One of the showcase pieces of our southeastern Pennsylvania landscape is the Susquehanna River. Not only is this the largest body of water in our region, it also provides us with some scenic views that have been drawn by artists and captured by photographers for many years. 

But before we get into some particulars about the river itself, let’s look at some general facts. First of all, the original spelling was “Sasquesahonough” and named for the Indians that called the river valley its home. Can anyone guess where the headwaters of the Susquehanna River are located? I discovered the answer myself by accident in the 1980s. Returning home on a trip to a world-famous mineral collecting site, I passed a historic marker stating that this location was the starting point of the river. 

My reaction was “wow.” 

The Susquehanna River is actually better known for a sports attraction — the National Baseball Hall of Fame in Cooperstown, N.Y. Lake Ostego in Cooperstown is where the water starts its journey southward. The main branch and the west branch of the Susquehanna drain a total of 444 miles to the Chesapeake Bay, draining a total of 27,500 square miles. This makes the “Susky” the longest river on the East Coast and the 6th-longest non-commercialized river in the world.

That’s another “wow.”

Topographic map of the Susqueahanna River through the Applachian Mountains

When rivers are developing their original flow across a landscape, the water obeys one principle: It wants the easiest route possible with no resistance. In other words, the water doesn’t want to work for a living. When one imagines the landscapes surrounding the Susquehanna River, there are many ridges and hard rocks that the water currents cut through. For example, picture the drive from Harrisburg north to Selingsgrove, a trip through part of the Appalachian Mountains. If you examine an aerial photograph of the area, you will find that most of the gaps you travel through on your drive are actually the same ridge that was bent by great tectonic forces millions of years ago. Only as you get close to Selingsgrove does that ridge not traverse the river.

View of the river from Pinnacle Rock in Lancaster County

The amount of rock and dirt removed from one of the water gaps is mind-boggling. I did a quick calculation on the amount of material removed from the water gap between Hellam Point in York County and Chickies Rock in Lancaster County for a recent program. I estimated over 349 million cubic yards of rock and dirt carved out by the Susky. That number alone is hard to imagine; now, think about the number of water gaps along the route of the entire river! 

These water gaps are awesome. Think about how much material has been carried into the Chesapeake Bay and the Atlantic Ocean thanks to erosion.  Near Bainbridge and Perryville, Md., there are exposures of sediment deposited by the river that are about 30 feet high.  The Susquehanna River has been nationally recognized for its water gaps and has attracted college students from across the country.

The Susquehanna River from Shull’s Rock York County looking north toward York Haven

Just how old is the Susquehanna River? The answer has been debated since the beginning of modern science. Some scientists believe that the river was in place prior to the formation of the Appalachian Mountains some 340 million years ago. If that were the case, the river would have been flowing in the opposite direction because the highlands were located to our east and the ocean was migrating to the west. If it were formed after the great collision between Africa and North America and the supercontinent Pangaea, the Susquehanna River would have flowed in its current direction.

It is mentioned in the classic “Geology of Pennsylvania,” published by the Pennsylvania Geologic Survey and the Pittsburgh Geologic Society in 1999, that the Conewago Member of the New Oxford formation in York County was formed by the Susquehanna River. That rock unit is about 210 million years old and forms the Conewago Mountain north and east of Dover.

Quartz pebbles found along Cherry Hill Road

If one were to write a book about the history of the great river, the largest chapter would concentrate on the Ice Age period, between 100,000 and 10,000 years ago. Yes, it is true that glaciers did not cover any of York County. In fact, the closest wall of ice was located about 50 miles to the north of Harrisburg. However, during this time, with oscillating periods of warm and cold weather, the ice would retreat and advance with change. When the mile-high wall of ice would melt during the warmer intervals, additional water would be sent down the river valley. Additional water would increase rates of erosion and downcutting. It was during this time that great downcutting occurred through our crust in Pennsylvania. Earlier geologic works have mapped some of these river terraces, which indicate a previous location of the river. Some of these terraces are 300 feet above the Susquehanna River’s current elevation. For example, along Cherry Hill Road in southern Lancaster County on a hill about a mile from where the river flows now, there are water-worn quartz pebbles marking one of these terraces.

