Fox Island Alliance

Guide to the Fox Island Geogarden

By Dr. Jack A. Sunderman, Geologist

Introduction

Welcome to the Fox Island Geogarden. The Geogarden at the entrance to the Nature Preserve is a display of igneous, metamorphic and sedimentary rock boulders that were transported from Canada to this area by continental glaciers about 10,000 years ago, during the so-called “Ice ages.” The Geogarden also displays local bedrock from nearby quarries. The Geogarden was established in 1989 by the Fox Island Alliance, in cooperation with the Allen County Park and Recreation Board. The Hanson Aggregates Corporation (Formerly May Stone/France Stone and StoneCo) provided machinery for transporting the specimens found in their quarries and nearby roadsides to the Park.

Development of the Geogarden

Most of the boulders in the Geogarden were found adjacent to Fox IslandPark property, along Yohne Road just east of the park entrance. A few boulders are from the Lower Huntington Road Quarry of Hanson Aggregates (formerly StoneCo), just south of the Park, and a few specimens are from the bedrock of the Ardmore Avenue Quarry of Hanson Aggregates, located just a few miles northeast of the Park. Small cranes and front-end loaders were used to put the boulders in place.

Origin of the Geogarden Rocks

Most of the rocks in the Geogarden came from Canada and are very old. All of the rocks except No’s. (1) and (9A-E) belong to the Precambrian age and formed between 1 and 3 billion years ago, at a time when there were large mountains in central and eastern Canada. The mountain-building forces produced igneous rocks such as granite and basalt, and formed metamorphic rocks such as gneiss and quartzite. Much later, glaciers picked up large masses of these rocks and transported them to Indiana. Rock specimens (1) and (9A-E) are sedimentary rocks found near Fox Island. They formed in shallow seas that covered this area during Paleozoic time, a few hundred million years ago.

Transport of The Boulders To Indiana

All the rocks of the Geogarden, except numbers (9A-E) were transported into Allen County, Indiana by continental glaciers and meltwater streams. this occurred about 10,000 years ago, during Late Pleistocene time. Boulder No. (1), with the large plaque, best displays evidence of glacial transport. Its flat, shiny and scratched surface was produced by grinding against other rock during its transport to Allen County, probably from northern Indiana or nearby Ohio. Note that other boulders are rounded, suggesting tumbling, rounding and abrasion by other boulders in glacial meltwater streams.

Rock Types Found In The Geogarden

Igneous Rocks form from molten (melted) rock material called magma. The pink and gray rocks of the Geogarden are granites and related rocks that formed by slow cooling of magma, deep underground. The shiny crystals with flat cleavage surfaces are feldspar , the most common minerals in the earth’s crust. Their large size (some more than an inch across) indicates they crystallized very slowly.

The dark, smooth rocks (No’s. 4 and 11) were poured out onto the earth’s surface as lava, during volcanic eruptions like those in Hawaii today. The lava cooled very rapidly, forming very small crystals. These igneous rocks all formed in Canada, during Precambrian time when plutonic (subsurface) and volcanic igneous activity were extensive.

Sedimentary Rocks form from sediment that settled on land or in water. Only Geogarden rocks No’s. (1) and (9A-E) are sedimentary. They are of Paleozoic age, and contain ancient marine fossils. Rock No.(1), with the large plaque, is a sedimentary dolomite rock of probable Devonian age (about 400 million years ago) that was soft enough to be polished by the glacier.

The other sedimentary specimen is actually five rocks, numbered (9A-E).These rocks are dolomite from an ancient reef bank of Silurian age that formed in Allen County and northern Indiana about 420 million years ago. These rocks contain fossil coiled snails, a cephalopod, corals, and a sponge-like animal the white layers) called stromatoporoids. These fossils are all marine, indicating that a warm shallow sea covered this area during Silurian time.

Metamorphic Rocks form from other rocks through the application of heat and pressure. Some of the Geogarden metamorphic rocks have been made stronger by the application of pressure, and some have been rearranged into layers and veins of different minerals.

A typical metamorphic rock called gneiss is coarsely-banded with layers of brightly-colored quartz, feldspar, and mica minerals. Rock No. (2) has bands that indicate the rock was compressed under very high pressure deep beneath the earth’s surface, probably during a mountain-building event.

