While its exact location is still a mystery, Lucan’s cursory mention of the Tarpeian rock along with his description of the Temple of Saturn (Phars. III,154) has led some to identify it with the cliff that overlooks the modern-day Piazza della Consolazione at the south-west corner of the Capitoline. The height of the cliff is 60 feet, although it may have been greater in ancient times, when the rock face would have continued down past the current level of the road. A Google Maps view of the site can be found here.
You know, I hadn’t noticed until I started reading through your vocabulary notes just how much the early sections stress the rock’s sheerness and describe it in so many different ways.