JAMES RIVER PARK SYSTEM

BEFORE YOU BEGIN: • The walk along the Geology Trail provides a hands­on learning expe- rience about how rocks were formed and how you can read their sto- ries. The walk is ideal for family groups touring the park with children above age 10 and for small groups of students studying geology in high school. • Begin your walk at the 22nd Street Entrance Parking Lot (near the Lee Bridge on Riverside Drive). • Note: Read the introduc- tion first! It helps every- thing else make sense, and it will give you a winning vocabulary in Geology Walk Scrabble. An Interpretative Guide to the Geology of the James River

When the planet Earth was formed about four billion years ago it resembled a ball of hot liquid and bubbling gases. As the surface cooled a crust developed, but the bubbling liquid rocks un- derneath (called magma) cracked the crust into huge chunks and moved them about. This is the essence of the Theory of Plate Tectonics. North America and Africa were once a part of the same huge “cracker” or crustal plate. Columns of hot liquid rocks and gases rising out of the core of the Earth cracked this huge crustal plate and pushed the pieces apart about 600 million years ago. (Look on a world map.) The two continents look like pieces of a puzzle that fit together. As they slowly moved, mud and sand (sediments) washed off the land were deposited along the edge. Geology Walk JAMES RIVER PARK SYSTEM

Introduction About 500 million years ago, the movement of the plates reversed and the continents began to come back together. The plate which became America also had most of the ocean over it. That made it more dense (hard and heavy) than the African plate with mostly land. The dense American crust then plunged under Africa. The intense pressure and heat that re­ sulted from these crusts rubbing together caused Begin the trail from the east end of the parking lot. the sediments to melt and change their makeup. Head towards the stairs and the stone wall. Stop at the Rock and sediments changed by heat and pressure wall made of cobblestones to the left of the steps. are called metamorphic rock. 1 Quarried Granite The crashing together of these crustal plates caused These rocks were cut from slabs of granite quarried in the park. Can cracks and weak areas that also allowed liquid you notice that the black specks are more concentrated in some rock from inside the Earth to squeeze through in blocks than in others and that some grains and crystals are larger places. These rocks, formed from the liquid core than others? Rock that cools quickly is usually near the surface of of the Earth, are called igneous rocks. (Liquid rock is called magna.) By 350 million years ago North the Earth and consists of a finer mix of parts, while rock that cools American and Africa had fully collided. They have slowly is usually deeper underground and contains bigger crystals. been slowly drifting apart ever since. (In perspec­ (It’s like pancake batter: if it stands for a long while, the parts will tive, if the history of the Earth is compared to a separate out, but if cooked into a pancake right away, the ingredi­ year, the history of man on Earth is about one ents stay together.) second.) How fast did this granite cool? How many different colored miner­ When liquid or semi­liquid rock squeezes through als can you find? Try to find these four: Biotite (black mica) is shiny soft spots in the crust, it forms huge blobs of rock and black. Quartz is clear and shiny. Feldspar is white or yellow. called plutons. They are found in a variety of Hornblende is brown, black or green. shapes and sizes, but are usually longer than they are wide. Some of the largest plutons are several Now walk up the stairs and about 10 steps beyond. hundred kilometers long. (A kilometer is a little Stop at the rock to your right with loose sandy mate- 6 longer than a half mile,­ about /10 of a mile.) Rock rial on top. that has squeezed through and pushed aside 2 Effects of Acid Rain existing rock is said to have “intruded” into the “Faith will move mountains,” it is said, but so will raindrops. Water surrounding “country rock,” as when strangers falling through the air picks up carbon dioxide and makes a weak intrude upon a private party or a secret meeting. acid called carbonic acid. Bits and pieces of country rock that did not fully melt into the new intruded rock look like blobs This natural acid dissolves some of the minerals that act like glue, and swirls of chocolate in a marble cake. They are causing others to break away as individual grains of sand. (Note: called xenoliths. Rocks are a collection of minerals. Sand and gravel are broken Geology Walk JAMES RIVER PARK SYSTEM

down rocks. Soil is sand and gravel mixed with organic tiny plant colony called a lichen (pronounced like­en). matter like dead leaves, rotten wood, and the remains of Lichens create a weak acid water product as they grow. animals.) Do these grains resemble some of those in the What do you think this might do to the surface of the granite blocks in the wall? (Please feel only a pinch on the rock? ground or others will not be able to enjoy this investiga­ Feel the black colored places on the rock to the left. How tion stop.) do they feel? What might dissolve away some of the min­ Walk about another 10 steps, as far as the erals and leave only these bumpy ones? From what you Beech tree roots across the trail. know about how the rate of cooling affects crystal size, did this cool slowly or quickly? 3 Root Force Many forces in nature can shape rocks. Much faster than Walk onto the pedestrian bridge which crosses rain is the pushing and expanding power of roots. They the railroad tracks. Stop at the lookout on the wind their way down tiny cracks in search of water and other end of the bridge. a firm base to support the plant. As the plant grows, the 5 A View of the River roots get thicker Look at the river. Can you pick out the flat area on either and longer and side and the steep edges where hills overlook the river? the cracks are pushed apart. Look upstream. Over the centuries (eons) the river has cut its way through the land. Freezing water also widens Floods swell the banks of the river every year. The flat these cracks. land on either side gets covered with water regularly. It This was one is therefore called a flood plain. It is flat due to layers of way colonists mud left after floods. quarried or cut The many rocks exposed in the river give this area of the out big chunks river its name, The Falls of the James. Rapids are formed of rock. (If you don’t think ice expands, look at the tops of because the hard granite does not wear down as fast as ice cubes. Notice the raised edges where there was once the rest of the river bed. Seven miles long, they begin at only flat water.) Bosher’s Dam about six miles upstream and end at Mayo’s Walk another 20 or more steps to the next set Island a mile downstream. This probably marks two sides of stairs. of the pluton of granite that is exposed here in the park and stretches all the way from New Jersey to Georgia! 4 Colors and Patterns Underground it is much larger still and it may be a differ­ The rock ahead and on the right has a modern painting­ ent shape too. like mix of several colors: green, tan, black, and light Look slightly downstream. The nearest island is Belle Isle. blue or gray. Some of the colors are the work of Mother During the Civil War it served as a prison for captured Nature. Stop for a minute to take a closer look and admire Union soldiers. Before and after the war it housed a nail its beauty. factory and a rock quarry. It also marks a major geo­ Green plants come in all shapes and sizes. The trees graphic change in land types. To the west (upstream) are around you are the largest plants visible. The shades of the rolling hills of the Piedmont and to the east (down­ dark green along the base of the rock on the right are the stream) is the flat land of the Coastal Plain. Which do you smallest called algae. think used to be the bottom of a shallow sea? How far did The spattering of light gray or blue to the right is another that ancient sea come? Geology Walk JAMES RIVER PARK SYSTEM

