MD Rocks State Park: Boulder Field Hollow

Just across the Mason Dixon Line in Harford County is Rocks State Park which is known mostly for massive outcrop cliffs and meandering Deer Creek. It is not unusual to see the parking areas for the King and Queen Seat and the picnic groves on the creek quite crowded in summer and fall. I wait until winter to visit, however. One of my favorite places to explore is a small wooded, boulder-strewn  valley that sits a mile or more behind the cliffs. It's where springs seep into the hollow and becomes a tiny stream.

Mountain Laurel in the freeze-thaw talus boulder field

Anyone driving through the park can observe the freeze-thaw talus slopes. These moss and lichen covered tumbled-down boulder fields are found on the north facing shoulders of the big ridge. Relics of the Pleistocene, the boulder fields originated at a time when glaciers had reached their maximum sixty miles north and began their great melt. The environment was much colder and harsher than it is today and this form of freeze-thaw-shatter boulder making, a type of mechanical weathering, is a beautiful and biologically important habitat in the PA/MD Piedmont region for mosses, lichens, ferns, and amphibian life.

Black Walnut in the old farmstead.

The entrance to the hollow is on a small nature trail loop across the road from the Hills Grove Picnic Area. A farm once stood here and the ground was cleared for livestock pasture. A signature tree of the old farmstead are the Black Walnut and Tulip Poplar, overtaking the flats where a barn and house once stood. Higher up are the White Pines that today are shushing in the wind. 

Tulip Poplar where a cattle shed once stood.

White Pine in the old cow pasture. 

The trail climbs up into the hollow as boulders crowd the landscape. White Pine and oaks dominate the woods beyond the reach of the old farm's clearing and the elements of a wetland forest begin to take shape. Mountain Laurel thickets, twisted and old, grow boldly on the boulder field while Witch Hazel occupies the seepage that mark springs.  

A very old Mountain Laurel.

Witch Hazel, the last (or first) flower of the year. 

This is where I leave the trail and pick my way higher into the hollow, careful to weave through the mossy boulders and not lose my step and I am only partially successful at this and come down hard on both knees - thankfully on moss and not stone. It is a maze of frost-shattered rock. 

Sphagnum Moss.

Plume Moss

Crustose lichen in a "dead bed" 

Where boulder mosses have dried up or gone dormant and formed  a "dead bed," crustose lichens find a niche. I poked my sketching pen into a dead bed on the top of a boulder and it went in several inches before hitting rock. This must have been hundreds of years of top growth sealing the rock beneath.

Hair Cap Moss.

Ground Pine Lycopodium.

The boulders are so tightly packed that there little space for a tree or shrub to put down roots.  But there's plenty growing in between. Fallen leaves trapped in the angles and gaps between boulders make rich habitat for clubmosses and ferns. Matted leaves also make bowls to hold water that seeps up from the cavities. A Carolina Wren splashed and bathed in a leaf bowl just below me. Broader mats of leaves between boulders form steps of water that overflow into the crevices and begin to trickle downhill.  

Seepage pools atop fallen leaves and begins to run downhill.

I sat in the upper reaches of the boulder field as the sun dipped behind the ridge and listened as water dripped and slipped under the mossy rocks into the fold of hillside that becomes a tiny stream in the valley. The micro-habitats of the boulder field are mostly dormant for the winter now. There's nothing left of the luxurious fern groves of summer, just stems of fronds and root clumps. And where the trees can grow, they push through the rocks in clumps of hangers-on where roots can penetrate. The freeze-thaw cycles of bitter arctic temperatures have long ago stopped working on the ridge and the boulders have been settled in place for millions of years, but on this afternoon - closing in on winter solstice - the low sun illuminates the violet-hued air breathed out from the buried springs and seeps, rich and cold and ageless.   

Notes:  Science Friday has this cool little video "This Field Rocks" about mechanical weathering and the creation of the Hickory Run Boulder Field about two hours north of Rocks State Park. Though the mechanics of freeze-thaw of seep water operated in the same way to make boulders,  the Rocks talus slopes are older and well grown over.   https://youtu.be/higFSvxyKRo