Other rocks that began as quartz sands or gravels on beaches or in streams first have been compressed into the sedimentary rocks sandstone or conglomerate by deep burial. They have been metamorphosed later by the application of even greater pressure (during mountain-building?) into the metamorphic rock called quartzite , named because of its high content of quartz in the original sands and gravels.

Tillites are metamorphic rocks with a very unique origin. They are ancient glacial clay tills that have been metamorphosed into a strong and cohesive rock. Rock No. (12) has a mostly small grain size, but it also contains rounded pebbles and cobbles of light-colored rocks of various types (including granite) embedded in it. This rock therefore must have had a very complex history that probably includes: 1) origin of the granite, 2) erosion and rounding of the granite to form the cobbles, 3) transport and deposition of the granite cobbles mixed with clay by an ancient glacier to form till, 4) burial and metamorphism of the till to form tillite, then 5) erosion and transport of the tillite boulder from Canada to Indiana!

Use Of The Guide

The descriptions of the Fox Island Geogarden rocks below are designed to help school groups and other visitors to the Park understand the origin of the igneous and metamorphic rocks that formed in Canada millions of years ago, and to provide an introduction to the glacial events that led to their transport to Indiana about 10,000 years ago. The descriptions also deal with quarried sedimentary rocks that formed in this area about 330 million years ago.

The rocks are numbered from (1) to (23), starting with the large dolomite slab with an explanatory plaque (No. 1), at the west end of the Geogarden display. The numbering continues eastward from (2) to (11) along the south path, then back westward along the north path from (12) to (20). Specimens (21) and (22) are the large blocks north of the Geogarden paths, and (23) is the large block just south of the Geogarden paths. Note that No. (9) is really five rocks obtained from ancient reef bedrock in the Hanson Aggregates quarry on Ardmore Avenue, and described below as (9A) to (9E). These small blocks are arranged counterclockwise, starting with the largest block (9A).

The Geogarden Rocks, By Specimen Numbers

(Click on any image to see a larger version)

No. 1: DOLOMITE, Sedimentary.
Mineral: Dolomite
Age: Middle Devonian (~380 million years ago)

This boulder is a fine-gained sedimentary dolomite rock probably formed northeast of Fort Wayne, in northern Ohio. It was carried here by a Pleistocene glacier about 10,000 years ago. Notice its faceted (smoothed), polished, and striated (scratched) surface - evidence that it was carried here by glacial ice. The glacier moved it firmly against other rock to make the flat surface. This boulder was obtained from the glacial drift at the top of the Hanson Aggregates Lower Huntington Road Quarry, just south of Fox Island Park.

No. 2: GRANITE PORPHYRY, Igneous.
Minerals: Feldspar, quartz, biotite
Age: Precambrian (1 to 3 billion years ago)

This boulder contains two grain sizes, giving it the name porphyry . It began to form from a molten magma deep underground, where slow cooling allowed the large mineral crystals to form. The magma then probably worked its way closer to the earth’s surface, where it cooled more rapidly, forming the smaller crystals.

Note the large, shiny, pink crystals. These are feldspar , the most abundant mineral in the earth’s crust. Note that many of the feldspar crystals have been broken, or cleaved, along flat planes, the typical way in which all feldspar minerals break. these pink feldspar crystals contain potassium and are called orthoclase feldspar because they cleave at right angles. Note also the clear gray quartz grains and the small masses of dark biotite mica.

No. 3: GABBRO, Igneous.
Minerals
Age: Precambrian (1 to 3 billion years ago)

This rock formed deep underground as indicated by the medium to large grains. The dark grains are augite , an iron-rich silicate mineral, and the lighter lath-like grains are feldspar. A small curved slab of rock broken from the boulder exhibits a brown rusty color due to a natural rust called limonite ,an iron oxide mineral formed by weathering of the augite.

