The description below is an electronic reprint excerpt from: "Caves of the Jacks Fork Valley", Journal of the Missouri Speleological Survey, Volume 22, Numbers 1-2, Pages 34, 35, 37.

JAM UP CAVE (SHN 020)
The large river entrance to Jam Up Cave is one of the most impressive entrances in Missouri. The primary reason for its great size is breakdown enlargement, the great masses of stone partially obstruct the large opening. The blocks obstruct the stream, but water does manage to bypass them to gain access to the inner part of the cave, it is necessary to ascend to the top of the breakdown hill and go down the other side. A fairly easy path descends and eventually reaches water level at a gravel beach. This beach is roughly in the center of the entrance room, a high elongated passage several hundred feet long. It is possible to proceed a short distance farther on a large block of stone, but this block marks the end of the dry passage on the river end of the cave. Because of the large entrance, daylight penetrates back to this point and illuminates the far end of the room dimly. The rear termination of the room consists of a wall 15 to 20 feet high extending almost to the ceiling. Above this wall, the passage can be seen to extend back still farther. In the distance, probably several hundred feet away, light from the large sink entrance is visible. On the right side the stream cascades down to the pool through a breach in the wall.
The passable portions of Jam Up Cave vary with the water level. During our visit (Tucker, April 1963), there was no water flowing in Jam Up Creek, and the water level in the cave was probably low. The stream entrance is 30 feet wide and 18 feet high, with Jam Up Creek, when flowing, coming in along the west wall. Along the right wall at the entrance there is a small spring, and the passage beyond it was not explored. This portion of the cave may be followed 150 feet until it becomes necessary to get wet above the waist to continue. Further on the ceiling appears to come to within 9 inches of the water level, with water at least 4 feet deep. Many large logs clutter this entrance, and it obviously floods during heavy rains. The passage from the entrance averages perhaps 30 feet wide and 10 feet high. The floor is covered with water and breakdown. Frogs and fish were common in the water.
The sink entrance is a ponor, 15 to 20 feet wide and 50 feet long, and 70 feet deep. It is possible to enter without a rope. At the bottom there is a large pile of breakdown and debris which has fallen in, along with debris carried in during floods by the stream. To the left or upstream the passage averages 20 feet wide and 25 feet high for approximately 200 feet. Here it turns to the right, and exploration was halted by deep water. A small alcove straight ahead does not lead anywhere. At this point the passage is 5 feet high above the water level, and 20 feet wide. Apparently the passage continues and connects with the passage leading from the upstream rear entrance. Perhaps the passage is penetrable with a rubber boat, but this was not determined. The floor of this section is covered by water from 2 to 5 feet deep, which contained frogs.
The passage downstream from the sink entrance is complex. Along the right wall there are a few domes, evidently joint controlled, and some minor side passages which were not completely explored. One portion of the passage comes out at the top of a high passage which leads to the river entrance, which may be seen from here. The view from this point is quite impressive, as it is 400 feet to the river entrance, and 50 feet down to the stream below, the entire area being illuminated by daylight.
In the central portion of the passage to the downstream side of the ponor is a large pile of breakdown, through which the cave stream flows. The stream cascades down over a series of drops, and finally enters a pool below the point mentioned above. This pool is apparently quite deep, being at least 8 feet to the bottom.
Along the left wall of the main passage there are several minor pockets and side passages, none of which were completely checked. One small passage leads to a dome which reaches to the surface 70 feet above, and thus is a fourth entrance, though it is doubtful that it has ever been used as such. From the bottom of the dome a passage 5 feet high and up to 7 feet wide leads 75 feet to a point on the left wall above the pool mentioned above. This is 10 feet below the vantage point previously mentioned, and the view of the entrance chamber is not as impressive.
The river entrance is an arch approximately 100 feet high and perhaps that wide too. This entrance leads immediately to a chamber 400 feet long, 50 to 75 feet high and 75 to 100 feet wide. Most of the floor of this chamber is covered with breakdown, some of which appeared to be quite recent. From the pool mentioned before the cave stream flows along the east wall of the chamber, under breakdown piles, and finally out the river entrance to enter Jacks Fork 150 feet outside. A small side passage along the east wall of this chamber was followed to its explorable end, 100 feet from the chamber. It averages 4 feet high and 3 feet wide, with a small stream on the floor. There are a few formations in this passage, none being of note. Two gray salamanders with black or dark gray eyes, presumably the Ozark Cave salamander, and one Lana-tailed salamander (Eurycea 1. longicauda) were seen, along with several frogs.
Jam Up Cave is obviously another case at stream piracy by a previously existing cave. Jam Up Creek was captured by the lower cave, diverting the stream flow underground. Since then there has been considerable vadose alteration of the original cave, which must have been phreatic. Presently the cave may be traversed from the sink entrance out through the river entrance, but a 50-foot pool of cold cave water must be crossed to do so. The traverse to the stream entrance may be possible at low water with a rubber boat.
This cave, although relatively short, is 1 of the most impressive in the Ozarks, and is well worth a visit. It is doubtful that the cave would suffer from overuse, as there are no speleothems of note, and most of the cave is flooded at times anyhow. The biology of the cave may be disturbed, however. Only one bat was seen in the cave, and it was flying in the large river entrance chamber. It appeared to be a red bat (Lasiurus borealis), but this is a rare bat in caves, the identification being open to question.

Tom Tucker-April 1963 and Lang Brod-August 1967