On the first page for The Big Valley, in the upper left-hand corner, there is a picture of the cast on the boxed set that you can purchase. If you look closely, Lee Majors is on there twice. Richard Long's picture is missing.
The series is apparently set during the administration of Ulysses S. Grant. However many of the years and dates mentioned during the series conflict with this setting.
Women during this time period did not have shoulder length hair. They did not cut their hair thus their hair should have been down almost to their waist. They always wore it in a bun and when going on in public they always wore hats. Yeah both the mother and the daughter of the show are seeing wearing their hair down constantly. The mother's hair was way too short for this time period.
The majority of the weapons used in the series were manufactured far later than its mid-1870s setting. In fact, many are from the 1890s and after the turn of the 20th century.
The fashions worn in the series reflected the 1960s filming era rather than its 1880s setting. For example, a proper woman such as Victoria Barkley would have never worn pants and nearly all of the men's clothing is period inappropriate.
The majority of the women on the series had their hair set using hairspray as well using techniques that were not available in 1870s America. In fact, hairspray was not invented until WWII and only came into regular use in the post-war years.
The majority of the males in the series were clean-shaven which was not only unfashionable for the time, but relatively rare in areas where access to clean water was limited.
Throughout the series, both Victoria and Audra wear pants clearly designed for the shape of a woman's body. They are quite obviously NOT men's pants. Women wearing pants before World War II would have been quite shocking for most of society. Thus, there would have been no reason OR market for women's pants. But women working in the war industries during World War II broke through this taboo. However those women wore pants constructed for men and designed for a man's body. It would not be until the 1960s that designers started to make pants designed to conform to the shape of a woman's body. Therefore, when Victoria and Audra wear WOMEN'S pants in The Big Valley, what they are wearing conforms more accurately the 1960's (the era that the series itself was filmed), and NOT the 1870's (the era in which the series is set).
Despite being "ladies" neither Victoria Barkley, nor her daughter Audra rode side-saddle. Both women rode on horseback like men, not how proper women of the time would ride.