No. 4: BASALT, Igneous.
Minerals: (Augite, biotite mica.)?
Age: Precambrian (1 to 3 billion years ago)

The minerals that make up this boulder are too fine grained to be identified without a microscope, but the dark rock color suggests iron-bearing minerals such as augite and biotite , as in the gabbro of rock No. (3). During a volcanic eruption, the basalt probably was injected into a near-surface crevice (producing a basalt dike ), or was poured out onto the earth’s surface as a lava flow . The lava then cooled so rapidly that large crystals did not have time to form. This same type of rock is found in most of the world’s ocean floors and oceanic islands, including the Hawaiian Islands and Iceland.

No. 5: GNEISS & PEGMATITE, Metamorphic & Igneous.
Minerals: Quartz, feldspar, biotite
Age: Precambrian (1 to 3 billion years ago)

This boulder is a combination of two rock types: metamorphic gneiss (the gray banded part) and igneous pegmatite (the brightly colored front part with large mineral crystals). Note that the gneiss bands are truncated by the pegmatite, showing that the gneiss was there first (is older) and the pegmatite was intruded later across the gneiss, probably as a small igneous dike . Pegmatite is similar to granite, but is distinguished by very large crystals, suggesting very slow cooling and crystallization, and/or a very fluid magma.

No. 6: GRANITE PORPHYRY with DIKES, Igneous.
Minerals: Feldspar, quartz, biotite
Age: Precambrian (1 to 3 billion years ago)

This boulder is similar to boulder No. (2) (the granite porphyry ), but it also contains small intersecting granite dikes . Note that one dike appears to be cut across and offset by another. The offset dike was intruded first, and was later cut across ( crosscut ) by the younger dike.

No. 7: PEBBLE QUARTZITE, Metamorphic.
Minerals: Quartz
Age: Precambrian (1 to 3 billion years ago)

Structures in this simple rock tell an interesting part of its history. The rock consists of quartz sand grains and small pebbles that started out as sand and gravel, probably deposited along a beach or stream bed. Later the sand and gravel were converted into the sedimentary rock called pebble conglomerate . Eventually, however, the conglomerate became deeply buried and was subjected to high pressures and metamorphosed to the very cohesive rock called quartzite . However, the original quartz sand grains and pebbles were so chemically and physically resistant to change that they are still visible.

Note that a faint layering is visible in the rock, and that the layering is at different angles in different parts of the rock. This structure is called cross-bedding , and it allows us to determine the original “top side” of the rock. The upper side of one cross-bed usually is cut off (truncated) by the next younger cross-bed layer above it. Has this rock been placed right side up, or is it upside down?

No. 8: BANDED SLATE & QUARTZITE, Metamorphic.
Minerals: Quartz, hematite
Age: Precambrian (1 to 3 billion years ago)

This rock originally consisted of alternating layers of fine sand and silt, probably deposited in relatively quiet water, perhaps offshore in the sea, or in a lake. It was later converted to layers of sedimentary sandstone and shale, and finally was subjected to moderate pressures during metamorphism, producing alternating layers of metamorphic quartzite and slate . Note the small quartz grains in the slightly lighter-colored layers. These are the quartzite layers. The slate layers contain grains too fine to identify and are so thin they have not developed the typical “slaty cleavage” of commercial slate. The red-purple color of the rock probably is due to the presence of the red iron oxide mineral hematite

No. (9A-E): DOLOMITE with FOSSILS, Sedimentary.
Mineral: Dolomite
Age: Middle Devonian (\~420 million years ago)

These rocks are the only specimens in the Geogarden that are truly native to the Fort Wayne-Allen County area. They are from the bedrock in the Hanson Aggregates (formerly May Stone/France Stone) Company Quarry along Ardmore Avenue, in south Fort Wayne. The fossils are all marine, which indicates that a warm, shallow sea covered this part of the Midwest during Silurian time, about 420 million years ago. Careful examination shows the rocks contain large fossil snails , cephalopods , corals and stromatoporoids

Fossil Key: The largest block (9A), the one farthest east, contains sponge-like fossils called stromatoporoids , shown as light-colored streaks and lenses. Stromatoporoids took the place of corals in building the reefs of Silurian time. In counterclockwise order, block (9B) contains the colonial coral Favosites , and block (9C) contains a planar-coiled snail ( gastropod ) and molds of two spirally-coiled snails. Block (9D) contains a large open-coiled fossil cephalopod , which is related to snails, clams and octopus. One variety of coiled cephalopod, the ” Pearly Nautilus ,” still lives in the Pacific Ocean today. Block 9E contains a planar-coiled snail.

