Maryland Department of Natural Resources

Twiggs Cave, Allegany County

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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 Straight Way in Twiggs Cave (photo by Dennis Slifer)
The Straight Way in Twiggs Cave

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.


The entrance to Twiggs Cave is through a vertical shaft, 25 feet deep, 4 to 8 feet long, and 1 foot wide. At the base of the shaft is a "Z" shaped passage about 40 feet long and sloping 45º which connects with the first room. This room has two "mud glaciers" at its south end that are slowly moving and covering the floor. The cave continues in a northerly direction from the is room as a passage, the Straightway, that is 10 to 15 feet wide and 10 feet high. Just north of the first room is a pile of broken rock leading down 25 feet to the level of the Straightway. The floor of the Straightway is made of fallen rocks to a considerable depth and many minor passages exist beneath the main level. The Straightway, 85 feet long, ends in two chimneys at the base of which is a deep well. A narrow, crevice-like passage leads off the base of the first chimney and continues into a small room that ends in a mud wall. A small crawlway, 4 feet in diameter, leads off the west side and curves around to a narrow clay shelf at its junction with the second major fissure passage. Here is a row of six pits leading to a lower tunnel. The largest pit, the Kings Chair, affords access to the lower level that is a low tunnel 3 feet wide, 2 to 5 feet high, and 40 feet long. The passage continues along the base of a high sloping crevice. The passage is 15 feet wide and slopes steeply for 100 feet to a drop of 14 feet. This point, 192 feet below and 375 feet from the entrance, is at the stream level of the cave.

The stream enters this section of the cave from below by a steeply sloping and curving shaft over 75 feet deep. The stream forms three shallow pools, each about 10 feet long, and then flows along the floor of the cave. Twenty feet beyond the drop to the water level is a shallow pit into which the stream plunges with a loud roar. The cave continues as an ever narrowing passage, decreasing from 20 feet high and 10 feet wide to 3 feet high and 2 feet wide. In places the route of traverse is in the stream bed, in other places along a clay ledge above the stream. At 120 feet this passage ends in a small tunnel 2 feet high and 1½ feet wide, the floor of which is occupied by the stream. This passage, which necessitates a crawl, is 25 feet long and ends in a narrow crevice-like passage 10 feet high, 4 feet wide, and 10 feet long. At the southwest corner is a narrow slit in the floor down which the stream plunges to a lower level. This falls is over 50 feet deep. Bottom was not reached in soundings.

In 1946 a small tunnel was excavated through packed clay and gravel to a low passage that leads 30 feet to a pit which is 40 feet deep and 5 feet in diameter at the top. The base of the pit is a slit 1 foot wide which leads off into a crevice passage less than a foot wide. Traverse of this passage was not possible, but the cave appears to open up somewhat 30 feet beyond. The floor at the base of the pit is covered by a thin veneer of yellow clay of recent deposition indicating that the stream backs up to this level. The base of the pit is 290 feet below and 750 feet from the entrance.

Above the Straightway is another passage resulting from the fall of large blocks of limestone that form the ceiling of the Straightway and the floor of the upper level. The two levels are connected by a number of openings between the rocks, but access to the upper level is afforded at only one point. At the south end of the Straightway, near the 25 foot drop, is a flow-stone ledge on the west wall that slopes steeply to the east. A traverse diagonally across the flowstone leads to a small hole that gives access to the upper level. The upper level passage is 10 feet wide and extends for 100 feet as a fissure similar to the lower levels. The fissure is 30 feet high and tapers out at both ends. Several massive stalactite formations are developed on the east side of the passage, and one of them gives beautiful musical tones when struck.

An upper level exists above the Kings Chair. It is a ledge 10 feet wide that lies 30 feet above the base of the fissure passage. At the north end this level develops as a separate passage extending over 100 feet.

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