Karst & Glacial Landforms

6.5 Groundwater — The Hidden Sculptor

The interest of geomorphology in groundwater is not as a resource; it is in groundwater's work in the erosion of landmasses and the evolution of landforms. Surface water percolates well when the rocks are permeable, thinly bedded and highly jointed and cracked. After moving vertically downward to some depth, the water under the ground flows horizontally through bedding planes, joints, or through the rock material itself. This downward and horizontal movement of water is what causes rocks to erode.

The physical or mechanical removal of materials by moving groundwater is insignificant in developing landforms. That is why the work of groundwater cannot be seen in all types of rocks. But in rocks like limestones or dolomites rich in calcium carbonate, surface water as well as groundwater chemically attack the rock through the twin processes of solution (CaCO₃ dissolves in carbonated rainwater) and precipitation (the dissolved load is later redeposited when the carbonated water loses its carbon dioxide). These two processes together can carve out an extraordinary range of landforms.

6.6 Erosional Karst Landforms

Pools, Sinkholes, Lapies & Limestone Pavements

Small to medium-sized round to sub-rounded shallow depressions called swallow holes form on the surface of limestones through solution. Sinkholes are very common in limestone/karst areas. A sinkhole is an opening that is more or less circular at the top and funnel-shaped towards the bottom, with sizes varying from a few square metres to a hectare and depths from less than half a metre to thirty metres or more. Some sinkholes form solely through solution (solution sinks); others might begin as solution sinks and, when the bottom forms the roof of an underground void or cave, the roof might collapse, leaving a large hole opening into the cave below — these are called collapse sinks. Sinkholes are often covered up with a soil mantle and appear as shallow water pools — anyone stepping over them sinks down as in desert quicksand.

The term doline is sometimes used to refer to collapse sinks. Solution sinks are more common than collapse sinks. Quite often the surface run-off simply goes down swallow and sink holes and flows as underground streams, re-emerging at a distance downstream through a cave opening.

When sinkholes and dolines join together — through slumping of materials along their margins or through roof collapse of caves — long, narrow to wide trenches called valley sinks or uvalas form. Gradually most of the surface of the limestone is eaten away by these pits and trenches, leaving the surface extremely irregular with a maze of points, grooves and ridges called lapies. Lapies form especially due to differential solution activity along parallel to sub-parallel joints. The lapie field may eventually turn into smooth limestone pavements.

Caves & Tunnels

In areas where there are alternating beds of rocks (shales, sandstones, quartzites) with limestones or dolomites in between — or where limestones are dense, massive and occur as thick beds — cave formation is prominent. Water percolates down either through the materials or through cracks and joints, then moves horizontally along bedding planes. Along these bedding planes, the limestone dissolves and long, narrow to wide gaps called caves result. There can be a maze of caves at different elevations depending upon the limestone beds and the intervening rocks. Caves normally have an opening through which cave streams are discharged. Caves having openings at both ends are called tunnels.

6.7 Depositional Karst Landforms

Many depositional forms develop within the limestone caves. The chief chemical in limestone is calcium carbonate, which is easily soluble in carbonated water (rainwater that has absorbed atmospheric carbon dioxide). This calcium carbonate is redeposited when the water carrying it in solution evaporates or loses its carbon dioxide as it trickles over rough rock surfaces inside the cave. The result is a beautiful catalogue of dripstone formations.

Stalactites, Stalagmites & Pillars

Stalactites^? hang as icicles of different diameters from the cave roof. Normally they are broad at their bases and taper towards their free ends, showing up in a variety of forms. Stalagmites^? rise up from the floor of caves. In fact, stalagmites form due to dripping water from the surface or through the thin pipe of the stalactite immediately above. Stalagmites may take the shape of a column, a disc with either a smooth, rounded bulging end or a miniature crater-like depression. The stalagmite and stalactite eventually fuse to give rise to columns and pillars of different diameters. Slender side branches that grow at odd angles are called helictites.

6.8 Glaciers — Rivers of Ice

Masses of ice moving as sheets over the land are called continental glaciers (or piedmont glaciers if a vast sheet of ice spreads over the plains at the foot of mountains). Linear flows of ice down the slopes of mountains in broad trough-like valleys are called mountain or valley glaciers. Both are simply called glaciers. The movement of glaciers is slow, unlike water flow — a few centimetres to a few metres a day, sometimes less and sometimes more. Glaciers move basically because of the force of gravity.

Erosion by glaciers is tremendous because of the friction caused by the sheer weight of ice. The material plucked from the land by glaciers — usually large-sized angular blocks and rock fragments — gets dragged along the floors or sides of the valleys and causes great damage through abrasion and plucking. Glaciers can damage even un-weathered rocks and reduce high mountains into low hills and plains.

