Tyler Arboretum is filled with all sorts of unique natural features to explore. Sure, there is a lot of excitement building up to the treehouses reopening this spring, and the last of the snow and ice has melted away from the multiuse trail. But let’s not forget about the gems that are outside of the fence and available for exploration – like the Pink Hill Serpentine Barren (warning – fascinating geologic knowledge coming your way!).
The beautiful “Phlox subulata” from Tyler Arboretum’s Pink Hill. Image provided by Laura McPhail.
You may have already traveled across this site, certainly if you have hiked along the Pink Hill Trail. It is not a coincidence that the trail has this name, for starting this month, you will see the native wildflower Phlox subulata blooming in beautiful pink. This moss phlox does very well across this grassland – which is fascinating scientifically, as native grasslands in the northeastern United States are uncommon.
But wait – there’s more! Even more fascinating is the fact that this grassland is really a serpentine barren, the last of its kind in Delaware County! What is a serpentine barren, you may ask? Let’s start with serpentine… Serpentine is a group of related minerals that are relatively soft and feel greasy to the touch that together form a metamorphic rock called serpentinite. These minerals typically have the chemical formula X2-3Si2O5(OH)4 where X = Mg, Fe2+, Fe3+, Ni , Al, Zn, or Mn. There are two main forms of serpentine – antigorite represents the more solid forms, and chrysotile represents the fibrous forms. Serpentine is created from extremely high temperatures and pressures deep in the Earth (but still just at the upper mantle layer) at plate tectonic boundaries. In fact, it is not uncommon to have existing olivine and pyroxene minerals in an igneous rock called peridotite be subjected to water that becomes superheated when ocean crust is near tectonic boundaries (the incredible chemical reactions that happen because of hot fluids – pretty incredible!).
Where might you find serpentine now (at least exposed at the surface)? This quote from a 1903 publication shows that scientists have had a fascination with mapping serpentine deposits for some time.
“Parts of Montgomery, Delaware, Chester and Lancaster Counties, Pennsylvania, are noted from a geologic standpoint for the presence of outcrops of serpentine rock. This rock formation is confined to the district southwest of the Schuylkill River, extending in a somewhat southwestward direction into Maryland, near the lower Susquehanna River. The largest outcrops near Philadelphia occur in the neighborhood of Lima, Delaware County, at Newtown Square, at places north and southwest of West Chester, while isolated patches exist south of Bryn Mawr and northwest of Media. There seems no doubt but that all of the serpentines in southeast Pennsylvania are altered igneous rocks, either pyroxenites or peridotites.” — John Harshberger, The Flora of the Serpentine Barrens of Southeast Pennsylvania, Science, volume 18, number 454, page 339-343. Published September 11, 1903.
Serpentine rock (serpentinite) is throughout eastern North America in isolated rock units from Canada down to Alabama (as well as out west, but I’ll just stick with the eastern exposures); however, the most diverse and botanically-significant outcrops are in southeastern Pennsylvania and northern Maryland (see State Line Serpentine Barrens), including Delaware County. The eroded serpentinite forms the foundation for a soil that is called – you guessed it – serpentine. And when you hear the phrase “serpentine barren,” this typically refers to not just the exposed and eroded serpentinite but also to the ecosystem that has formed and established in these shallow soils. You see, Eric Jones from Cal Poly explains “that serpentine soil carries the unique chemical composition containing high amounts of magnesium and low amounts of calcium. The pH of the soil greatly varies, ranging from 6.1 (slightly acidic) to 8.8 (slightly basic). In many cases heavy metals, such as chromium, nickel, and iron, are present and give the soil its red color. Serpentine soil is typically a shallow and rocky, not suitable for retaining water. These conditions inhibit plants from developing deep roots, also making serpentine soils highly vulnerable to erosion. The abnormal chemical composition of serpentine also restricts the growth of normal plant habitats, instead supporting its own unique flora which have adapted over time.”
DID YOU KNOW… Nearly half of the serpentinite found in North America is located in California, making it a natural choice for the state rock.
I hope I haven’t bored you with the science behind serpentine and serpentine barrens, but here is why serpentine barrens are important, and why we should care. They contain very unique biological communities that are not found elsewhere on our planet. Many of the key species in these communities are threatened and in danger of going extinct – and one of the causes of the endangered status is human development and encroachment. But human activity can also protect, conserve, and educate others about this really special place right here at Tyler Arboretum.
One reason I’m excited about Tyler’s Pink Hill Serpentine Barren as it provides an excellent site for me to bring my students to visit. There are not many geological areas in this immediate area that are safe for us to explore (too many outcrops are along a busy road or difficult to get to), and being able to take students to this local site will help them learn more about their own space and place in the local environment. By showing students the serpentine barren, I can help students learn more about the ground on which they live, work, and go to school. I can also introduce students to different time scales, differentiating between what happens in human timescales versus geologic time (millions of years). I think the serpentine barren is an excellent site underutilized by teachers and students, and I will be making sure that my courses explore the history and geological significance of this incredibly unique site.
To view the hill and meadow known as Tyler Arboretum’s Pink Hill Serpentine Barren, start on the multiuse trail and exit Gate 7 in the deer enclosure fence. Walk across Painter Road, then cross Dismal Run stream via two bridges to view Tyler’s Chestnut Nursery and hilltop views of the Arboretum and Pink Hill.
To read more on this subject, check out the publication Pink Hill Serpentine Barrens Restoration and Management Plan, by Roger Latham (a free PDF downloadable from Tyler Arboretum’s website).
AND, if you are STILL interested in learning more, you should mark on your calendar a speaking engagement by Roger Latham. He will speak at an open meeting about Pink Hill and the importance of its preservation at the Middletown Township Land Conservancy Meeting on April 22, 2014 at 7:30 p.m. at the Middletown Township Building, located at 27 N. Pennell Road in Lima, PA.
Journeys of Dr. G at Tyler Arboretum 
July 15, 2014 at 3:41 PM
Thank you for your very informative explanation of the Serpentine Barren at Tyler! I’m extremely intrigued by the Mg / Ca substitution activity in the plants that grow there! Very curious about what effect if any that mineral substitution has had on the animals that evolved in that environment, too!