North of Echert, hills scattered with pink,
blocky boulders
of gneiss, and granite are readily seen from the highway, which is the last of the central Precambrian portion of the Llano uplift.
North of the F .M. 965 junction to Llano, Texas 16 is in Valley Spring gneiss and Packsaddle schist terrain. Watch for low roadcuts where the light-colored gneiss looks like contorted, banded granite, whereas the schist is black and has platy minerals that sparkle in the sunlight.
Looking east from the junction of Texas 16 and F.M. 965 (the side road to Enchanted Rock State Park) the prominent skyline hills are a block of Paleozoic sedimentary rocks, which occupy a graben (down-dropped fault block) that was dropped into the Precambrian rocks at the southeast comer of the Llano uplift. The Paleozoic sedimentary rocks were actually more resistant to weathering than most of the surrounding Precambrian crystalline rocks, so they stand elevated here as a ridge line.
North of Echert, hills scattered with pink,
blocky boulders
of gneiss, and granite are readily seen from the highway, which is the last of the central Precambrian portion of the Llano uplift.
Look west to see the topographic edge of the Edwards Plateau, where the Cretaceous sandstone and limestone form a distinct cliff. The road ascends from the Llano basin proper climbing through a short segment of Hensell sand. Watch carefully for brown sandstone roadcuts for about a mile on either side of the small hamlet of Echert. This is Cambrian Hickory sandstone preserved in a faulted, down-dropped block on the edge of the Llano uplift.
North of Fredericksburg, Texas 16 traverses Cretaceous rocks, crossing about eight miles of poorly exposed, flat-lying, thin beds of tan-colored Hensell sand seen in hills on either side of the road. A nice view north into the Valley of Palo Alto Creek comes up at about 2 miles north of town.
mile 0.0 (30o 45.590; 98o 40.510) - Junction of 29, 16, 71
mile (30o ; 98o ) - Llano River Bridge. Turn right before the bridge and go to the park behind the Llaneaux Restaurant. Walk to the rocks southeast of the park. This is the Valley Spring Gneiss, a gray- and pink-banded gneiss with mineral elongation lineations on foliation surfaces that define broad, open folds . The gneiss is intruded by two generations of granite that either cross-cut or are folded with the foliation.
return to highway included in road log
mile (30o ; 98o ) - turn left to park on south bank of the Llano River; east of the bridge.
A virtual field trip of this location can be seen at the 2005 Field Trip, Lafayette Geological Society field trip.
return to highway included in road log.
mile (30o ; 98o ) - highway 71 departs to the east; this is the route of the Llano to Marble Falls trip
mile (30o ; 98o ) - highway 2323 departs to the southwest; this is the route of the Llano to Fredricksburg highway 2323 trip
mile (30o ; 98o ) - low roadcuts where the light-colored gneiss looks like contorted, banded granite, whereas the schist is black and has platy minerals that sparkle in the sunlight
mile (30o ; 98o ) - highway 965 to Enchanted Rock departs to the southwest. This trip will run from Fredricksburg to Enchanted Rock Park to this intersection.
mile (30o ; 98o ) - blocky boulders of gneiss and granite
mile (30o ; 98o ) - Hensell Sand in outcrop
mile (30o ; 98o ) - Hickory Sandstone , Cambrian in roadcut
mile (30o ; 98o ) - Hensell Sand in roadcut
mile (30o ; 98o ) - From Fredericksburg, the highway heads north across clayey, silty, sandy Cretaceous sediments to the limestone terrain of the Edwards Plateau. Lower Cretaceous limestones, deposited in the shallow seas that once spread across Texas 100 million years ago, form hillside ledges all around.
mile (30o ; 98o ) - A few miles north of town a vista to the north comes into view; to the left, west, is a ridge of flat-bedded limestone, but to the right, east, is a dark red, bouldery hill looking distinctly out of place.
mile (30o ; 98o ) - This steep- sided hill is Bear Mountain , which is composed entirely of pink granite, which is mined here for architectural stone.
The granite knob appears to be at about the same level as the limestone across the road. Did the granite penetrate the limestone? Or was Bear Mountain a huge knob on the Cretaceous seafloor against which the limestone was piled? To answer this question, we need to know the ages of the two rocks, limestone versus granite. As it turns out the granite crystallized a billion years ago, while the limestone is "only" about 100 million years old. So the granite being much older, could not logically have penetrated the limestone. That means the granite knob must have been a hill on the Cretaceous seafloor, and the limestone beds lapped onto it, eventually covering it. Erosion has removed the relatively softer limestone from around the granite, which now stands high as it continues to resist erosion.
mile (30o ; 98o ) - Stop at the picnic area north of Bear Mountain to get a close look at blocks of this beautiful, fine-grained granite.
