Stop #1 - Purgatory Chasm

History:

According to legend, Hobomoko (the Native American devil) carried a woman to Purgatory Chasm after she had murdered a "white man". When the woman began to fight, Hobomoko hit her head against a boulder and attacked her with a tomahawk. The bowl-like depressions show where her head hit the boulder, the ax-marks where the tomahawk struck, and the footprints in the vein of stone where he carried his victim’s body to the edge of the fissure.

Nearby, the spires of St. George’s School Chapel (1928) can be seen.

Description:

Purgatory Chasm is the result of weathering of closely-spaced, quartz filled, ‘joints’ that may mark edges of large rectangular-shaped boudins. Similar features in the same orientation, crop out along the northern Cliff Walk where conglomerate forms rectangular, sharp-ended boudins separated by quartz. Exposed in the northern chasm face is a N-trending fault. Faults and Chasm boudins are the only evidence of D3 and D4 events.

This exposure extends beyond the park boundary southward as a continuous outcrop around Easton’s Point to First Beach. Immediately south of the Chasm (on private property), numerous vertical E-trending rock faces beautifully display the interpenetrative nature of conglomerate clasts. At that point, units structurally, and presumably stratigraphically, below the Purgatory Conglomerate crop out and the massive conglomerate interfingers with Rhode island Formation sandstones and siltstones that form the more distal portions of the alluvial fan complex. Cross-beds, pebble lag deposits, and fining upward sequences are abundant.

At this outcrop the approximately 440-ft thick (135-m), massive, clast-supported Purgatory Conglomerate units (Figure 1) are interbedded with thin sandstones and magnetite-rich sandstone lenses. The conglomerate represents a proximal to very proximal facies of a wet alluvial fan that formed off the southeastern block-faulted margin of the basin. Clasts are generally prolate triaxial ellipsoids and range from pebbles to boulders in size (majority are cobbles). Clasts are predominantly quartzite, with rare granite and schist cobbles. The outcrop forms part of one of several elongate, N-trending ridges that mark the positions of major F1 and F2 fold limbs (Figure 1, 2). Long cobble axes trend N10E parallel to fold axes. Most deformation was caused by D1 and D2; only minor indications of D3 and D4 are present. Metamorphism to chlorite grade was syn- to post-D2.

Cobble deformation was achieved by pressure solution. Cobbles have tangential, almost planar, and deeply embayed contacts. Thin sections of cobble contacts show no evidence of quartz or mica deformation within cobbles nor any distortion of internal cobble bedding. Some cobbles contain numbers of microstylolitic seams parallel to their long axes, which give cobbles an internally deformed appearance in the field. Large, fibrous, quartz pressure shadows can be seen at long axis terminations of most cobbles. Where cobbles are in close contact, matrix is 1 to 3 mm thick, depleted in quartz (<3%) and enriched in residual material remaining after pressure solution. Shear fractures offset the margins of some cobbles. Substantial redistribution of cobble volume (V) has been measured for the conglomerate throughout the southeastern basin (23% hinges; 55% overturned limbs). Much of the apparent strain is caused by original cobble shapes; intercobble rotation during D1 and D3 realigned already ellipsoidal sedimentary cobbles into their present position. Real strains are constrictional (Cobble strains, Percival quarry: ea = -11%, eb = -20%, ec = 0%, V= -23%; west Easton’s Point: ea =-26%, eb = -28%, ec = 0%, V = -41%). Although pressure solution features can be observed everywhere on this outcrop, the best localities are: Second Beach near large out-of-place conglomerate block, hilltop near Chasm, and just east of Chasm parking lot along N-trending gravel pathways. On hilltop near the Chasm parking lot, conglomerate beds interfinger with sandstones (bedding: N10E, 55SE), showing right-side-up cross-bedding and both flat-laying (N4E, 22E) S1 and steep (N24E, 70NW) S2 foliations.

