An appraisal in the field of the engineering geology problems associated withthe Southern Basin Synclinorium was undertaken on April 23, 1989. The field trip focused onslope instability along the Moruga Road and the sedimentology of exposures in the Marac area.

Landslides are gravity-induced, relatively shallow mass-movements restricted ordinarily to the outermost 300m of the earth's crust. They range from tiny 'pop-outs' on soil slopes to enormous slide complexes involving cubic kilometers of material. They range in shape from sheets and slabs to blocks and tongues, in general sense of displacements from vertical to nearly horizontal, in rate of movement from meters per second to millimeters per year, and in duration of activity from seconds to years. Thus they are difficult to treat systematically.

Landslides can be distinguished in terms of their dominant styles of behaviour - falling, sliding and flowing. Falls involve very rapid dropping or toppling of bedrock or surficial deposits from near vertical slopes, whereas slides move very slowly to very rapidly along one or more surfaces of shear failure. Flows involve very slow to very rapid travel in which particles become intermixed. Many simple to highly complex combinations of these movements may occur in any one episode of activity.

Landslides and mass earth movements in general have caused severe damage to properties in several areas in Trinidad and Tobago. Among the areas affected by land sliding the Moruga area stands out. In the Moruga area landslides are generally associated with the northern and southern flanks of the Ortoire/Siparia/Erin Synclinorium, and with particular lithology types. On this trip we will have a first-hand look at different types of earth movements with a view to:

Stop I - Observation of roadside slides/creep occurring between the 1 and 4 mile posts on the Moruga Road. In particular the damage to houses between the 1.6 and 2.6 mile marks (Plates 1 and 2). Geological Formations are Upper and Lower Lengua and Lower Cruse. Type - Mainly Creep.

Stop 2 - Observation of roadside and hillside slides occurring between the 9 and 13 mile posts on the Moruga Road. Extensive damage to asphaltic pavement provides evidence of several episodes of movements. Geological Formations are Cruse, Lengua and Karamat (mainly Karamat). Hillside slides are present off the road. Type - Rotational Slide.

Stop 3 - Marac Bay: Near vertical beds of poorly graded, well-sorted sandstones with clays in the Casa Cruze Sandstone member of the Moruga Formation (Plate 3). Vertical thinly bedded sands with dark grey clay outcrop on western side of bay with gently dipping sandstone beds on eastern side of bay.

Main features are heavy erosion, coarse cobbles, gravels and sands on beach (cobble beach), high energy environment, wrench tectonics, microfolding and faulting.

Stop4 - Marac Quarry: Fossiliferous massive limestone blocks with congolomeratic sandstones of the Soldado Formation. Weathering of lithologies to terra rosa is observed.

The mass movements observed in the area are a complex combination of several types of movements and result from a combination of factors. The structural and lithological relationships are important contributing factors. The geological map indicates that a major thrust fault, the 'Dunmore Hill Thrust', dominates the structure of the area. The lithologies are mainly clays and silts which, because of their mineralogy, structure and chemistry, are prone to movement. The topography is dominated by steep hillsides which in some instances are inclined at an angle greater than 200 and would naturally tend to be unstable. This is further aggravated by precipitous embankment cuts during construction. Given these factors the area would be naturally prone to movement. However, when these natural conditions are aggravated by the activities of man, the area can be described as a 'disaster waiting to happen'. Inadequate or poor drainage systems (both private and public), poor construction techniques e.g. inadequate foundation design for existing structures and steepening of slopes, both by natural and man-related activities are some of the mechanisms triggering movements in the area.

Main conclusions are as follows:

(1) Deep seated movements along the faults could be contributing to surface movements.

(2) Lithology is important as evidenced from the fact that movements occur in a particular lithology type.

(3) The smaller wooden structures that represent the typical dwelling found in the area are more in harmony with existing conditions with larger concrete structures not only manifested greater structural distress but are contributing to the over-loading of the slopes and hence their instability.

(4) Inadequate drainage is clearly a major contributory factor.
(5) Some of the remedial and corrective measures taken, particularly gravity
(6) The Marac area is of particular geological interest and needs to be further investigated.

It was suggested that improving the drainage and upgrading the construction techniques can go a long way towards reducing and preventing the damage caused by earth movements. Properly designed and constructed retaining walls, gabion baskets, redesigning of slopes, planting of specific grasses e.g. bamboo, as well as a reduction of slope angle can be employed to arrest movement. In conclusion we would like to emphasise the need for the following:

(a) Adequate site investigation.

lb) Good design and construction of retaining structures.

(c)Effective drainage, both private and public, including drainage design for retaining structures.