The Hatch Site: Why It's Here; Of Landforms, Soils and Sites

The Hatch Site is one of a complex of stone tool workshops all focused on the use of the Jasper quarries up the valley.  What makes it unique and important is a happy accident of topography.

The site sits in a small, bowl-shaped area at the foot of some steep slopes that lead up-valley.  These surrounding slopes are ultimately, the reason the site is so well preserved.  To understand that point, it's necessary to first grasp exactly where what we call soil (and what a geomorphologist calls sediment) comes from. Once you understand how soils form, you can begin to understand how the archaeological sites within them form as well.

Of course, ultimately ALL soil comes from interactions of weather, gravity and bedrock geology.  But that interaction can happen in many different ways.  In the vicinity of the Hatch Site, there are three basic types of soil formation going on.

On the relatively level uplands west of the site, soil is forming in place as a residue of the slow weathering and decomposition of the underlying limestone.  This residue is called, unsurprisingly, residuum.  It's not usually very deep, and consists of a dark, organically enriched surface layer (the topsoil or A Horizon), a lighter colored clay or silt layer beneath the topsoil (the B horizon), and often coarse, stony, and even lighter colored material decaying from the bedrock (the "parent material" or C horizon). In residuum, artifacts and site features are only found in the topsoil (which in Pennsylvania, an agricultural breadbasket for over two centuries, is almost always plowed) and on the very top of the underlying B horizon.

Water, wind and gravity are always removing material from the surface of the A horizon, even as the underlying B horizon becomes enriched with organic material via the actions of roots, earthworms, and other animals and insects, and creates new A horizon from beneath.  The material eroded from residuum creates new kinds of soils, called depositional soils. One of the ways that happens is via transport by gravity down hill on slopes.  This produces deep soil deposits at the base of the slopes called colluvium.

Another kind of depositional soil is produced when sediment is picked up, carried and deposited by streams.  The banks of streams are often mantled with silt and sand left behind by floods.  These level, stream-side areas are called floodplains and the soils that comprise them are alluvium. 

Both colluvium and alluvium have the potential to bury archaeological sites, sometimes to surprising depths.  As colluvial and alluvial deposition continue over centuries and millennia, they can also preserve superimposed layers of artifacts and features from successively older and deeper human occupations. These are called stratified sites, and they are the most valuable kinds of archaeological sites.

The goal of archaeology is the detailed reconstruction of human behavior from the objects and features left behind by our predecessors. Stratified sites make it relatively easy to tease apart the evidence from specific occupations, because they're separated from each other like layers of icing in a layer cake.

At the Hatch Site, a thick layer of colluvium eroded from the surrounding hill slopes when the little valley was first logged and farmed in the mid-19th century. This colluvium has buried and sealed an old topsoil or A horizon, that was plowed, but only during the 19th century. That A horizon contains thousands of jasper flakes, tools, and other detritus that documents the stone tool production process.  Beneath the A horizon is a B horizon that appears to be alluvial, and contains artifacts deposits in sediments that that have not been plowed.  The site has the potential to produce some very fine-grained pictures of day-to-day life at this stone tool workshop as it was used repeatedly for centuries.  Thanks to the cap of colluvium at its surface, you would never even know the site is there!

A note: The first week of excavations was completed at the site yesterday.  In the next installment I'll report on their progress so far, and I'll try to describe what we've learned about the process of making stone tools from modern experimental archaeology.

The Hatch Site: Beginnings

The Hatch Site: Underpinnings

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