Getting there: From south MoPac, take the Loop 360 exit and head east. The first light includes a left turn lane which leads you into a parking lot for the office building next to the trail entrance.  Try to park as far to the left, close to the greenbelt entrance as possible.
The Hike: 
The start of the trail is marked on the map with the waypoint labeled "Trail 
Head". Here you'll find a large sign with a map of the entire Greenbelt. In 
total there are probably about 15 miles of trails listed. There are some 
additional trails not shown on the map, as you'll soon see.
The first part of the trail curves toward the creek itself and when you get 
there you'll be directly across from the creek from a large vertical rock face, 
marked on the map with a waypoint called "Rock Face". On this particular hike 
there were two groups of mountain climbers scalling different sections of the 
rock face.
The trail along this stretch of the creek is easy. Rather flat and wide 
enough for at least two to walk side by side. The established trail crosses the 
creek at a point on the map where "Gus Fruh Park" is printed (sorry, I forgot to 
set a waypoint). The creek was flowing VERY well that day, so we decided to stay 
on this side of the creek and turned towards a trail headed up along the hill 
slope. You can see this as our track turns to the right.  Note that the slope of 
the land on the other side of the creek (where the marked trail lies) is far 
more flat.
This part of the trail is a little tricky. It's obvious that many others have 
gone this way before, but it is not an officially marked greenbelt trail. Some 
portions are along very steep slopes where grabbing hold of a nearby tree branch 
is a good idea. Occassionally when the trees thinned out you're afforded great 
views of the creek below from 100-200 feet up (or so it seemed). It turns out 
that this is not the only way to continue hiking along this side of the creek, 
but it is the more challenging.
For the next 1/3 to 1/2 mile or so the trail hugs rocky slopes and shelves 
along the creek. Once or twice we thought the trail may have ended since it is 
not always obvious from a distance where it is. You'll be rewarded for 
continuing though.
Just before getting the last waypoint ("Water Falls") you'll come to two 
large rocks in the trail placed very close together through which you'll have to 
squeeze. If you're a big guy, breath in. Right after that you're at the bottom 
of a small valley fed by a stream. Another hiker told us about the waterfalls up 
hill so we followed the path of the stream up hill a good distance and we found 
half a dozen small waterfalls all along it's path. Some of the path is steep and 
started to get very muddy (read: slippery), so we stopped what appears to be 
less than half the distance from the top of the valley.
On the other side of the valley the trail continues up a very steep slope. 
Grab onto a tree and take your time. We stopped our trip at this point as we 
thought the trail ended (a couple was sitting on top of the bluff overlooking 
Barton Creek and we could not see any continuation. Looks like we were wrong 
though as another hiker on the way back told us about a rock bridge of some sort 
a little further along the trail that we thought ended. Next time we won't stop 
there.
On the trip back (not included in the track map) we hugged the creek bed more 
and found the easier route back. On our track just draw an imaginary line along 
the creek where we had earlier turned right towards the steeper slope.
The hike was only about a mile out, but it was hilly, challenging and very 
entertaining. If you hike Barton Creek be sure to take some time and turn off of 
the beaten path.