Contemporaneous advice from Colorado was to use a well-graded gravel, not exceeding  and including 25–40% fines passing a  sieve, but not more than 5% passing a #10 (2.0-mm) sieve.
Colorado recommended employing a well-graded gravel, characterized by a particle size distribution where no more than 40% of the material can pass through a certain sieve, with an additional constraint on the percentage passing through an even finer sieve.