| MACROCAVERN | Includes all cavities in rocks greater than 20 cm in diameter. This includes all the cave passages that are accessible to people. Cf Mesocavern, Microcavern. Cave - Kinds of. Ref EH |
| MACROINVERTRBRATE | An invertebrate large enough to be easily visible to the naked eye. Fauna. Ref AC |
| MAGNESITE | Magnesium carbonate rock. Usually considered as a mineral, and may have various impurities such as Calcium or Iron and may be susceptible to karst solution processes. (cf dolomite).Example is the magnesite karst at Savage River in NW Tasmania. Karst - Kinds of. Ref AC |
| MAGNETIC NORTH | The direction to the north magnetic pole. The position of the north (and south) magnetic poles undergoes continuous change year by year. Survey & Mapping term. Ref JJ |
| MARBLE | Limestone recrystallised and hardened by pressure and heat. Karst - Kinds of. Ref JJ |
| MARL | A calcareous clay, or impure fine grained limestone. Karst - Kinds of. Ref KG |
| MASS MOVEMENT | Dislodgment and downslope transport of soil and bedrock under the influence of gravity. Geology. Ref MM |
| MASSIVE | A rock mass that is not obviously fractured, or with individual strata more than 100mm in thickness. Geology. Ref MM |
| MAZE CAVE | A complex pattern of connecting passages often on two or more levels in a cave. Syn Labyrinth, Network. Speleology term. Ref JJ |
| MEANDER | An arcuate curve in a river course due to the stream eroding sideways. Geology. Ref JJ |
| MEANDER NICHE | A hemispherically roofed part of a cave formed by a stream meandering and cutting down at the same time. Speleology term. Ref JJ |
| MEROKARST | Having bare surfaces on thick deposits of limestone that extend below sea level and little or no surface drainage. They have karren, dolines, uvalas and cave systems. cf holokarst. Karst - Kinds of. Ref WM |
| MESOCAVERN | Includes all cavities in rock that are smaller than 20 cm in diameter and larger than 0.1 cm in diameter. Not large enough to be considered as a cave in the usual sense. Cf Microcavern, Macrocavern. Cave - Kinds of. Ref EH |
| METABOLIC RATE | The rate at which an organism transforms food into energy and body tissue; most cave animals, particularly the obligates in the dark zone have a reduced metabolic rate. Fauna. Ref MM |
| MICROCAVERN | All cavities in rock that are smaller than 0.1 cm in diameter down to and including microscopic size. Whislt not considered a cave, such caverties may be important biological niche sites. Cf Mesocavern, Macrocavern. Cave - Kinds of. Ref EH |
| MICROCLIMATE | The climate (temperature, humidity, air movement etc) of a restricted space, eg of a cave, or of a portion of a cave. see Microhabitat. Speleology term. Ref JJ |
| MICROGOUR | Miniature rimstone dam with associated tiny pool of the order 10mm wide and deep. Most commonly occurs on flowstone, but may occur in other situations. Cave - Depositional Features. Ref JJ |
| MICROHABITAT | The individual faunal habitat or niche within a larger (cave) environment. Fauna. Ref AC |
| MICROKARST | Karst topography in which all surface features are small. An area dominated by minor karst features. Karst - Kinds of. Ref WM |
| MOONMILK | Often a cauliflower shaped mass of soft, white, moist, plastic speleothem consisting of calcite, and hydrated minerals such as hydrocalcite, hydromagnesite or huntite. May become hard or powdery if they dry out. Syn Rockmilk. Cave - Depositional Features. Ref JJ |
| MORPHOLOGY | The study of the physical form of lands or regions. Also the form and structure of any natural phenomena, eg of plant or animal design. Geology. Ref MM |
| MOTTLING | Blotchy or patchy colour patterning. Speleology term. Ref KG |
| MUD PENDULITE | A pendulite with the knob coated in mud. Cave - Erosional & Solutional Features. Ref JJ |