Looking at the Marble Falls map I linked to above, it shows a VERY bizarre twist between topo maps and Google maps. Google maps implies there is a road to Crystal Cave that climbs straight up the incredibly steep mountain slope. It appears they have drawn a straight line between the Marble Falls trail and the old abandoned Colony Mill road that appears as a trail on the topo maps.

To compare, look at the Marble Falls map link, and switch between the "t4 Topo High" choice and "t1 Terrain". The Terrain (or "Map" or "Hybrid") are Google Maps version of the area with this road superimposed.

Here are two embedded Gmap4 maps of the same location:









Here's the Google Maps "Get Directions" to the Crystal Cave location.