ABSTRACT
With the aid of Global Positioning System (GPS) and Satellite Aerial Photo, a comparative
characterization and mapping of soil gully erosion features on two geological formations
were carried out in Nsukka area of eastern Nigeria. The two geological formations were
Ajali and Mamu formations. The study involved the use of base map created using a
Geographic Information System (GIS) (GPS Track Marker) and Satellite Photo downloaded
from the internet using the same GIS (GPS Track Marker). This aided the field work for
erosion site study and data collection. A total of seventy (70) erosion sites with an average
length of about 1606.5 meters, average width of about 64.2 meters and average depth of 8.6
meters were visited in Ajali formation. On the other hand, only nine with an average length
of about 484.2 meters, average width of about 6.5 meters and an average depth of about 3.7
meters were visited in Mamu. In Ajali formation, forty three new erosion sites were
identified to add to the twenty seven old sites while in Manu formation only five new sites
were identified to add to the four old sites. Three profile pits were dug in each formation to
represent the soils. They are sites of Ada (Mpt1), Agu-Orba (Mpt2) and Agu-Ekwegbe
(Mpt3) on Mamu Formation while Iheaka (Apt1), Ede-Oballa (Apt2) and Aku (Apt3) were
sites on Ajali Formation.

The soils from the pits were sampled and analyzed for some
physical and chemical properties. The properties were, colour, texture, soil reaction, organic
matter, exchangeable bases, total nitrogen and available phosphorus, exchangeable acidity,
cat ion exchange capacity, and aggregate stability. There was a significant difference in the
value of gully length and width while the depth was statistically the same. There was a
positive significant correlation between length and width (r = 0.409), depth and width (r =
0.862), but non significant correlation between length and depth (r = 0.188) in Mamu
Formation, while a positive and significant correlation was found between length and depth
(r = 0.635), length and width (r = 0.578), depth and width (r = 0.689) in Ajali formation.
The results of the soil percentage state of aggregation (PSA) and percentage aggregate
stability (PAS) was low at both soils. There was no significant difference between their
mean weight diameters (MWD). Their low MWD values (1.1mm) in the soil of Mamu
formation and 1.2mm in the soil of Ajali formation were indication that the soil were highly
susceptible to erosion. Both soils of the studied area have low silt content (8% in the soil of
Ajali and 5% in the soil of Mamu), but moderate to high fine sand values (18% in Ajali and
49% in Mamu). These could be one of the factors promoting the soil erodibility. At micro