Brush Mountain and Sliding Rocks

Strange things can happen when we mess with something that hasn't moved or changed in a quarter of a billion years.  The recent rock slide behind Logan Town Centre marks the third notable environmental problem that has arisen on Brush and Bald Eagle Mountains since the completion of I-99.  Two of the three have resulted in considerable expense to remediate, while the third was widely ignored since it was difficult to prove its cause.

The acid water contamination (from exposed iron pyrite rock) over Skytop near State College was the most newsworthy, adding a whopping $80 million dollars to an already expensive chunk of highway.  Earlier in the construction process, well water supplies west of the interstate highway were also affected when the aquifers that supplied homeowners with water were "beheaded" or cutoff when the highway cuts were made into the side of Brush and Bald Eagle Mountains.  The Brush and Bald Eagle ridges are actually geologically the same mountain and have two different names only because the Little Juniata River created a water gap in Tyrone.  The ridge actually wraps around the end of Sinking Valley and is still called Brush Mountain on the opposite side of the valley.  The rocks and slopes of all these mountains are nearly identical.

Brush Mountain is a plunging anticline.  In simple English, that means that an upwardly folded bunch of rocks ends or plunges downward, in Brush Mountain's case just north of Hollidaysburg.  Frankstown Road climbs up and over the southern end of the anticline.  Like the rest of the Appalachian Mountain system, the 2,500 foot high Brush Mountain was once much larger than it is now.  What we see today are stubs of what were Himalayan-sized peaks about 250 million years ago.  A quarter billion years setting out in the weather will do that kind of damage, even to hard rocks.

The mountain has survived that test of time better than other rock formations because the Tuscarora Sandstone is so hard (and more resistant to erosion).  If the Tuscarora and its surrounding rocks are left in place, they don't usually go anywhere and the water that passes through it flows uninterrupted.  But because of the still relatively steep slopes, it does not take well to major earth moving.  The eastern sides of the mountains (moving down into Sinking and Nittany Valleys) the rocks are closer to perpendicular to the surface.  One of those rock layers contains rocks with iron and sulfur and once exposed they produce acidic runoff and groundwater.

On the Altoona side of the mountain, the rock layers run close to parallel to the surface and the water in the sandstone aquifer flows toward Altoona.  When I-99 was first built, the road cut ripped apart the base of the Tuscarora formation, beheading the aquifer.  This has long been visible at the cut at the Graizerville interchange but could also cause the failure of wells supplied by that rock formation.  The same thing happened when Logan Town Centre was built and we can see this in the form of waterfalls behind the plaza during wet spells.  The orientation of the rock layers also contributes to the likelihood of slides, since the rocks will slide on each other.  Given the weight of those rocks above the plaza, it's easy to see why fractures and slides could occur.  Once we understand the geologic forces at work, these occurrences don't seem quite so unexpected after all.