(source: Mosher, Burks, and Rech, Geological Society of America Centennial Field Guide–Northeastern section, 1987, p.192-193)

The major structure of the peninsula is anticlinal (Figure 1), upon which are smaller warps and drag folds, some with vertical to overturned limbs. The extraordinary elongated boulders that appear at numerous places will be inspected along the shoreline north of Purgatory chasm. The boulders range in length from several inches to over four feet (one is 13 feet long). They are mostly quartzite, some containing considerable mica and feldspar; a few are granite and a few are felsite.

In the coarser beds the boulders lie tightly packed and little matrix material is present (Figure 3). They are elongated along N10E, parallel to the fold axes (b, Figure 2) and a lineation is present which appears as a faint striation on the thin coatings of micaceous material (muscovite and chlorite). This lineation may be seen at the south end of the east side of the isolated outcrop several hundred feet along the beach to the northeast of the main exposure.

Several thin, dark gray sandy beds indicate bedding dips about 60° to the east. The flat (ab) plane of the triaxial boulders does not lie in the bedding plane, but generally follows a foliation or cleavage direction which strikes parallel to bedding but dips gentler or steeper in different parts of the structure (Figure 2). At this stop the cleavage plane dips about 60 to the west. Although the boulders show their long axes consistently oriented north-south and horizontal, the flat a-b planes are less uniformly parallel, except where shearing in the a-b is very marked.

The tendency of the flat surfaces of the elongated boulders not to be parallel to bedding suggests that the boulders may have been rotated individually about b during folding and also sheared along cleavage planes inclined to bedding. Evidence for such movement is observed in the area.

Figure 3: A) Outcrop at Purgatory Chasm in planview; circles indicate presence of quartz pressure shadows. B) Purgatory outcrop in cross section; circle marks location of sheared boulder and arrows indicate tightly packed boulders (matrix <3mm thick).

The axial ratios (b:a:c) of the boulders average approximately 4.1:1.5:1. Where shearing was more intense it may be 5.7:2.4:1. The tapered ends commonly consist of intensely crushed material. Some boulders show an elongated rhombic or polygonal outline, in which the straight sides lie along cleavage and shear directions (Figure 4). Many boulders show indentations or pinching where they were pressed against adjacent boulders the compressive stress acting east-west but not necessarily horizontally.

One or more sets of fractures may be present in the boulders. Some of these are regional joints transecting many feet of the country rock, but others are small fractures limited to the boulders. Transverse (ac) joints are abundant. They are steep to vertical and trend east-west, essentially perpendicular to the fold axes. These are tension joints; many of which display a north-south separation. Some are occupied by quartz veins (Figure 4e). Quartz veins, from one-eighth of an inch to more than a foot thick also follow several joint directions. Purgatory chasm has been carved by wave surge and erosion along a prominent cross-joint striking perpendicular to the shore.

Longitudinal fractures (Figure 4c) may represent a foliation or a cleavage direction, or possibly the original bedding within the boulder. It may be emphasized that faint streaks, apparently bedding, are oriented transversely in some quartzite boulders and have not yet opened up as tension fractures.

Oblique shear fractures striking NE-SW and NW-SE may appear as a single set or as a conjugate system. Slip commonly appears along these joints as a series of stepped offsets producing extension along the major axis, b (Figure 5). Some oblique fractures, striking at an obtuse angle to b, have opened up because of post-fracturing extension along b (Figure 4d).

(source: Agron, New England Intercollegiate Geological Conference Guidebook, vol. 55, p22-26, 1963)

Transverse veins occupying tension fractures tend to be wider than the other sets. Several groups of short en-echelon veins are present on the flanks of the folds, showing an E-W strike, indicate E-W compression and N-W extension. The quartz veins are penetrated by joints and cleavage of the host rock, indicating their syntectonic age. This is also indicated by the folded, contorted, and sheared appearance of some veins. Under the microscope the vein quartz shows strong strain shadows and Boehm lamellae. Magnetite grains are abundant in some of the sandier beds especially along the western side of the outcrops (Figure 6). They appear in whisps and layers up to one inch thick, as well as in scattered grains following certain beds.