Cross section of the Susquehanna River from Harrisburg south to the Chesapeake Bay

Holtwood Dam

To examine the forces that the Susquehanna River has placed on soft and hard rocks in our crust, travel to what is called the “Deeps,” south of the Holtwood Dam. Park at the Lock 12 Holtwood Dam parking area and walk south on the trail. After passing the restored Lock 12 and the heavily overgrown Lock 13 under the Norman Wood Bridge, you’ll find yourself out on the island. Here you can see and hear the forces of water hitting upon the metamorphic rocks. Look for places where the rock has been carved away, the formation of potholes, and channeling. This is a wonderful place to go to have a picnic lunch and admire the beauty of one of the most famous sections of the Susquehanna River.

The “Deeps” south of Holtwood Dam

Lock 12 at Holtwood Dam

The rapid downcutting of the Susquehanna River during the Ice Age affected the tributaries flowing into the river. These tributaries did not have additional water flowing in their channels to increase their erosion. The end result was that the tributaries were not able to keep up with the downcuttting of the river, thus resulting in what geologists call a “reverse” profile of these tributaries.

Normally, the steepest section of a stream would be at the headwaters and the channel would become flatter as it came closer to the river. Not the case here. The tributary is steeper closer to the river, resulting in scenic waterfalls near the junction of the Susky. Locations such as Wildcat Falls, Accomac Gorge and Otter Creek in York County and Tucquan Glen and Shenks Ferry in Lancaster County are great representatives of such a case.

A waterfall at Accomac Gorge

I have touched on only a few of the outstanding features of the Susquehanna River here.  One of the field trips I’ve designed and conducted for groups is a tour around the river between Columbia and Holtwood Dam. Drive River Road in Lancaster County to Holtwood Dam, cross the Norman Wood Bridge and travel up Pa. Rte. 74 and Pa. Rte. 425 to Indian Steps to explore the river and its hills.  Much human history (prehistoric, historic and industrial) is found around every turn of the road.

How many of you have taken a walk either in a developed area or in the quiet woods and stumbled over an interesting-looking rock?  Have you ever wondered how the rock was formed or how it got here?  Interesting rocks can be found anywhere and all it takes is to reach over and pick it up.  Every rock has a story to tell and this blog will tell these stories.

Each day everyone comes in contact with both geology and history.  This blog will explore some of the most fascinating locations in York County. This column will introduce stories about the origin of some interesting rocks, how we use natural resources and how we live with our geology.  Related topics are certainly welcome from the readers.

What better place to start than to introduce you to the general layout of the land and rocks in the county.

The highest point in York County is Stone Head, high atop South Mountain west of Dillsburg.  This exposure of quartzite is at an elevation of 1,371 feet above sea level.

The lowest point is along the Susquehanna River at the Mason-Dixon Line at an elevation of 200 feet.  That means that York County has a total relief of 1,171 feet.  Because of its magnificent view, some people regard Same Lewis State Park as one of the highest points.  Actually, this location and elevations in the Pigeon Hills near Hanover and the Hellam Hills in the eastern section of the county are just over 800 feet above sea level.

In the northern part of the county, the steep-sided hills of Roundtop (Ski) and Nell’s Hill are elevations of 1,355 and 1,330 feet respectively.  The rolling hills of southern York County are generally 800-900 feet in elevation with one ridge near Airville capping off just over 1,000 feet.

York County hosts igneous, sedimentary and metamorphic rocks.  These rocks range in age from about 600 million years old to as recent as 150 million years. The southern third of the county is composed of mostly metamorphic rocks that tell us of a vast ocean and several continental collisions.

The historic U.S. Route 30 lies within a limestone valley from near Thomasville eastward into Lancaster County. The limestone and surrounding rocks on the bordering ridges reflect a breakup and construction of an ancient continent.  Proof of a beach can be seen in the rocks in the now abandoned York Silica Sand Co. quarry behind the Harley-Davidson plant.

The northern third of the county is underlain with sedimentary and igneous rocks that are the youngest in our area.  These rocks tell us the amazing story of when York County was an Everglades swamp followed by the breakup of a large continent known as Pangaea. It is hard to realize that over a course of one billion years, York  has seen several episodes of continental collisions and breakups, an ocean environment, volcanic activity and dinosaurs walking through the mud of an ancient swamp.  York County has it all in the eyes of a geologist and offers many adventures that will be included here.