. Bulletin. Science; Natural history; Natural history. MIOCENE PHOLADID BORINGS 85. Fig. 2. Infilled clavate borings along basal contact of the Isidro Formation at CSUN loc. 1495 (where a large block of rock has fallen from the cliff face). Hammer is 30 cm in length. in clusters) with significant numbers of small-sized (up to 3 cm height) and large- sized (up to 8 cm height) pectinids, internal molds of Turritella and unidentified bivalves, and rarer components of clypeasteroid echinoids, large-sized (up to 17 cm length) oysters, and bone fragments. A nearshore depositional environment is indi
. Bulletin. Science; Natural history; Natural history. MIOCENE PHOLADID BORINGS 85. Fig. 2. Infilled clavate borings along basal contact of the Isidro Formation at CSUN loc. 1495 (where a large block of rock has fallen from the cliff face). Hammer is 30 cm in length. in clusters) with significant numbers of small-sized (up to 3 cm height) and large- sized (up to 8 cm height) pectinids, internal molds of Turritella and unidentified bivalves, and rarer components of clypeasteroid echinoids, large-sized (up to 17 cm length) oysters, and bone fragments. A nearshore depositional environment is indicated by the fossil content and lack of significant post-mortem transport of the fossils. A late middle Miocene age for the Isidro Formation at this locality has been reported by Squires and Demetrion (1993) on the basis of the presence of the sand dollar Astrodapsis bajasurensis Squires and Demetrion, 1993. The horizontal contact of the two formations at the study site is clearly delin- eated by a remarkable trace-fossil assemblage. The contact is accessible in two ways. A number of spurs have been created by erosion of prominences along the cliff face. They are rather steep and precarious. Safer, but more limited, access is possible at several places where huge blocks have fallen from the cliff face. Some of these fallen blocks provide excellent specimens of the trace fossils (Fig. 2). In situ, the contact presents a limited three-dimensional view due to differential weathering of less highly indurated Eocene substrate penetrated by Miocene bor- ers. Ichnofossils Sediment-infilled clavate Z)on>2^5. — Infilled borings are predominantly smooth but rare individuals (Figs. 3A, B) display basal concentric, serrated tool marks. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Southern California Academy of S
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