Part 2

1-C. Cryptolithus tesselatus, Arthropod (Trilobite), Middle Ordovician (Trenton Stage). Top view of the Cephalon. (X2)

1-D. Cryptolithus tesselatus, Arthropod (Trilobite), Middle Ordovician (Trenton Stage). Side view of the Cephalon. (X2)

2. Isotelus gigas, Arthropod (Trilobite), Middle Ordovician. Top view of specimen.

3. Flexicalymene, Arthropod (Trilobite), Ordovician to Silurian. Top view of an enrolled specimen.

4-A. Dinorthis pectinella, Brachiopod, Middle Ordovician (Trenton Stage). Exterior view of the brachial valve.

4-B. Dinorthis pectinella, Brachiopod, Middle Ordovician (Trenton Stage). Exterior view of the pedicle valve.

5. Reuschella edsoni, Brachiopod, Middle Ordovician. Exterior view of the pedicle valve.

6. Lingula, Brachiopod, Ordovician to Recent.

7-A. Prasopora, Bryozoan, Ordovician. Top view. (X0.5)

7-B. Prasopora, Bryozoan, Ordovician. Side view. (X0.5)

7-C. Prasopora, Bryozoan, Ordovician. Vertical thin section showing the nature and growth of part of a bryozoan colony. (X18)

8. Bryozoan, “twig-like” type, Ordovician to Devonian.

9. Sowerbyella, Brachiopod, Middle and Upper Ordovician. Exterior view of brachial valve. (X2)

10. Rafinesquina, Brachiopod, Middle and Upper Ordovician. Exterior view of the pedicle valve.

11. Platystrophia trentonensis, Brachiopod, Middle Ordovician (Trenton Stage). Anterior or front view.

12. Hesperorthis tricenaria, Brachiopod, Middle Ordovician (Black River and Trenton Stage). Interior view of the pedicle valve.

Brachiopods. Brachiopods are abundant in the Park rocks (see Plate 1, 4A, B; 5; 6; 9; 10; 11; 12). These invertebrates are small marine animals which generally live in waters no deeper than 600 feet. The two valves of their shell are joined at the back (posterior) end of the body along a hingeline of interlocking teeth and sockets. The shell of the brachiopod is opened or shut by muscles attached to the inside of each valve. Brachiopods are found in the oldest rocks containing definite and abundant fossils. Brachiopods are still living today.

Bryozoans. Bryozoans or “moss animals” are very small marine animals which live in colonies. The bryozoans construct their mutual home or colony of lime which is commonly preserved for the fossil record. Large colonies of the fossil Prasopora (prah-sop-OR-ah) are commonly seen on the weathered surfaces of many of the rock layers in the Park (see Plate 1; 7A, B, C; 8 and Figure 5). Individuals of one genus common here, can be recognized by their chocolate drop shapes. Bryozoans first appear in lower Paleozoic rocks and are still living today in clear well-circulated shallow to deep marine water. Considering all of the fossils found in the Park rocks, the past environment is thought to have been a relatively shallow and warm sea.

THE ROCKS AND THEIR HISTORY

Approximately 75 vertical feet of the Glens Falls Limestone occur along the Park beach. The rocks are black or blue-black on a fresh surface, gray or grayish-white on a surface which has been exposed to the weather. Most of the individual beds or layers are 5 to 7 inches thick (see Fig. 6) with the thickest being just under 5 feet. The beds are separated by thin “partings” of rock, many of which contain abundant fossils. The beds consist of massive limestone, shaly limestone or limy shale; the partings are generally limy shale or shaly limestone.

Explanation for Figure 7

1. Glens Falls and younger sediments were deposited on the Ordovician sea floor.

2. Sediments hardened into Glens Falls Limestone and younger rocks.

3. Rocks were tilted during the late Ordovician Taconic Disturbance and the younger rocks and part of the Glens Falls Limestone were removed by erosion. Erosion continued for some 350 million years.

4. During the Pleistocene Epoch, which started some 1 million years ago, glacial ice overrode the beveled layers of the Glens Falls Limestone. Hard rocks frozen to the underside of the glacial ice produced scratches or striations in the exposed layers of the Glens Falls Limestone.

5. Glacial lakes Vermont form as the glaciers retreat northward. In between the glacial lakes Vermont and present Lake Champlain, marine waters flooded the valley and formed an arm of the Atlantic Ocean. Clay, silt, sand and gravel were deposited on glaciated Glens Falls Limestone (Fig. 1a).

6. Present-day Lake Champlain formed when relatively greater uplift in the north dammed the Champlain valley.

The rock types found in the Park lead to certain conclusions regarding the environment which existed during their formation. Most of the rocks are composed of lime (limestone) or a mixture of lime, fine sand and mud (limy shale or shaly limestone). The mineral pyrite (FeS₂) is present in many of the rocks. Most of the rocks contain abundant amounts of organic matter. The sediments which make up these rocks were carried to the Ordovician sea by streams flowing primarily from the east. As these streams entered the quiet sea waters the larger followed by the smaller particles began to settle to the bottom. Lime was slowly precipitated from the warm sea water and pyrite formed under stagnant bottom conditions. Organic material accumulated on the bottom and intermixed with the sediments. The poor life-sustaining qualities of much of the bottom waters prevented rapid or complete bacterial action on the accumulated debris and the sediments remained “organic black” in color. Slowly, as the weight of overlying sediments increased, the lower layers were compacted and cemented into the hard limestone and shale which we see today.