As glaciers continue to move, debris is removed, divides are lowered, and eventually the slope is reduced to such an extent that glaciers stop moving — leaving only a mass of low hills and vast outwash plains along with other depositional features.

6.9 Erosional Glacial Landforms

Cirques — The Cradles of Glaciers

Cirques^? are the most common landforms in glaciated mountains. They are quite often found at the heads of glacial valleys. The accumulated ice cuts these cirques while moving down from the mountain tops. They are deep, long and wide troughs or basins with very steep concave to vertically dropping high walls at their head as well as at their sides. A lake of water can quite often be seen within the cirques after the glacier disappears — such lakes are called cirque lakes or tarn lakes. There can be two or more cirques one leading into another in a stepped sequence.

Horns and Serrated Ridges (Arêtes)

Horns^? form through head-ward erosion of the cirque walls. If three or more radiating glaciers cut headward until their cirques meet, the high, sharp pointed and steep-sided peaks formed are called horns. The divides between cirque side-walls or head-walls get narrow because of progressive erosion and turn into serrated or saw-toothed ridges sometimes referred to as arêtes^? with a very sharp crest and a zig-zag outline.

Glacial Valleys / Troughs — The U-Shape Signature

Glaciated valleys are trough-like and U-shaped with broad floors and relatively smooth, steep sides. The valleys may contain littered debris or debris shaped as moraines with a swampy appearance. There may be lakes gouged out of the rocky floor or formed by debris within the valleys. There can be hanging valleys at an elevation on one or both sides of the main glacial valley — tributary glaciers cut shallower valleys than the main glacier, so when the ice melts, these tributary valleys are left "hanging" high above the main valley floor, often with a waterfall plunging down. The faces of divides or spurs of such hanging valleys opening into the main glacial valley are quite often truncated to give them an appearance like triangular facets. Very deep glacial troughs filled with sea water and making up shorelines (in high latitudes) are called fjords/fiords^?.

6.10 Depositional Glacial Landforms

The unassorted coarse and fine debris dropped by melting glaciers is called glacial till. Most rock fragments in till are angular to sub-angular in form. Streams form by melting ice at the bottom, sides or lower ends of glaciers and some amount of rock debris small enough to be carried by such melt-water streams is washed down and deposited. Such glacio-fluvial deposits are called outwash deposits. Unlike till, outwash deposits are roughly stratified and assorted. The rock fragments in outwash deposits are somewhat rounded at their edges.

Moraines

Moraines^? are long ridges of glacial-till deposits. Terminal moraines are long ridges of debris deposited at the end (toe) of the glacier. Lateral moraines form along the sides parallel to the glacial valleys. The lateral moraines may join a terminal moraine, forming a horseshoe-shaped ridge. There can be many lateral moraines on either side in a glacial valley. These moraines partly or fully owe their origin to glacio-fluvial waters pushing materials to the sides of glaciers. Many valley glaciers retreating rapidly leave an irregular sheet of till over their valley floors — such deposits, varying greatly in thickness and surface topography, are called ground moraines. The moraine in the centre of the glacial valley flanked by lateral moraines is called medial moraine; medial moraines are imperfectly formed compared with lateral moraines and are sometimes indistinguishable from ground moraines. Recessional moraines are smaller terminal-type ridges left where the glacier paused during its retreat.

Eskers

When glaciers melt in summer, water flows on the surface of the ice or seeps down along the margins or even moves through holes in the ice. These waters accumulate beneath the glacier and flow like streams in a channel under the ice. Such streams flow over the ground (not in a valley cut in the ground) with ice forming their banks. Very coarse materials like boulders and blocks, along with some minor fractions of rock debris carried into this stream, settle in the ice-walled valley beneath the glacier — and after the ice melts they can be found as a sinuous ridge called esker^?.

Outwash Plains

The plains at the foot of the glacial mountains, or beyond the limits of continental ice sheets, are covered with glacio-fluvial deposits in the form of broad flat alluvial fans which may join to form outwash plains of gravel, silt, sand and clay.

Drumlins

Drumlins^? are smooth, oval-shaped, ridge-like features composed mainly of glacial till with some masses of gravel and sand. The long axes of drumlins are parallel to the direction of ice movement. They may measure up to 1 km in length and 30 m or so in height. One end of the drumlin facing the glacier — called the stoss end — is blunter and steeper than the other end called the tail. Drumlins form due to dumping of rock debris beneath heavily loaded ice through fissures in the glacier. The stoss end gets blunted because of pushing by moving ice. Drumlins give an indication of the direction of glacier movement.