Was it excavated by Native Americans, and perhaps enlarged by Andrew Clemens?
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on picture for higher resolution image
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| Sand Cave is located near the base of the St.
Peter Sandstone in an area of spectacular iron stains and cements. Was it excavated by Native Americans, and perhaps enlarged by Andrew Clemens? |
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Ordovician Strata; Bridal Veil Falls Down to Sand
Cave, The Pictured Rocks Area by Brian Witzke and Bill Bunker, Senior Research Geologists Geological Survey Bureau, Iowa, Department of Natural Resources |
| Note: Well maintained trails lead to the upland drainages above Bridal Veil Falls, and there is also an easy trail access to the area beneath the rock overhang which forms the lip of the falls. However, the trail which leads from the falls down the steep drainages and ravines to Sand Cave is no longer maintained, and the trek down to Sand Cave is not recommended for those trip participants who may have difficulties on the steep and sandy slopes below. This trek covers nearly 300 feet of vertical section, so be forewarned that it is more than just a casual climb coming back up the trail slopes after we visit Sand Cave. In addition, the steep slopes in the park are often covered by a thin veneer of fragile woodland plants, so off-trail hiking is strongly discouraged. The stream drainage that begins above Bridal Veil Falls cascades and downcuts its way down the flanks of Pike’s Peak to the Mississippi River below. The picturesque bluff and slope exposures of St. Peter Sandstone along this drainage-way have been termed the “Pictured Rocks,” a name whose origin should be evident as we proceed down the trail. |
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Figure 1: Geology along the unnamed drainage that includes bridal veil falls and the Pictured Rocks / Sand Cave area. |
The upland drainages above the lip of Bridal Veil Falls expose ledges of limestone which belong to the Decorah Formation. The drainage along the trail about 200 feet south of Bridal Veil Falls exposes some of the basal shale layers of the Decorah Formation (Spechts Ferry Shale), including an altered volcanic ash bed (Deicke K-bentonite). Most of this basal shale, however, is covered by limestone talus and vegetation. This shale interval forms a layer that is relatively impervious to the movement of groundwater, and several small springs can be seen issuing from near the base of the overlying Guttenberg limstone ledges. The more resistant wavy-bedded Guttenberg limestones are better exposed above. These distinctive fossiliferous limstone beds deserve a closer look, and some of the less weathered or vegetated surfaces reveal a variety of brachiopods and other fossils. We even identified a complete trilobite fossil (Isotelus) in this area. Farther up these drainages are seen limetone ledges of the upper Guttenberg and Ion members.
Bridal Veil Falls straddles a portion of the Platteville Formation. The overhanging ledge which forms the lip of the falls as well as the underlying recessive cave-like re-entrant underneath the falls are developed within the upper Platteville Formation, McGregor Member. The typical thin wavy-bedded limestone layers are well displayed here. The base of the waterfalls occurs near the base of the McGregor Member, and underlying lower Platteville strata of the Pecatonica Member are not well exposed below. However, some dolomite ledges of the Pecatonica Member are seen in the cascading drainage below the falls, and near the base of the member the dolomite beds contain scattered embedded grains of quartz sand.
Unfortunately, the thin interval of green-gray Glenwood Shale is completely covered at this locality. This shale interval can be seen to cap the St. Peter Sandstone at other nearby localities.
St. Peter SandstoneA remarkably thick succession of St. Peter Sandstone can be accessed following the drainage below Bridal Veil Falls downstream to Sand Cave. Portions of the St. Peter cannot be safely accessed, however, especially the steep-walled areas beneath the next waterfalls below Bridal Veil Falls.
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Figure 2. A cliff exposure of St. Peter Sandstone below Bridal Veil Falls. |
The St. Peter Sandstone is a relatively monotonous and homogeneous succession of poorly cemented quartz sandstone. It is commonly so soft and friable that the term “sandstone” almost seems a misnomer. Nevertheless, the sandstones “harden up” a bit on outcrop as the sandstone exposure surfaces “case harden” by the relatively recent (and probably ongoing) precipitation of iron oxide and calcium carbonate cements. Because of this surface hardening the St. Peter sandstone beds can be held up in nearly vertical cliff faces in places. The tremendous overall homogeneity of the sands further serves to incorporate thick intervals of sand into massive units that lack clear bedding planes. Because bedding planes commonly follow surfaces marked by lithologic (rock type) breaks (for example, limestone to shale), the overall lithologic homogeneity of the St. Peter is further underscored by the paucity of clear bedding within the sandstone succession.
St. Peter sandstone lithologies in the northeast Iowa area are dominated by very fine- and fine-grained sand with lesser quantities of fine- to medium-grained sand. Coarse sand (and sometimes gravel) is locally present in the basal part, and thin horizons (especially in the upper part) may contain coarse and even very coarse sand (associated in part with pyritic hardground surfaces). A detailed grain-size distribution within a sandstone succession commonly requires considerable trenching and destructive sampling. Because of this, it was deemed unacceptably destructive to the delicate mosses, liverworts, ferns, and other plants that grow on the picturesque sandstone faces to proceed with such a study.
| Figure 3. An exposure of the basal portion of the St. Peter Sandstone in the Pictured Rocks area near Sand Cave. The red and orange banding created by iron oxide staining and cements can be seen in the face exposed in the upper center of the photo. (click on image for full- screen view). A close-up of this area is reproduced in color in Figure 3a (below). |
.The St. Peter Sandstone at Pikes Peak State Park includes the thickest known interval of the formation exposed anywhere in the Midwest, and thicknesses are known to vary between 90 and 223 feet in thickness within the park. The St. Peter succession we will examine for this field trip stop exceeds 200 feet in thickness. However, even thicker St. Peter Sandstone sections are known from the subsurface (known from well penetrations) at places in Illinois and Iowa (to 400 feet thick in Jackson County). As discussed previously in this guidebook, these thick St. Peter sections are known to fill deep valleys that were incised into the underlying dolomites of the Prairie du Chien Group (and locally into Cambrian strata) during a long period of erosion which preceded St. Peter deposition. The lower valley filling phase of St. Peter deposition, which includes the bulk of the succession seen at Pikes Peak, probably represents fluvial and estuarine aggradation of sands. The later stages of St. Peter deposition (termed the Tonti Member in nearby Wisconsin) occurred within shallow-marine environments as the interior seaway encroached across the region.
