The Borrow Pit

The Borrow Pit, located right off the Salado Greenway Trail and Geology Trail, is a great spot to see the formation of upper rock layers and soil. This area received its name from when the Voelckers “borrowed” gravel to build roads on their dairy farm, which was part of the “Buttermilk Hill” region of Bexar County.

The pit consists of a scooped out half circle on the northern part of the Geology Trail 6 to 8 feet down from the surface, as the terrain gently slopes downward toward Blanco Road.

The Borrow Pit walls consist of uneven layers of cobbles made of limestone and marl which have been lithified (stuck together) to form conglomerates. These stones found their way into the area after the Balcones Escarpment uplift caused the land west of San Antonio to rise and wash materials downstream. The outcrops we see along the hillside were deposited between 1 and 2 million years ago. The deposits sit on top of the Buda Limestone formation, which can be seen on the surface of the Salado Creek bed off the trail just past the Voelcker farmhouse.

If you observe the sides of the Borrow Pit closely, you’ll notice some layers are grayish and others rusty, some almost solid, and others crumbling into sand at the pit’s base. The differences result in part from minerals leaching out of the rocks. There are also some little “caves” in the deposits which formed as the result of erosion.

Rocks continuously erode in the pit, exposing live oak roots. You can trace the roots down from the base of one large oak tree to see how they both grow around the stone and force their way through it, creating further erosion. Amazingly, the pit contains one root which extended horizontally beyond the hillside and transformed into branches!

Micro-climate

The Borrow Pit’s walls create a warm and wet micro-climate. The walls shelter plants and animals from high winds. Due to this shelter, two hackberry trees have grown approximately 50 feet tall — which is very rare! Hackberries have notoriously shallow roots and are usually blown down by storms before they reach 20 feet in height.

Water flows through the stone from the thin, shallow soil up to the surface. The high walls create shade, which slows down water evaporation. Thus, sedges remain green in the pit even in winter. Indian mallow, Turk’s cap, prairie vervain, soft-haired marble seed, Texas baby blue eyes, and zexmenia also grow inside the pit, but not as thickly as in other parts of the park due to lack of sunlight.

Moisture on the sides of the pit provides the perfect place for lichens, moss, and even the occasional fern to grow. Overhangs and “caves” provide habitat for large colonies of insects, such as harvestmen, springtails, crickets, and beetles.

This area is an excellent place to explore and many classes and field trips have been held here.