No. 10: TILLITE, Glacial & Metamorphic.
Minerals: Feldspar, quartz, biotite
Age: Precambrian (1 to 2 billion years ago)

This rock has a complex history, partly outlined above, and is similar to rock No. (22). Most of the light-colored pebbles are granite, with mineral grains of feldspar and quartz. This rock originated as an ancient glacial deposit called till . It probably is one two two billion years old, and has been compressed and metamorphosed to convert it into the solid rock we see here, called tillite . For a more complete description, see that for rock No. (22).

No. 11: BASALT, Igneous.
Minerals: Too fine to identify
Age: Precambrian (1 to 3 billion years ago)

The dark color and very fine grain size of this rock indicate it is an ancient basalt lava similar to rock No. (4).

No. 12: GRANITE GNEISS, Metamorphic.
Minerals: Quartz, feldspar, biotite
Age: Precambrian (1 to 3 billion years ago)

This rock has the composition of granite, with typical brightly colored feldspar minerals, but it has been metamorphosed to the metamorphic rock, gneiss . Note the glassy quartz, the white or pink feldspar, and the black, flaky biotite. The prominent banding was produced during mountain-building, in Canada, by pressure applied at great depth. The relative directions of pressure can be estimated from the orientation of the bands of coarse minerals, which grew perpendicular to the pressure directions.

No. 13: HORNBLENDE GNEISS, Metamorphic.
Minerals: Hornblende, feldspar, biotite
Age: Precambrian (1 to 3 billion years ago)

This rock is similar to the granite gneiss of No. (12), but it obviously has different minerals. The most abundant mineral is a black, shiny silicate mineral called hornblende . Note that iron oxide (natural rust) is forming on apart of the rock. The rock itself is dark because of the presence of iron in the hornblende and biotite.

Also note that this rock contains small inclusions of even darker rockmaterial, possibly the original rock from which the metamorphic hornblende gneiss originated. The black inclusions similar to the basalt of rock No. (4) indicate that the original rock probably was a basalt lava that was later metamorphosed to the present hornblende gneiss. The inclusions are parts of the original basalt parent rock that were not completely metamorphosed.

No. 14: GRANITE GNEISS, Metamorphic.
Minerals: Feldspar, quartz, biotite, muscovite.
Age: Precambrian (1 to 3 billion years ago)

This rock is similar to rock No. (12), but also contains a few grains of the light-colored mica, muscovite , named after occurrences near Moscow, Russia, and whose common name is isinglass . Also note the bands of quartz (and some feldspar) that stand out on the rock surface. Differential weathering removes other less resistant minerals (the micas, biotite and muscovite ) because they are less resistant to weathering than adjacent bands of quartz and feldspar. Biotite is generally brown to black, and muscovite is clear and shiny. Both micas consist of very thin layers that easily peel off.

No. 15: QUARTZITE, Metamorphic.
Minerals: Quartz
Age: Precambrian (1 to 3 billion years ago)

This rock is similar to No. (7), but is finer grained. Nearly all of the grains are quartz. At one time the grains were part of a sandy beach or stream bed. The sand was first converted into the sedimentary rock, sandstone , then metamorphosed to its present form, quartzite . Quartzite is a very “tough” or resistant rock because its grains are nearly all quartz (a very hard and resistant mineral), and the grains have been interlocked and cemented together by more quartz during metamorphism. Note that the original sand layers are now vertical, indicating that the rock has been turned up on end for display, perpendicular to its original orientation.

No. 16: QUARTZITE, Metamorphic.
Minerals: Quartz
Age: Precambrian (1 to 3 billion years ago)

This rock is very similar to No’s. (7) and (15). Note that it is cross-bedded like No. 7, but that it is fine grained like No. (15). The cross-beds indicate its original “up” direction.

