In the vicinity of Twiggtown the Helderberg Formation is comprised of three members totaling about 325 feet in thickness. Twiggs Cave is developed in a series of dark gray to black crystalline limestone beds that lie about 25 feet below the base of the Coeymans Member and 250 feet above the base of the Keyser Member. The limestone is somewhat knobby and is high in clay content. The cave is developed on the western side of an anticline. The beds dip 60º W and strike N 40º E. A series of master joints trends N 40º E and dips 60º E. A series of subordinate vertical joints trends N 50º W.
      The cave is developed as two large parallel fissure openings along the master joints. Bedding planes are relatively unimportant except at the entrance where they form a small sloping passage. In cases where collapse has occurred the bedding planes are reflected in the walls of passages. The subordinate joints show up only in a few small side passages and in the low tunnel connecting the parallel fissures.
      The large amount of clay in this cave is of considerable interest. In the first room at the base of the entrance slope a large amount of clay in the form of two "mud glaciers" is encountered. The clay is entering the cave through two chimney-like passages in the south end of the room and is a result of the accumulation of surface-derived material. These mud glaciers have characteristics similar to normal ice glaciers although their power to excavate material is practically nil.
      The clay encountered throughout the rest of the cave cannot be so easily accounted for. The high clay content of the original limestone (12% by weight) contributes considerably. In the ceiling the process of leaching of the lime and development of residual clay is easily seen. The solid limestone forming the ceiling grades into a soft, light gray to white, somewhat coarse clay that has a thickness up to one inch. Outside of this is a zone of fine, wet, light brown clay about one quarter inch thick. The surface layer, less than an eighth of an inch in thickness, consists of somewhat coarse dark brown to dark gray clay. The surface of the clay has the shape of rough, blunt, clay stalactites, each stalactite being less than an inch long and about one quarter inch in diameter. The clay of the cave floor and lower walls is entirely different, and its origin is open to various interpretations. It is generally dark brown to dark gray in color, 6 inches to 1 foot thick, and laminated. The individual laminae average 2 to 4 millimeters in thickness and are distinguished by minute changes in grain size. Two methods of origin appear possible. The clay may be a result of deposition from the underground stream which in wet weather may flood considerable portions of the cave. However, since the laminations are often found a considerable distance above the floor with the laminations following the contour of the walls, this hypothesis seems unlikely. The other hypothesis ascribes the source to the parent rock which is high in insolubles. These insolubles are deposited by thin films of water as the limestone is dissolved. The water transports the material from the upper part of the walls and deposits it in the lower sections. The laminations reflect varying conditions in the amount of water seeping into the passages.

(from ES3 -Caves of Maryland)