated 1881 and beyond are from a variety of engineers and businessmen to TPR, including John Marston Goodwin and Eban Brewer. Rough drafts and carbon copies of outgoing letters from TPR cover a wide variety of topics, including family matters, engineering problems, and the naming of the engineering building at Montana State University for his father. The most recent letters concern Roberts's descendants. All have been chronologically arranged.

Five small manuscript diaries created by William Milnor Roberts while engaged by Brazil's Dom Pedro II for the construction of a railroad in that country. The diaries contain personal information as well as technical notes on the project. The final diary, created by WMR in 1865, is actually a small memoranda book that contains random notes composed during his voyage back to the United States after the completion of his stay in Brazil. The Thomas P. Roberts diary contains detailed entries of his day to day activities in 1867, a time when he served as an assistant engineer under his father while engaged in a comprehensive survey of the Ohio River for navigation improvement.

Contracts, financial notes, appointment certificates, and miscellaneous financial notes pertaining to Thomas P. Roberts or his father, William Milnor Roberts. Documents include partnership agreements and property settlements for WMR in his Brazilian work, court documents for lawsuits involving TPR as a witness, contracts and property statements for TPR's personal property, and miscellaneous financial receipts for business and personal expenses. All have been chronologically arranged, and oversize materials removed to Box 5 have been noted.

Manuscript essays and reports composed by Thomas P. Roberts and others. The documents created by TPR cover a wide variety of topics, including his abiding interest in the causes of the Ice Age and its effects in North America, local history, religion, and engineering projects along the Ohio and Monongahela Rivers. TPR also gathered material dealing with his family's genealogy, and even wrote a novelette about Ice Age people. One essay, written toward the end of his life, gives TPR's views on Einstein's theory of relativity. Three additional essays, written by William Milnor Roberts, John Marston Goodwin, and Emile Low, present thoughts on engineering projects in Brazil and Pennsylvania. The essays have been arranged in chronological order, with those by TPR preceding the last three created by others.

Printed reports, pamphlets, broadsides, and newspaper clippings written by Thomas P. Roberts and others, collected by TPR or his descendants. Many of the materials in this series were either written by TPR or mention him and concern engineering projects such as the Lake Erie and Ohio River Ship Canal, the navigation exploitation of the Allegheny and Monongahela rivers, railroad construction, and the general topic of early American civil engineering. The first seven folders contain offprints of journal articles or pamphlets written by TPR. The following folders have been arranged in alphabetical order by the names of the creators and include serial publications that include articles written by TPR and others on the above mentioned topics. The newspaper clippings and scrapbooks are extremely fragile and have been removed for separate storage in oversize box 5. Photocopied surrogates are available for research use.

A formal 1912 portrait of TPR, with detailed information regarding its creation written on the back; several views of TPR with colleagues; images of TPR's stepmother and brother-in-law; an image possibly of the steam boat T.P. Roberts; several views of engineering and industrial sites in Pennsylvania.

Manuscript and printed maps, including a detailed 1824 chart of the route of a Pennsylvania canal and other canals and railroads. An unidentified house plan, which may be that of TPR's home, details landscaping plants, and two later dated printed maps show early automobile routes through western Pennsylvania. Fragile oversize scrapbooks and two large printed appointment commissions are also in this series.