Which text is valuable for gaining context about a historical event but is unreliable as evidence

Secondary historical sources.

Explanation:

Two types of sources, primary and secondary, have generally been represented in history. The primary sources are artefacts, documents, records, and other raw, source material, and they serve primarily as a source of information for the historian.

Secondary sources include quotes, comments and the like. For many historians, secondary sources create problems, they may be useful, but they need to be correctly interpreted. Because of their form, they are subject to change (because the quotation can be distorted over time and get another meaning), so they should be carefully handled. Therefore, they are distrustful of context and unreliable for proof.

the answer to your question is a. the salt plains

in 1811, he led an expedition, known as the george c. sibley expedition, to improve relations with the pawnee and kansa tribes, and also to locate the rumored jefferson's salt mountain

a. always outside of government control.