Geology is not an exact science – the Martic controversy
Like most sciences, geology is not an exact science. Much of the understanding of how the Earth System works and why it operates the way it does is dependent upon many scientists’ thoughts or theories. Thankful for these scientists, we have evolved as far as we did in this understanding of how our landscape has developed through repetitive cycles of crustal collisions and riftings. But this series of theories was not easy to come by, and matter of fact, still written in stone. Remember theories are ideas supported by evidence that are acceptable to the science world. These theories are always open for discussion and change. When I was a college student and reading a research paper on an area I was working in, the author’s paper started out by saying:
“If you don’t accept my thoughts disclosed in this paper, write your own.”
One of the largest controversies in geology in southeastern Pennsylvania is centered on what is today known as the Martic Line or Martic Fault. This geologic structure is easily traced on a geologic map from at least Maryland into eastern Pennsylvania where it connects with the Doe Run Fault near Philadelphia. In York County, the Martic Line is found at the southern end of Long Level at the Susquehanna River, runs through the north edge of Dallastown and Jacobus, about 1 mile north of Jefferson and occupies one of the long sections of Lake Marburg in Codorus State Park. If one would stand on the Black Rock Road bridge at the western end of the state park and look east, you are looking along the Martic Line.
What makes the Martic Line so intriguing? On a geologic map, the structure stands out. The Martic Line divides the metamorphic rocks to the south and generally sedimentary rocks to the north. Phyllites, slates and schists making up the Marburg and Octoraro formations are separated with limestone of the Conestoga Formation. Even a non-science brain would ask why is there such a sharp division there between metamorphic and sedimentary rocks?
A section of the geologic map of York County by George and Anna Stose (1944) showing the Martic Line near Dallastown.
We have to go back into history a short distance to introduce this Martic Line controversy. What helps to make this such a controversy is the lack of good rock exposures to study. First of all, faults are not uncommon, particularly in a region like ours that has gone through major continental collisions and separations. Often, we have a good rock exposure like in a quarry to do our examination and measurement of a fault to interpret what occurred there. However, for the Martic Line is poorly exposed along its line of existence. Even from its type site at Martic Forge in Lancaster County, there is not a good exposure to study. The best exposure is found along the Enola Low Grade Bike Path in Provenance Township near Quarryville, Lancaster County. More details about that later.
Stose and Jonas (1939) map of the geology of York County showing the Martic Line south of Hanover, now the location of Lake Marburg.
The recognition of the Martic Fault was first published by Eleanora Bliss Knopf and Anna I. Jonas in 1929. These two females were laying the foundation for women in geology and had graduated from Bryn Mawr College in Bryn Mawr, PA. Their professor was Florence Bascom, who was nationally recognized as the first woman in geology. Florence not only has a crater on the Moon named for her but also an asteroid. Her life is worth its own blog sometime.
Eleanora and Anna recognized in the Coatesville area that a group of rock formations known as the Glenarm Series was sitting on top of a slightly metamorphosed limestone known as the Conestoga Formation. They believed that the Glenarm Series metamorphic rocks (schists) were much older than the Conestoga Formation rocks. Eleanora and Anna’s thinking about the age of these two rock units is what created a separate argument with Florence Bascom. Eleanora and Anna believed the Martic Fault could be traced from at least Virginia eastward to Coastesville where it connects with the Doe Run Fault which continues into New Jersey. They also hypothesized that the rocks of the Glenarm Series were moved approximately 20 miles to the northwest over younger-aged rocks. Well, let me tell you, this published work generated a bunch of controversy (that is only an editorial note since there was not way I was around here yet).
One of the leading non-believers in the controversy was J. Hoover Mackin who published an article in the Journal of Geology in 1935. He presented evidence not supporting the Martic Fault theory. Many supporters against the theory also rose to the top. Was it because these two trend-setting females came up with an idea on their own and women weren’t too well accepted into the geologic world yet? In their two classic woprks in York County geology, George Stose and Anna Jonas Stose identified this structure as the Martic Overthrust. (See an earlier blog regarding George and Anna’s life together). So this debate continued for many years. Various publications and field trip guides centered on this controversy with no “clear-cut” decisions. Researchers used a variety of tools to attempt to provide conclusive evidence such as geomagnetic work, the degree of metamorphism across the “line” and the structural orientation in the rock. I would have loved to be present for some of these classic discussions on a field trip – that would have been a classic to have recorded.
