Field Trip to Baraboo 

Baraboo area is an ideal location to study the region geology. Here, we can observe igneous, sedimentary, and metamorphic rocks. We can investigate the geologic structure. We can also study the structures associated with sedimentary and metamorphic rocks.

The figure below show our routes and stops for this Baraboo trip. For an enlarged figure, click the map below.

A topographic map of Baraboo area can be seen below. You may click the image to view a larger image.

A geologic map is made on top of topographic map. Each geologic unit is represented by a unique pattern together with a unique color. The geologic map of Baraboo area can be seen below. You may also click the map to see an enlarged map. However, the file is quite large (1 MB). Thus, if you are using a slow modem you may experience long time loading.

STOP 1 -- LOWER NARROWS -- East Side.

From the roadside at this point one can see through the Lower Narrows of the Baraboo River into the basin to the south formed by the Baraboo syncline.  The Lower Narrows is one of several "water gaps" in the area.  There is much evidence that many present valleys in the area are exhumed early Paleozoic ones.  It is probable that the Lower Narrows is such an old valley, although clear evidence is lacking here. This gap is cut in the south-facing and near-vertical or steeply north-dipping quartzite beds of the north limb of the Bamboo syncline.  Based on subsurface data north-south fault through the gap was proposed. 

There is a historical marker on the road side. There are two huge rocks,  sitting on each side. One is the Barabbo quartzite and the other is a rhyolite. They all have reddish color. Look carefully, one can seen that one has white quartz veins (the rhyolite) and the other show sand grains with cross-bedding (the quartzite). 

On the north slopes of the North Range on either side of the Narrows are the most extensive outcrops of rhyolite in the area. The rhyolite structurally overlies the inverted quartzite, thus is thought older than the Baraboo quartzite. But the contact is not visible, and no rhyolite pebbles have been positively identified in the quartzite. 

Cross the bridge - and proceed to rhyolite  outcrops in low cliff to the east of the house. The red, quartz-veined rhyolite at this location (elsewhere it is black) can be taken for quartzite at first glance.

Continue on Wisconsin Highway 33 across bridge over Baraboo River. Junction County U at end of bridge; turn on County U and then immediately onto Fairfield Road to the left. Proceed. We are now on the north side of the overturned Baraboo quartzite and we can see outcrops of black rhyolite and later Paleozoic Galesville Sandstone.

STOP 2 -- FAIRFIELD ROAD -- North Side.

Black rhyolite can be seen here. Notice that this stop is on the north side of the North Range. The range is made of Baraboo quartzite. The layer of the Baraboo quartzite is nearly vertical on the north limb of the Baraboo syncline. If one try to unfold the syncline, the rhyolite  would occur below the Baraboo quartzite. 

STOP 3 -- FAIRFIELD ROAD -- North Side.

Yellow colored massive sandstone can be seen here. The cement of the sandstone is not as strong as that of the quartzite. Thus, the sandstone undergoes physical weathering more easily. Pout some water onto the sandstone, notice that the sandstone would absorb water easily. This is due to the porosity of the rock. While for quartzite, the porosity is cemented by re-crystallized quartz. Sandstone is normally a good aquifer to store and transmit groundwater.  

STOP 4 -- UPPER NARROW -- Van Hise Rock.

The rock consists of two current-bedded massive vertical quartzite beds, which face south and sandwich a darker, fine-grained phyllitic layer. 

Well developed cleavage dipping gently south in the quartzite is refracted into a phyllitic cleavage dipping north at about 40° in the darker bed. X-ray patterns of powdered samples from the phyllitic layer here and others in the Baraboo Quartzite reveal an assemblage quartz-pyrophyllite.  This is indicative of the lower greenschist facies of regional metamorphism and temperatures not greater than 410-430°C.

Is Van Hise Rock in place?  Examining the outcrops west of the road along strike reveals no equivalent phyllitic bed.  However, two massive quartzite beds wall a vertical cleft and pinch together westwards.  Presumably the phyllitic layer formed a lens of sedimentary or tectonic origin, which has been weathered away.  The structural elements in Van Hise Rock have exactly the attitudes of their counterparts elsewhere in the North Range. 

The phyllitic and quartzite cleavages in Van Hise Rock are correlated with the prominent cleavage surfaces seen in phyllitic and quartzitic strata respectively throughout the syncline and here designated S1 and S1’.  Their attitudes are typical of the North Range and there can be no doubt that they are closely related to the formation of the Baraboo Syncline.

STOP 5 -- ROCK SPRING QUARRY -- Ripple mark, Buttress Contact, and Breccia Zone.

At the "Rock Springs City Limit" sign, walk up lane to right (west) into an old quartzite quarry. Above and to the right is a ripple-marked vertical face in the quartzite.  To the far right is a curious breccia zone in the quartzite.  This is best developed on the spur separating this quarry from the larger one to the south (0.25 miles south of Van Hise Rock).

The ripple mark shown in this picture occurs vertically, indicating the the original bedding has been tilted about 90°.

Breccia zone.  The occurrence of brecciated quartzite at this locality has been known for many years.  Slacks of red quartzite from 1/4 inch up to two feet across and mostly very angular, are set in a stockwork of white quartz crystals.  The latter are encrusted with dickite, a two layered, monoclinic mineral of the kaolinite group, especially in vugs.  Some of these vugs now form cavities a few feet deep in the quarry walls. The breccia zone is over 100 yards wide and runs vertically to the top of the cliff.  Similar breccia occurs 100 yards further north on the cast side of the Upper Narrows leading to suggestions of a north-south fault through the Narrows.  There are also occurrences one mile to the west in an inlier on the south side of the North Range, in the Chicago and Northwestern Railroad quarry, and at the southeast end of the Narrows Creek gorge.  In the past faults have been drawn along the North Range in the upper Narrows-Narrows Creek area, and through the Upper Narrows, on the basis of the breccia occurrences. There is no strong evidence of faulting in the breccia zones.