mile (30o ; 98o ) - The road continues north in hill-arid-valley limestone terrain for another ten miles, only to turn an abrupt corner, where a gorgeous panoramic view of Enchanted Rock and the surrounding countryside is laid out from skyline to skyline.
mile (30o ; 98o ) - With an unimpeded view, the road twists downward across the topographic edge of the Edwards Plateau, toward the Sandy Creek valley below. It is clear here how the Cretaceous sediments were laid down directly on the surface of the Precambrian granites. But, it is also clear that quite a bit of topography existed on that surface, considering the large granite domes seen ahead.
mile (30o ; 98o ) - As the road crosses over Sandy Creek, look for black boulders in fields and along fences. These rocks are black schists which are the metamorphosed products of the shaly part of the original Packsaddle sediment wedge. These metamorphic rocks are full of platy minerals which formed when the shale was intensely heated far below the surface.
mile (30o ; 98o ) - The road continues on Packsaddle schist for a few more miles, paralleling the granite domes , which loom ever-larger as you approach the entrance to Enchanted Rock State Park.
mile (30o ; 98o ) - Enchanted Rock State Park is a marvelous piece of Texas landscape, where geologic features are clearly laid out in raw-rock profusion, devoid of soil or vegetation-or concrete cover. The view from the summit of Enchanted Rock, elevation 1,825 feet and 445 feet above Sandy Creek, is a gorgeous 3600 panorama . Most obvious at first view of the park is the dome shape of the rocks. However large as they may seem, Enchanted Rock's granitic domes are but a small side piece of a huge, round globe of granite which rose through the Packsaddle schist like a giant, hot balloon about a billion years ago. The circular shape of the granite body, a batholith , can be seen on the maps, but is too big to make out from road level. The overall geologic history of the Llano country, which includes Enchanted Rock, is discussed in the Llano uplift section.
The Enchanted Rock name comes from old Indian legends and pioneer observations of strange sounds and lights. Common creaking and groaning noises could easily be granite blocks grinding against one another as they expand and contract from heating and cooling between daytime and nighttime. Low light at dawn and sunset shimmers off crystals in the granite and the sparkling reddish domes blend almost magically with the early morning or late evening sky. Thus, geology explains legend, but should not reduce our sense of wonder and awe over natural phenomena.
The dome shape of the hills is the product of interaction of erosion with the geologic peculiarities of the granite. Note on the sketch map how the domes are separated by linear zones of fractures that intersect at nearly right angles. Search around in the low area between Enchanted Rock and Little Rock along the Echo Canyon Trail to find areas where blocky fractures show up quite nicely. Water and erosion attack these fractured areas with greater ease than the solid granite areas, creating valleys along the fractures, while leaving solid granite to form the high areas.
The rounded shape of the domes is caused by exfoliation . Granite forms deep within the earth's crust; as thousands of feet of rock overburden are removed by erosion over geologic time, pressure caused by the weight of this pile of rock is reduced. The granite expands a little in response to the lessened pressure, which in turn causes the granite to split in curved sheets. Weathering cracks the sheets, creating blocks and slabs, which slide downslope. Exfoliation sheets and blocks are beautifully displayed on the north flank of Little Rock, and can be seen from the flanks of Enchanted Rock.
As you climb around, look for odd-shaped rock pedestals, created by the differential disintegration of the wetted, lower portion of boulders, where chemical weathering attacks the rock faster than the dry boulder top.
Note also the round weathering pits, or pools, where standing water causes chemical weathering to disintegrate the granite. These pits, filled with water after rains, are important water sources for animals in many desert areas of the world.
The surface of Enchanted Rock is crossed by long linear bumps and rills . These are caused by dikes which filled cooling cracks in the early granite batholith. As it crystallized, the granite shrank, cracks developed, and hot liquid from the last phase of the magmatic mush rushed in to fill the cracks. If the filling material is finer-grained than the surrounding granite, it is slightly harder, so it stands up to erosion forming a little ridge. But, if the granitic dike material is coarser grained, it weathers more easily, creating a little depression in the surface rock. These dikes are particularly noticeable on the face of Enchanted Rock.
Many other fine points of erosion, weathering, and granite rocks are to be seen by the careful observer. For those interested in more detailed information to help the search, purchase the well-written and illustrated pamphlet by James F. Peterson, entitled Enchanted Rock State Natural Area - A guide to the Landforms, published in 1988 by Terra Cognita Press, San Marcos, Texas. The book is available for a few dollars at the State Park entry building. And, while you are there, spend some time viewing the displays of Enchanted Rock geology. They are extremely well presented and informative.
mile (30o ; 98o ) -
mile (30o ; 98o ) -
mile (30o ; 98o ) - Looking eastward at the junction with Texas 16, the prominent skyline ridge ahead is a fault-block of Paleozoic rocks that has been downdropped into the Precambrian in the southeastern part of the Llano uplift.