Since the turn of the century, thinking among the geologic world began to change on the origin of the Martic Line. Further research involving the theory of plate tectonics and establishing a picture of how our landscape evolved placed the Martic Line has the edge of what has now been called terranes. Terranes are pieces of the Earth crust that had a origin separate that that on our continent and later attached itself onto our landmass. Different terranes have been identified in the Mid-Atlantic states by geologists. More recent dating procedures have been conducted by Gale Blackmer of the Pennsylvania Geologic Survey across the Martic Line to determine the age and degree of metamorphism.
Don Wise (2006) map showing the terranes of our region.
Today, the Martic Line is widely accepted as a low-angle fault known as a thrust fault. Many maps label the structure as the Martic Fault. In the mid 2000’s, Don Wise, a former professor geology at Franklin and Marshall College and now at Amherst College in Massachusetts, created a new regional map of southeastern Pennsylvania and eastern Maryland showing the various terranes. Using this theory, the Martic Fault has a definite role in the history of plate tectonics in our region.
It is surprising to me that a number of residents have heard of the Martic Fault. This stemmed from the April, 1984 earthquake centered near Martic Forge, Lancaster County. While being interviewed about the earthquake, Dr. Charles Scharnberger of Millersville University was asked by a media writer if a faults occur in this area. Dr. Scharnberger responded yes we have the Martic Fault. The reporter mis-interpreted that the Martic Fault was the key player in the origin of the earthquake. The readers of the article believed that the Martic Line was active and the center of earthquake activity which is totally a false statement. The earthquake was actually along another fault not directly associated with the Martic Line.
THe Enola Low Grade Railroad bike trail exposure showing the Martic Fault. The Octoararo SChist is on top of the Conestoga Formation limestone. Dr. Charles Scharnberger, my dear friend and field trip host poses for the photographer.
Finally, if anyone would like to view the Martic Fault, travel to the Enola Low Grade Bike Trail in Providence Township just west of Quarryville, Lancaster County. On the west side of the path just north of the Sawmill Road bridge (mile marker 3.8), there is a large rock exposure. If you examine this outcrop, the Octoraro Schist lies on top of the Conestoga Formation limestone. This is the best exposure of the Martic Fault known in Pennsylvania.
Looking for fossil collecting sites – the Kinzer/Cambrian formation – we live in Phoenixville PA and would like to do some collecting in York County. Not sure where the Locust Lane site is but have some general directions from the google maps – any other sites nearby where we could look? Plan to visit this spring as soon as the weather warms. We are frequent visitors to St. Clair and other fossil sites. Thanks.
THere will be a blog up anytime now describing some sites.
The Conestoga Limestone in the bottom of that outcrop is just barely limestone. 10% HCl dissolves the Calcite leaving 50 to 75 percent of the rock intact. The remainder appears to be Muscovite, Quartz and maybe feldspar. Is this normal for the Conestoga or is it a transitional phase between phyllite and marble? Also, I was expecting to see more fault gouge or techtonized material. It could be that it has been so long since there was,any movement on the fault that any gouge has been metamorphosed to phyllite or that the whole lower part of the outcrop is gouge! This is a topic that interests me considerably since the line is drawn as being 20 feet north of my house near Downingtown. I know, the lines are only approximate.
Hi Bob:
You are pretty good on your evaluation of the Martic Line. The Conestoga is not a normal limestone but has been metamorphosed and in many places resembles a phyllite than a limestone. Any gouges have been eroded or destroyed by later tectonic events. As you are aware that the Martic Line has been a controversy but recent work in the Piedmont is starting to set some things about it clearer. Generally, depending upon what scale map you are using, the lines on the map aren’t too far off. I know Bill Kochanov of the PA Geologic Survey has been doing some mapping in your home area.