Note the Cambrian conglomerate veneer on top or the south and west walls.  It is only a few feet thick for the most part, but fissures as much as 5-10 feet deep are filled with boulders (see south face).  Clasts range up to three feet in diameter, and are rounded.

STOP 6 -- HIGHWAY 12 -- Ripple Marks, Cleavages, and Boudins.

At eleven o'clock is the mouth of Baxters Hollow, wherein a granite and diorite complex lies structurally beneath the Baraboo quartzite. Relative age relations are somewhat uncertain, as are Rb-Sr dating results.

Large outcrop of quartzite to right of road, with exceptionally clear current bedding, some of which is the trough type.  In the south end of the exposure, there are several zones of contorted cross stratification formed by synsedimentary deformation (probably resulting from loose packing and shear by a sudden current pulse). This outcrop exemplifies the north-to-south paleocurrent pattern of the Baraboo quartzite as a whole. 

Above the small quartzite cliff  (and also on the west side of the road) the phyllitic zone in the upper Baraboo quartzite has well developed structures of all phases of deformation clearly exposed. Very thin phyllitic partings occur in the quartzite and S1 cleavage is near parallel to the gently north-dipping bedding and at right angles to the steeply south-dipping Sl' cleavage in the quartzite.  This is an excellent locality at which to observe the essentially parallel down-dip lineation the surfaces of the phyllitic cleavage and bedding.  DO NOT HAMMER THIS ROCK!
Above the prominent quartzite cliff is the phyllitic zone of the upper Baraboo quartzite.  This can also be seen on top of the hill west of the highway 100 yards northeast of the farm. Locally - as at road level to the north of the quartzite out one can see the conjugate north-dipping crenulation cleavage. The north and south-dipping S2 surfaces have an interesting relationship to boudins in thin quartzitic layers within the phyllite.  Note that some of these boudins are deformed by the chevron folds and therefore related to an earlier phase of the deformation.

At the extreme upper (and southeastern) corner of the outcrop there is a fascinating and perplexing structure in the phyllite (picture below).

Ripple marks can be seen on the surface of quartzite. Dr. Li is explaining the use of ripple mark to tell the direction of waves that generated the ripple marks. A close look of the ripple marks on the south side of the Baraboo syncline reveals that the north side of the ripples (downward) are relatively flat and the south side (upward) of the ripples are relatively steep, indicating that the current was moving from north to south.

STOP 7 -- DEVILS LAKE STATE PARK -- Talus, conglomerate.

This stop provides an introduction to South Range geology and rounds out thte highlights of geomorphology. Devils Lake occupies a portion of a pre-glacial valley now dammed at both ends by terminal moraine. The lake is fed by two small streams. The only outlet is by seepage.

In the rest area one can see the talus formed on slopes on both sides of the lake. Large accumulation of talus was attributed to the cleavage, joints, and bedding within quartzite. It is felt that most of the talus formed as a periglacial phenomenon, and the abundance of large trees growing on the slope indicates little new accumulation at least during the past century.  The terminal moraine dam at the north is apparent in the wooded ridge directly north of the lake­shore pavilion.  

 

Park headquarters; the Nature Center in the park contains excellent dioramas and other geological and biological displays.

West Bluff Trail.  At the top of the bluff just north of the trail is a large patch of Cambrian conglomerate with boulders up to six feet in diameter.  

we climb the Last Bluff Trail by a stairway for a panoramic view of the geomorphology of the lake basin and an examination of well-exposed loser Paleozoic conglomerate and sandstone resting upon quartzite in Elephant Rock.

Along the trail in the cliff called Elephant Rock, we see a well-exposed example of Paleozoic basal conglomerate resting upon the quartzite in a low-angle unconformity.  Quartzite boulders up to 5 or 6 feet in diameter occur at the base, but size decreases upward, and above 15 feet, there is only tan quartz sandstone.  This locality is typical of the unconformity relationships along most of the north side of the South Range, that is the basal conglomerate is not very thick at any one point, and boulders more than about 2 feet in diameter occur only on the unconformity itself. Moreover, because of the low angle of the unconformity surface, the breadth of the conglomeratic facies in plan view may be more than a mile (unlike the buttress" effect in the Upper Narrows seen at STOP 5).  

The contact between Paleozoic conglomerate  and sandstone (yellow colored) and Precambrian quartzite (pink colored).

STOP 8 -- PARFREYS GLEN -- conglomerate.

Parfreys Glen Scientific Area is typical of many picturesque narrow gorges that notch the outer steep faces of the Baraboo Ranges, and contains the exotic yellow birch flora.  Because the present streams are very small in proportion to their valleys and are choked with boulders that they cannot move, it is assumed that most of the gorges were cut in the past by glacial meltwater.  Most of the cutting may be as young as the last 10,000 years. Remnants of large potholes in the upper gorge walls are relicts of the down cutting history.

Gorges such as Parfreys Glen provide excellent exposures of Cambria strata. Farther from the Precambrian contact, as at Parfreys Glen, largest conglomerate fragments rarely exceed four feet in diameter, but tend to be well rounded.  Associated finer pebbles, however, are less rounded, a contrast characteristic of normal gravels.

The Glen is so located as to expose conglomerate facies with a nearly one-to-one conglomerate-sandstone ratio.  Based upon elevation alone, the strata should represent the Trempealeau Group, but the southward dip of 6° to 10° here makes it probable that Tunnel City equivalents also are present; there is no basis for separating the two here at the lower end of the gorge. Note pocked weathering of sandstone due to irregular silica cementation.

This Page Was Last Modified On 7-26-02