The highest portion of the St. Peter Sandstone is partially exposed in the sloping drainage below Bridal Veil Falls (units 12 and 13 on Figure). Unlike lower strata, the sandstones in the upper part are slightly argillaceous (contains clay). The Glenwood shale occurs above these argillaceous sandstones, but the shale is covered at this locality. A precipitous change in slope appears below these upper sandstone units, and the steep-walled faces display a waterwall in the drainage immediately below Bridal Veil Falls. These picturesque but precipitous sandstone cliffs are very difficult to access and will not be attempted for this trip. Moving down the trail a series of sandstone exposures can be seen along the slopes and chasms of the stream drainage (units 9 and 10 on Figure). Faint low-angle crossbeds were observed in the upper part of this interval, formed by the action of water currents. Most of the interval is a homogeneous succession of massive sands, with little apparent internal structure.
| Figure 3a. Close- up view of St. Peter Sandstone exposure pictured in Figure 3. (click on image for full-screen view). |
The most interesting part of the St. Peter succession is found in its lower part (unit 8 on Figure) as Sand Cave is approached along the winding and precipitous trail. This lower interval is marked by peculiar but strikingly beautiful secondary red-colored alteration bands characterized by sweeping swirls and mottles of iron oxide mineral precipitates. These dramatic patterns of red are laterally discontinuous, and are expressed in varying abundance throughout the sandstone succession of Unit 8. Geologists term such red swirls “Leisegang bands,” whose origins relate to redox boundaries associated with groundwater flow along the eroding bluffline. Reduced iron in solution within the groundwater is oxidized and precipitated as iron oxides as these fluids approach the surface. These red-mottled sandstones of the lower St. Peter are well displayed within and around Sand Cave, an interesting shallow cave developed into a prominent overhanging sandstone face. The mouth of the cave is about 12 feet in diameter and extends into a portion of the sandstone that shows a particularly prominent development of red iron oxide cements. The development of a cave in these massive sandstones seems geologically anomalous, and its occurrence would not be predicted by any obvious geological conditions present at this particular site. However, the reason for the development of Sand Cave may not be directly geologic in scope. Of note, ancient peoples that inhabited this region used red ochre for ceremonial and mortuary purposes, and the red iron oxide pigments seen in Sand Cave could have been easily collected as a source for red ochre. It seems possible that generations of Archaic and Woodland peoples may actually be responsible for the creation of Sand Cave. Even in historic times, people continued to remove the beautiful red sands from Sand Cave. However, please leave the sand in place for others to enjoy (NO Collecting!).
The basal part of the Unit 8 sandstone succession below the mouth of the cave (8A) as well as the strata above and within the cave (8B) occur at the same elevation as a series of dolomite exposures of the Oneota Formation a short distance downstream. These relationships indicate that the pre-St. Peter erosional channel cut across the Oneota dolomites marking the margin of that channel in this area. Over 40 feet of local erosional relief along the Oneota-St. Peter surface is therefore interpreted in this small area. Based on the varying thickness of the St. Peter Sandstone, over 130 feet of relief occurs along this surface within Pikes Peak State Park. The actual erosional contact between the St. Peter channel-filling sandstones and the adjacent and subjacent Oneota dolomite ledges is not clearly shown in the Sand Cave area, but sandstone fills (probably within karstic openings associated with the ancient channel incision) are seen in the dolomites downstream from the cave (especially lower unit 6). In addition to the dramatic red mottles and swirls within St. Peter unit 8, the lower beds (8A) display possible ESE-trending crossbeds, and above the cave mouth (top of 8B) a prominent bedding surface (one of the few in the succession) is marked along a thin siltstone bed.
| Figure 4. Ledges of Oneota Fm. Dolomite below Sand Cave. |
Although we will probably not proceed any further than Sand Cave, the stream drainage continues down to the Mississippi River and exposes a series of dolomite ledges belonging to the Oneota Formation, lower Prairie du Chien Group (Fig. 4). The Shakopee Formation of the upper Prairie du Chien Group has been completely removed by erosion in this area. The Oneota ledges expose most of the stratigraphic succession below, but some portions of the formation are not visible beneath the local cover of colluvium (especially units 1 and 4). The dolomites are composed mostly very fine and fine dolomite crystals, and original sedimentary fabrics are hard to resolve. More coarsely crystalline dolomite beds are seen especially in unit 6. The dolomite beds are commonly vuggy (large open pores in the rock), and portions of the Oneota interval contain chert nodules in varying abundant (unit 5, lower unit 3). The lowest portion of the exposed Oneota section is dominated by finely crystalline dolomite (units 1, 2); quartz crystal lined vugs are found near the top. Some embedded quartz sand grains occur within the lowest dolomite ledges. The lower interval, although very poorly exposed, may represent part of the Coon Valley Member.
We will return up the drainage from Sand Cave to our starting point at Bridal Veil Falls. Please be as careful as possible not to further disturb the delicate plant communities and crumbling sandstone ledges along the way.
| Hike
back up the trail to Bridal Veil Falls then to Stop 7 at Hickory Ridge Mound Group |
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Hike to Hickory Ridge Mound Group
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