A field trip to Bringhurst Woods Park is appropriate for students in grades 5 and up (10 years and older), and provides an opportunity to observe intrusive plutonic igneous rocks that have intruded into country rock, which in this case is the blue rock or what geologists call the Brandywine Blue Gneiss. In addition, the minerals in the pluton are large, easily identified, and interesting. Mineral collecting is not allowed within the park, however permission may be obtained to collect along Shellpot Creek southeast of the park. Please do not use rock hammers on the rocks in the park.
The specific objectives of this adventure are:
- To observe an intrusive igneous rock and the country rock (Wilmington blue rock) it has intruded
- To identify the individual minerals in an igneous rock
The rocks along Shellpot Creek in Bringhurst Woods Park are intrusive igneous or plutonic rocks. Because of the good exposure in this park, geologists have named these rocks the Bringhurst Gabbro and mapped the pluton as a geologic unit within the Wilmington Complex (Figure 1). The Bringhurst Gabbro represents a magma flow that flowed into the Wilmington Complex and cooled deep underground. The rocks of the Wilmington Complex underlie the most City of Wilmington and Brandywine Hundred. During the 18th and 19th centuries all rock units within the Wilmington Complex were extensively quarried for building houses, fences, retaining walls, schools, churches, and factories. They were used wherever a building material was needed. The most common rock unit in the Wilmington Complex is a high-grade metamorphic rock called the Brandywine Blue Gneiss (commonly called the Wilmington blue rock). This "blue rock" was named for the bright blue color of the rock when it is freshly exposed. It is the Wilmington blue rock that the Bringhurst Gabbro intruded. The Bringhurst Gabbro exposed along Shellpot Creek has not been deformed or recrystallized by metamorphism, thus the rocks of the Bringhurst pluton lack the layering found in most of the other metamorphic rocks of the Delaware Piedmont. Because there are no fine-grained "chilled margins" at the contact between the pluton and the Wilmington blue rock, the pluton probably intruded the gneisses while they were still hot, sometime in the early Paleozoic between 500,000,000 and 400,000,000 million years ago.
Shellpot Creek in Bringhurst Woods Park is choked with large rounded boulders of Bringhurst Gabbro that have eroded out of the surrounding hills. A close look shows the minerals in the gabbro are between 1/4 and 2 inches in length and 1/4 to 1 inch in diameter (Figure 2). Blobs of fine-grained dark rock are common in the Bringhurst pluton. These dark blobs are chunks of Wilmington blue rock that were picked up and incorporated into the magma as it intruded into the gneiss. These inclusions are called xenoliths, a word derived from the root xeno- meaning foreign and lithos- meaning rock. Thus, a xenolith is a foreign rock enclosed within another rock. In this case the xenoliths are derived from the country rock, the Wilmington blue rock. Although the Wilmington blue rock is composed of both dark layers and light layers, all the xenoliths are derived from the dark layers. This is possibly because the light-colored inclusions melted at a lower temperature than the dark inclusions, and the light inclusions melted in the hot gabbroic magma of the pluton becoming commingled and no-longer recognizable.
A contact between the coarse grained rocks of the pluton and the Brandywine Gneiss occurs approximately 700 to 800 ft east of the park entrance (Figure 1). The gneiss at the contact is contorted and contains clots of quartz. Before a field trip to Bringhurst Wood Park, it is recommended the group visit one of the Wilmington Complex stops described in the Wilmington Blue Rocks Geologic Adventure, so the participants can recognize the Wilmington blue rock.
Minerals of the Bringhurst pluton (Figure 2) are plagioclase feldspar, pyroxene and olivine. The plagioclase is dark gray and glassy. Feldspar has two distinct cleavages, thus when a feldspar crystal is broken along a cleavage plane it will present a smooth shiny surface. The pyroxene crystals are elongated, black or bronze colored, and may have a distinctive schiller or iridescent luster on a fresh surface. The olivine grains are less common than the pyroxene, and in this pluton, the olivine grains are usually rusty and have a black rim. Individual minerals in the rims cannot be recognized in hand specimens, but microscopic study has identified an inner rim that is an intergrowth of orthopyroxene with spinel and an outer rim that is an intergrowth of hornblende with spinel. The olivine-bearing rocks are more abundant southwest of the park entrance.