MORPHOGENESIS OF BOULDER CAVES IN THE NORTHEASTERN UNITED STATES

There are numerous caves formed in the northeastern states within boulder deposits or beneath or within the mass of a large boulder. Also known as rock-fall caves or talus caves, these are openings within an accumulation of boulders that have become dislodged through mass wasting and slope failure or deposited by glacial ice. These caves may be found within scree along the bases of escarpments, on steep slopes, or in valleys. Caves may be formed beneath a single boulder, or in a large split rock where fragments have separated and moved, creating an opening.

Most of the more significant talus caves in the northeast are the result of a three-dimensional accumulation of clasts that vary in size, sorting, and shape. Clasts that comprise talus may be (1) spheroidal, discoidal, or rod-like boulders, (2) blocks, or (3) slabs or blades. The size and extent of openings (caves) among the clasts depends on the texture of the talus accumulation, including size, shape, and sorting of the particles, much in the same way as these parameters determine the degree of porosity in finer grained sediment (sand, gravel).

Talus caves have three stages in development. (1) In the primary stage, the host rock is fractured, creating blocks bounded by joints. The nature of fracturing, including extent, density (spacing), and orientation of fractures will determine the size, shape, and sorting of the clasts. (2) In the secondary stage, rock fragments become dislocated and moved, creating enterable openings (caves). Processes in this step include movement under the force of gravity (rock falls, rock slides, toppling, creep, and other mechanisms), fluid transport of clasts by fluvial or glacial action, and packing of clasts into their final orientation. (3) In the tertiary stage, diagenetic changes occur within the talus deposit, including physical and chemical weathering of clasts and changes in sizes of interstices through infilling of interstices with finer grained particles or removal of fill by erosion. Superlative talus caves (long, deep, or voluminous openings) occur where talus deposits are sufficiently thick, clast size is maximized, sorting of clasts is high, interstitial matrix of fill is initially minimal or has been erosionally removed, and openings remain large over time.