No. 17: GRANITE, Igneous.
Minerals: Quartz, feldspar, biotite
Age: Precambrian (1 to 3 billion years ago)

This rock is a good example of the common type of granite used for tombstones and decorative facings on buildings. The uniform and medium grain size of the quartz and feldspar indicate that the magma from which the granite crystallized cooled slowly but uniformly, deep beneath the earth’s surface.

No. 18: ANORTHOSITE, Igneous.
Minerals: Plagioclase feldspar
Age: Precambrian (1 to 2 billion years ago)

This rock is one of the most unusual rocks in the Geogarden display. It consists of essentially one mineral: a dark-colored, calcium-rich feldspar called plagioclase . Rocks similar to this specimen have been found on the moon! This specimen, however, formed in Canada and was transported here by glaciers, along with all the other igneous and metamorphic boulders of the display. Anorthosite is a relatively rare rock, most of which occurs in ancient “Precambrian Shields,” where it is associated with metal-bearing ores such as those of chromium, platinum and nickel.

No. 19: MIGMATITE, Metamorphic & Igneous.
Minerals: Quartz, feldspar, biotite
Age: Precambrian (1 to 3 billion years ago)

This attractive rock also is unusual: it is partly metamorphic and partly igneous. Note that most of the rock closely resembles the metamorphic gneisses of No’s (12) and (14), but that the bands of coarse crystals more closely resemble the igneous granites of No’s (17) and (20).

One possible explanation for the origin of this migmatite is that the mountain-building conditions that produced the metamorphic part of the rock exceeded normal metamorphic temperature-pressure conditions and partly melted the rock. This produced local “magmas” that recrystallized to form bands of “igneous” rock within the original metamorphic rock, producing the compound rock migmatite (half gneiss, half granite).

No. 20: GRANITE, Igneous.
Minerals: Quartz, feldspar, biotite
Age: Precambrian (1 to 3 billion years ago)

This rock is very similar to No. (17). Note, however, that this specimen contains small inclusions of a darker rock type, probably basalt. The basalt inclusions probably were incorporated into the granite magma as it worked its way up through basalt rock formed earlier near the earth’s surface.

No. 21: GNEISS, Metamorphic and Igneous.
Minerals: Feldspar, quartz, biotite, hornblende
Age: Precambrian (1 to 3 billion years ago)

This spectacular metamorphic rock is similar to No’s. (12) and (14), but it contains more dark minerals ( biotite and hornblende ) and more igneous veins of light-colored minerals ( feldspar and quartz ) than the other specimens. The veins probably were injected into weak zones in the gneiss soon after its metamorphism. Note that the feldspar and quartz of the veins are coarse enough to be easily identified.

No. 22: TILLITE, Glacial & Metamorphic.
Minerals: Too fine to identify, except in the pebbles and cobbles.
Age: Precambrian (1 to 3 billion years ago)

This spectacular rock is very similar to No. (10), but it contains larger cobbles of other rocks, some of which are granite (with visible feldspar and quartz grains), and a few are basalt. Most of the boulder, however, has grains too small to identify without magnification.

At one time this rock probably was much like the material in many of our yards and gardens in northern Indiana, where we have sticky gray clay with scattered boulders, called glacial till . This specimen also originated as till, then was compressed by burial, and metamorphosed during mountain-building to form the rock, tillite

No. 23: GRANITE PORPHYRY, Igneous.
Minerals: Feldspar, quartz, biotite
Age: Precambrian (1 to 3 billion years ago)

This striking rock is an excellent example of a rock that had a two-stage history of cooling. Note that the rock has mineral crystals an inch or more long, and that it also has smaller mineral grains. This indicates that the rock cooled in two stages, first slowly to form the large crystals (the porphyry ), then more rapidly to form the smaller crystals (the normal granite ).

The large crystals of this rock are orthoclase feldspar , a white to pink mineral that breaks along flat “cleavage” planes. During a sunlit day, the strong reflection of light from the cleavage surfaces make this mineral easy to identify. This attractive type of granite rock is used in tombstones and as facings on buildings.

Credits:

This Geogarden description is modified from a May, 1989 publication of the Fox Island Alliance (now out of print), also by Dr. Sunderman.

Photographs by Dr. John Schutt