While it was admittedly rare to find a woman playing in one of the heavy underground bands, there was no shortage of women in Rock as a whole in the early 1970's. Many bands, particularly of the Folk and Progressive variety, had a mixed gender line-up, usually because they wanted to make use of the strengths of both vocal styles. Renaissance had Jane Relf, and the always weird and exciting Gong had a variety of vocalists over the years (including drummer Mireille Bauer). Also on the Avant Garde side of things in a similar vein to Gong, the irascible Dagmar Krause was a core member of both the oddball outfit Henry Cow and the more traditional Slapp Happy. The rarely mentioned yet often applauded Fuchsia also had female members in the mix (Vanessa Hall-Smith, Janet Rogers, and Madeleine Bland), who provided backing harmonies and an excellent string section for their 1971 debut).
Of course, there were still plenty of Garage style bands out there. The Feminine Complex, an all-girl Psychedelic Pop band, put out their sole album near the end of the 1960's. The Ace of Cups and She were both still rocking as the 1970's rolled around, and each had their moments of wild heaviness, but both bands would eventually fold after the toll of years weighed them down, and as mainstream success constantly eluded them.
As the 1960's gave way to the 1970's, Psychedelic Rock, The Blues and Folk Rock all seemed to merge into what would be called "Progressive Rock". Of course, the bands that leaned more towards the Psychedelic and Blues side of things were dubbed "Heavy Metal", although most people now prefer to call them "Hard Rock" to avoid mixing them up with the modern spectrum of Metal. At this time, the Progressive band Curved Air began to make waves, with Sonja Kristina as their frontwoman. And of course, when mentioning female-fronted bands from this era, it would be a crime to not recognize the very talented veterans of Jefferson Airplane, with the iconic Grace Slick at the helm. In an interesting twist, rather than have their sole female member languish in the background, the Hard Rock band Ramatam had April Lawton in front and center as she stunned the world with her lead guitar skills. There was also the cult band Amon Düül II, with Renate Knaup as their lead vocalist. And lastly, there's the very Groovy and heavy band Affinity, with Linda Hoyle as their lead vocalist (thanks for introducing me to them, Psy-Fi!).
By the original definition of Heavy Metal, and the sound that the phrase originally referred to, almost all of these bands managed to capture it with one song or another. Ramatam's brand of Hard Rock, as well as Affinity's excellent guitar solo in "I Am and So Are You", Curved Air's "Stretch" (along with the other video above, which is a fantastic live recording, but you have to find it on youtube to hear it as it doesn't play after being embedded), and the Ace of Cups' "Stones", are as rough and Blues-y as much of what would come out of the underground scene of the time. Although they aren't quite as dark and heavy as what would be put out by game-changers like Black Sabbath, or as furious as the songs made by acts like May Blitz, Dust and The Pink Fairies, they still have the quintessential essence of the grit that caused Psychedelic bands to make the leap to Progressive Rock, and eventually to the original wave of Heavy Metal.