Palaeogeography around the Harappan Port of Lothal, Gujarat, Western India

Article excerpt

Introduction

Lothal, one of the most fascinating remnants of the ancient Harappan civilisation, covers an area of 64752[m.sup.2] (Rao 1985). The essential components of the town are a dockyard 37m in length and 21.8m wide to the west of which are located an 'acropolis', 'lower town' and a cemetery (Rao 1985) (Figure 1). Harappan Lothal has been interpreted to be primarily a port. Rao (1985) suggested that the modern silted creek extending up to Lothal represented the ancient river that was used by the Harappan people. This creek joins the river Bhogavo towards the south but can be seen today only as far as the town of Saragwala, south of Lothal. Satellite multispectral data can help in mapping ancient palaeochannels and understanding past geography (Jensen 2000). Here, we use Indian Remote Sensing LISS-3 multispectral data to argue for the former presence of a wide, tidally-influenced river channel adjacent to Lothal and we cite environmental evidence to relate it to the Harappan period.

Methods

Field surveys show that in the entire region the upper 20cm of the land surface is represented by dark brown to blackish coloured clays. This uniformity in sediment composition coupled with absence of topographic variation makes the task of identifying palaeochannels from the surface extremely difficult. Sediments were sampled and analysed palaeontologically from a trench north of Lothal in order to substantiate the depositional environment of the surface sediments.

To search for the palaeo river channels, geocoded IRS 1D LISS-3 (Linear imaging self scanning) images were used derived from satellite remote sensing data. The multispectral data is in four spectral bands: Green, Red, Near Infra Red (NIR), and Short wave Infra Red (SWIR) (Jensen 2000). Peak summer images were used, since visibility in other months of the year is obstructed by heavy cloud cover and vegetation. The satellite imagery was analysed using ERDAS Imagine [TM] image processing software. Various image-processing and enhancement techniques such as principal component analysis, de-correlation stretch and band ratios were used to obtain maximum contrast. Of these, de-correlation stretch was found to provide maximal tonal contrast and was used for palaeochannel mapping (Figure 2). Field checks were made during summer and the spectral signatures verified (by ground truthing).

Results

The enhanced images revealed a number of palaeochannel features as subtle tonal variations. The key features which were deduced include a sinuous channel adjoining Lothal to the north (Figure 2a), several single-thread palaeo-tributaries south-east of Lothal (Figure 2b) and a palaeoestuary east of Lothal and adjacent to its shoreline (Figure 2c).

The North palaeochannel

This palaeochannel was tracked in the satellite imagery for a stretch of 30km. It is a northern extension of the broad bed of a tributary to the Bhogavo River. The Bhogavo River tributary has a large width as compared to its depth, typical for tidal channels. It abruptly dies out west of the village of Saragwala. North of Lothal the sediments are brown to brownish black. The geomorphology of this region is featureless and devoid of topography, save for the mound of Lothal. In the satellite imagery, the palaeochannels are clearly identified by darker tones (Figure 2a). After extending the form of the modern Bhogavo River tributary for another 8 km, this palaeochannel reduces in width. Nine km north of Lothal the river becomes increasingly sinuous and shows scroll-bar complexes. The channel sinuosity for the upper segment is 2.5 as compared to 1.1 for the lower reaches (Figure 3), and the channel widths are small (10-300m), compared with the lower reaches which have widths of 1.2-1.6km. This suggests the presence of a strong tidal influence up to Lothal and 6km north of it. The scroll bar complexes have 4-5 ridge-swale structures, which is differentiated, based again on tonal variations. …