Caution: Do not attempt the following part of the tour if cut in the park. As the river washed away the soil and the river is flooding the area. debris, which at one time covered this pluton of granite, the pressure on top of the granite was decreased. The Walk down the stairs and at the bottom turn pressure underneath, pushing up on the granite, caused right toward the “Attention Rock Hoppers” horizontal stress sign. Behind the sign is a path. Follow the dirt lines to develop. path which becomes concrete until you reach a These made the large block. Go around the block and continue granite crack in on the path for 100 more steps. Turn left and layers. (It looks walk out onto the rocks in the river. like the layers of 6 The Rocks an onion.) It is Several forces which shape rocks, such as plant roots, termed exfoliation. Can you find some examples? freezing water, and lichens, have already been mentioned. The movement of the crustal plates, one plate pushing Another force that can shape rock is a fast moving river. against another plate, is responsible for the parallel frac­ The water in the river car­ ture lines found here. Look across the rocks towards the ries small particles of de­ opposite bank. Do all the fracture lines run in the same bris which become trapped direction? in the natural fractures in The occurrence of both parallel fracture lines and hori­ the rock. The force of the zontal stress lines make this an excellent site for a quarry running water causes these since the rock was already cut on two or more sides. small particles to move Curb stones found throughout the City of Richmond around in the fracture, were quarried from these sites and similar ones along grinding away part of the the James River. There are several sites between here and rock like sandpaper. As the hole increases in size, larger the old railroad bridge to Belle Isle. There is also a large debris becomes trapped and these also begin grinding. quarry at the 42nd Street Parking Lot. The result of this grinding away at the rock forms a pot­ hole. Get back on the cement path and continue walking toward the old railroad bridge. When Gently reach into one of the potholes (be careful of glass) you reach the bridge, and the end of the path, and pull out one of the stones. turn left and walk out on the rocks. How does this stone differ from one you would find on 8 Xenoliths the trail? Is it smooth or rough? What is its shape? Can you give an explanation for the shape? (Please put the As the liquid rock that formed this pluton squeezed rock back where you found it.) through the crustal plate the country rock was melted and mixed with the new rock. In some instances the mix­ Turn so that you are facing the river bank and ing was not complete and remnants of the country rock path. Walk three - fourths of the way to the can be found in the granite. These remnants or xenoliths path and a little towards the railroad bridge. are the darker patches of rock that you see here and are Look for a place where the rounded rock sur- responsible for the marble cake appearance of the rock. face changes to squared off edges and flat rock Locate one of the larger xenoliths. Can you determine the surfaces. direction this liquid rock was flowing? 7 Signs of Quarrying This concludes the geology walk. You have inves- This is one of the many quarry sites where rock slabs were tigated, at various steps, from the formation of the Geology Walk JAMES RIVER PARK SYSTEM

Earth’s crust to the use of granite for man’s needs, softer minerals that bind other minerals in the rock to­ but not necessarily in chronological order. As you gether. The dark colored places on top of the rock should walk back along the trail, think about what was feel bumpy. pointed out at each of the stops and try to place Softer minerals are dissolved by acids from lichens or by the information in order as it occurred in time. acid rain. Remember, rocks with small crystals cooled more quickly than rock with larger crystals. Answer Key for selected questions Stop #5: The Coastal Plain was the bottom of the shal­ low sea, and the ancient sea came as far as the 14th Street asked at each stop: Bridge where the Coastal Plain ends and the Fall Zone Stop #1: The speed of cooling is reflected by the size of the begins. mineral crystals observed. Stop #6: The stones in the potholes are usually smooth Smaller crystals indicate that the rock cooled quickly, and round because the rocks are swirled around by the while larger crystals indicate that the rock cooled more river water and rough edges are ground away. slowly. Stop #7: Not all of the fracture lines run in the same Stop #2: The grains of sand contain minerals that were direction. Most run northwest and southeast. once part of the granite. Feldspar is one of the first to Stop #8: The direction the liquid was flowing can be weather and break away from the rock, and it forms clay. determined by examining the pattern of dark bands in the Stop #4: Acids created by the lichens dissolve away the rock.

Written by Ralph White, edited by Nancy Figiel Published by Friends of the James River Park April 1993, Revised 2019